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CONTENTS

FOREWORD
PREFACE

1 Reproduction  in  Organisms

1.1  ASEXUAL REPRODUCTION
1.2 SEXUAL REPRODUCTION
1.2.1 Pre-fertilisation Events

1.2.1.1 Gametogenesis
1.2.1.2 Gamete Transfer
1.2.2 Fertilisation

1.2.3 Post-fertilisation Events
1.2.3.1 The Zygote
1.2.3.2  Embryogenesis



2 Sexual  Reproduction  in  Flowering  Plants

2.1 FLOWER – A FASCINATING ORGAN OF
2.2 PRE-FERTILISATION: STRUCTURES AND EVENTS
2.2.1 Stamen, Microsporangium and Pollen Grain
2.2.2 The Pistil, Megasporangium (ovule) and Embryo sac
2.2.3 Pollination

2.3 DOUBLE FERTILISATION

2.4  POST-FERTILISATION : STRUCTURES AND EVENTS
2.4.1 Endosperm
2.4.2 Embryo
2.4.3 Seed

2.5  APOMIXIS AND POLYEMBRYONY


3  Human  Reproduction

3.1 THE MALE REPRODUCTIVE SYSTEM
3.2 THE FEMALE REPRODUCTIVE SYSTEM
3.3 GAMETOGENESIS
3.4 MENSTRUAL CYCLE
3.5 FERTILISATION AND IMPLANTATION
3.6 PREGNANCY AND EMBRYONIC DEVELOPMENT
3.7 PARTURITION AND LACTATION


4  Reproductive  Health

4.1 REPRODUCTIVE HEALTH – PROBLEMS AND STRATEGIES
4.2 POPULATION EXPLOSION AND BIRTH CONTROL
4.3 MEDICAL TERMINATION OF PREGNANCY (MTP)
4.4 SEXUALLY TRANSMITTED DISEASES (STDS)
4.5 INFERTILITY

5  Principles  of  Inheritance  and  Variation
5.1 MENDEL’S LAWS OF INHERITANCE 
5.2 INHERITANCE OF ONE GENE
5.2.1 Law of Dominance
5.2.2 Law of Segregation
5.2.2.1 Incomplete Dominance
5.2.2.2  Co-dominance
5.3 INHERITANCE OF TWO GENES
5.3.1 Law of Independent Assortment
5.3.2 Chromosomal Theory of Inheritance
5.3.3 Linkage and Recombination
5.4 SEX DETERMINATION
5.4.1 Sex Determination in Humans
5.5 MUTATION
5.6 GENETIC DISORDERS
5.6.1 Pedigree Analysis
5.6.2 Mendelian Disorders
5.6.3 Chromosomal disorders




Biology in essence is the story of life on earth. While individual
organisms die without fail, species continue to live through
millions of years unless threatened by natural or anthropogenic
extinction.  Reproduction  becomes  a  vital  process  without
which species cannot survive for long. Each individual leaves
its  progeny  by  asexual  or  sexual  means.  Sexual  mode  of
reproduction enables creation of new variants, so that survival
advantage  is  enhanced.  This  unit  examines  the  general
principles underlying reproductive processes in living organisms
and then explains the details of this process in flowering plants
and humans as easy to relate representative examples. A related
perspective  on  human  reproductive  health  and  how
reproductive ill health can be avoided is also presented to
complete our understanding of biology of reproduction.

Born in November 1904 in Jaipur (Rajasthan) Panchanan Maheshwari
rose to become one of the most distinguished botanists not only of India
but of the entire world. He moved to Allahabad for higher education
where he obtained his D.Sc. During his college days, he was inspired
by Dr W. Dudgeon, an American missionary teacher, to develop interest
in Botany and especially morphology. His teacher once expressed that
if his student progresses ahead of him, it will give him a great satisfaction.
These words encouraged Panchanan to enquire what he could do for
his teacher in return.

He worked on embryological aspects and popularised the use of
embryological chracters in taxonomy. He established the Department
of  Botany,  University  of  Delhi  as  an  important  centre  of  research  in
embryology and tissue culture. He also emphasised the need for initiation
of  work  on  artificial  culture  of  immature  embryos.  These  days,  tissue
culture  has  become  a  landmark  in  science.  His  work  on  test  tube
fertilisation  and  intra-ovarian  pollination  won  worldwide  acclaim.

He was honoured with fellowship of Royal Society of London (FRS),
Indian  National  Science  Academy  and  several  other  institutions  of
excellence. He encouraged general education and made a significant
contribution to school education by his leadership in bringing out the
very first textbooks of Biology for Higher Secondary Schools published
by NCERT in 1964.

PANCHANAN MAHESHWARI

(1904-1966)

CHAPTER 1
REPRODUCTION IN ORGANISMS


Each and every organism can live only for a certain period
of time. The period from birth to the natural death of an
organism  represents  its  life  span.  Life  spans  of  a  few
organisms are given in Figure 1.1. Several other organisms
are drawn for which you should find out their life spans
and write in the spaces provided.  Examine the life spans
of organisms represented in the Figure 1.1. Isn’t it both
interesting and intriguing to note that it may be as short
as a few days or as long as a few thousand years?  Between
these two extremes are the life spans of most other living
organisms. You may note that life spans of organisms are
not  necessarily  correlated  with  their  sizes;  the  sizes  of
crows and parrots are not very different yet their life spans
show a wide difference. Similarly, a mango tree has a much
shorter life span as compared to a peepal tree. Whatever
be the life span, death of every individual organism is a
certainty, i.e., no individual is immortal, except single-celled
organisms. Why do we say there is no natural death in
single-celled organisms? Given this reality, have you ever
wondered how vast number of plant and animal species
have existed on earth for several thousands of years?  There
must be some processes in living organisms that ensure
this continuity.  Yes, we are talking about reproduction,
something that we take for granted.

BIOLOGY

4
4

Figure 1.1 Approximate life spans of some organisms

REPRODUCTION  IN  ORGANISMS

Reproduction  is  defined  as  a  biological  process  in  which  an
organism gives rise to young ones (offspring) similar to itself. The offspring
grow, mature and in turn produce new offspring. Thus, there is a cycle
of birth, growth and death. Reproduction enables the continuity of the
species, generation after generation. You will study later in Chapter 5
(Principles of Inheritance and Variation) how genetic variation is created
and inherited during reproduction.

There is a large diversity in the biological world and each organism
has  evolved  its  own  mechanism  to  multiply  and  produce  offspring.
The organism’s habitat, its internal physiology and several other factors
are collectively responsible for how it reproduces.  Based on whether
there  is  participation  of  one  organism  or  two  in  the  process  of
reproduction, it is of two types. When offspring is produced by a single
parent  with  or  without  the  involvement  of  gamete  formation,  the
reproduction is asexual. When two parents (opposite sex) participate in
the reproductive process and also involve fusion of male and female
gametes, it is called sexual reproduction.

1.1  ASEXUAL REPRODUCTION
In this method, a single individual (parent) is capable of producing
offspring. As a result, the offspring that are produced are not only
identical to one another but are also exact copies of their parent.
Are  these  offspring  likely  to  be  genetically  identical  or  different?
The  term  clone  is  used  to  describe  such  morphologically  and
genetically similar individuals.

(a)

Figure  1.2 Cell  division  in  unicellular  organism:  (a)  Budding  in

yeast;  (b)  Binary  fission  in  Amoeba

Let us see how widespread asexual reproduction is, among different
groups  of  organisms.  Asexual  reproduction  is  common  among
single-celled organisms, and in plants and animals with relatively simple
organisations. In Protists and Monerans, the organism or the parent
cell divides into two to give rise to new individuals (Figure1.2). Thus,

(b)

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BIOLOGY

(a)

(b)

(c)

(d)

Figure1.3 Asexual  reproductive  structures:  (a)  Zoospores  of  chlamydomonas;  (b)  Conidia  of

Penicillium; (c) Buds in  Hydra ; (d) Gemmules in sponge

6

in these organisms cell division is itself a mode of reproduction. Many
single-celled organisms reproduce by binary fission, where a cell divides
into two halves and each rapidly grows into an adult (e.g., Amoeba,
Paramecium).  In  yeast,  the  division  is  unequal  and  small  buds  are
produced  that  remain  attached  initially  to  the  parent  cell  which,
eventually gets separated and  mature into new yeast organisms (cells).
Members  of  the  Kingdom  Fungi  and  simple  plants  such  as  algae
reproduce through special asexual reproductive structures (Figure 1.3).
The most common of these structures are zoospores that usually are
microscopic  motile  structures.    Other  common  asexual  reproductive
structures are conidia (Penicillium), buds (Hydra) and gemmules (sponge).

REPRODUCTION  IN  ORGANISMS

(a)

Nodes

Buds

Adventitious

Root

(b)

Adventitious

Buds

(c)

(d)

(e)

Figure  1.4 Vegetative propagules in angiosperms: (a) Eyes of potato; (b) Rhizome of ginger;

(c) Bulbil of Agave; (d) Leaf buds of Bryophyllum; (e) Offset of water hyacinth

You have learnt about vegetative reproduction in plants in Class XI.
What do you think – Is vegetative reproduction also a type of asexual
reproduction?  Why  do  you  say  so?  Is  the  term  clone  applicable  to  the
offspring  formed  by  vegetative  reproduction?

While in animals and other simple organisms the term asexual is used
unambiguously, in plants, the term vegetative reproduction is frequently
used. In plants, the units of vegetative propagation such as runner,
rhizome, sucker, tuber, offset, bulb are all capable of giving rise to new
offspring (Figure1.4). These structures are called vegetative propagules.
Obviously, since the formation of these structures does not involve two
parents, the process involved is asexual.

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BIOLOGY

You must have heard about the scourge of the water bodies or about
the ‘terror of Bengal’. This is nothing but the aquatic plant ‘water hyacinth’
which is one of the most invasive weeds found growing wherever there is
standing water. It drains oxygen from the water, which leads to death of
fishes.  You will learn more about it in Chapters 13 and 14. You may find
it interesting to know that this plant was introduced in India because of
its beautiful flowers and shape of leaves.  Since it can propagate vegetatively
at a phenomenal rate and spread all over the water body in a short period
of time, it is very difficult to get rid off them.

Are  you  aware  how  plants  like  potato,  sugarcane,  banana,  ginger,
dahlia are cultivated?  Have you seen small plants emerging from the
buds (called eyes) of the potato tuber, from the rhizomes of banana and
ginger? When you carefully try to determine the site of origin of the new
plantlets in the plants listed above, you will notice that they invariably
arise from the nodes present in the modified stems of these plants. When
the nodes come in contact with damp soil or water, they produce roots
and  new  plants.  Similarly,  adventitious  buds  arise  from  the  notches
present at margins of leaves of Bryophyllum.  This ability is fully exploited
by gardeners and farmers for commercial propagation of such plants.

It is interesting to note that asexual reproduction is the common method
of reproduction in organisms that have a relatively simple organisation,
like algae and fungi and that they shift to sexual method of reproduction
just  before  the  onset  of  adverse  conditions.    Find  out  how  sexual
reproduction  enables  these  organisms  to  survive  during  unfavourable
conditions? Why is sexual reproduction favoured under such conditions?
Asexual (vegetative) as well as sexual modes of reproduction are exhibited
by the higher plants. On the other hand, only sexual mode of reproduction
is present in most of the animals.

1.2 SEXUAL REPRODUCTION

Sexual reproduction involves formation of the male and female gametes,
either by the same individual or by different individuals of the opposite
sex. These gametes fuse to form the zygote which develops to form the
new organism. It is an elaborate, complex and slow process as compared
to asexual reproduction. Because of the fusion of male and female gametes,
sexual reproduction results in offspring that are not identical to the parents
or amongst themselves.

A study of diverse organisms–plants animals or fungi–show  that
though they differ so greatly in external morphology, internal structure
and  physiology,  when  it  comes  to  sexual  mode  of  reproduction,
surprisingly, they share a similar pattern. Let us first discuss what features
are common to these diverse organisms.

All organisms have to reach a certain stage of growth and maturity in
their life, before they can reproduce sexually. That period of growth is

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REPRODUCTION  IN  ORGANISMS

called the juvenile phase. It is known as vegetative phase in plants.
This phase is of variable durations in different organisms.

The end of juvenile/vegetative phase which marks the beginning of
the reproductive phase can be seen easily in the higher plants when they
come to flower. How long does it take for marigold/rice/wheat/coconut/
mango plants to come to flower?  In some plants, where flowering occurs
more than once, what would you call the inter-flowering period – juvenile
or  mature?

Observe a few trees in your area. Do they flower during the same
month year after year? Why do you think the availability of fruits like
mango, apple, jackfruit, etc., is seasonal? Are there some plants that flower
throughout the year and some others that show seasonal flowering?
Plants –the  annual  and  biennial  types,  show  clear  cut  vegetative,
reproductive and senescent phases, but in the perennial species it is very
difficult to clearly define these phases. A few plants exhibit unusual
flowering phenomenon; some of them such as bamboo species flower only
once in their life time, generally after 50-100 years, produce large number
of fruits  and die. Another plant, Strobilanthus kunthiana (neelakuranji),
flowers once in 12 years. As many of you would be knowing that this
plant  flowered  during  September-October  2006.  Its  mass  flowering
transformed large tracks of hilly areas in Kerala, Karnataka and Tamil
Nadu into blue stretches and attracted a large number of tourists. In
animals, the juvenile phase is followed by morphological and physiological
changes prior to active reproductive behaviour. The reproductive phase
is also of variable duration in different organisms.

Can you list the changes seen in human beings that are indicative

of  reproductive  maturity?

Among animals, for example birds, do they lay eggs all through the
year? Or is it a seasonal phenomenon? What about other animals like
frogs and lizards? You will notice that, birds living in nature lay eggs only
seasonally. However, birds in captivity (as in poultry farms) can be made
to lay eggs throughout the year. In this case, laying eggs is not related to
reproduction but is a commercial exploitation for human welfare.  The
females of placental mammals exhibit cyclical changes in the activities of
ovaries and accessory ducts as well as hormones during the reproductive
phase. In non-primate mammals like cows, sheep, rats, deers, dogs, tiger,
etc., such cyclical changes during reproduction are called oestrus cycle
where as in primates (monkeys, apes, and humans) it is called menstrual
cycle. Many mammals, especially those living in natural, wild conditions
exhibit such cycles only during favourable seasons in their reproductive
phase and are therefore called seasonal breeders. Many other mammals
are reproductively active throughout their reproductive phase and hence
are called continuous breeders.

That we all grow old (if we live long enough), is something that we
recognise. But what is meant by growing old? The end of reproductive

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BIOLOGY

phase can be considered as one of the parameters of senescence or old
age.  There  are  concomitant  changes  in  the  body  (like  slowing  of
metabolism, etc.) during this last phase of life span. Old age ultimately
leads to death.

In  both  plants  and  animals,  hormones  are  responsible  for  the
transitions between the three phases. Interaction between hormones and
certain environmental factors regulate the reproductive processes and
the associated behavioural expressions of organisms.
Events in sexual reproduction : After attainment of maturity, all sexually
reproducing organisms exhibit events and processes that have remarkable
fundamental similarity, even though the structures associated with sexual
reproduction are indeed very different. The events of sexual reproduction
though  elaborate  and  complex,  follow  a  regular  sequence.  Sexual
reproduction is characterised by the fusion (or fertilisation) of the male and
female gametes, the formation of zygote and embryogenesis. For convenience
these sequential events may be grouped into three distinct stages namely,
the pre-fertilisation, fertilisation and the post-fertilisation events.

1.2.1 Pre-fertilisation Events

These include all the events of sexual reproduction prior to the fusion of
gametes.  The two main pre-fertilisation events are gametogenesis and
gamete transfer.

1.2.1.1 Gametogenesis

As you are already aware gametogenesis refers to the process of formation
of the two types of gametes – male and female. Gametes are haploid
cells.  In  some  algae  the  two  gametes  are  so  similar  in  appearance
that it is not possible to categorise them into male and female gametes.

10

(a)

(b)

(c)

Figure  1.5 Types  of  gametes:  (a)  Isogametes  of  Cladophora  (an  alga);  Heterogametes

(b) Fucus (an alga); (c) Human beings

REPRODUCTION  IN  ORGANISMS

They are hence, are called homogametes (isogametes) (Figure 1.5a).
However, in a majority of sexually reproducing organisms the gametes
produced are of two morphologically distinct types (heterogametes).  In
such organisms the male gamete is called the antherozoid or sperm
and the female gamete is called the egg or ovum (Figure1.5 b, c).
Sexuality in organisms: Sexual reproduction in organisms generally
involves the fusion of gametes from two different individuals. But this
is  not  always  true.    From  your  recollection  of  examples  studied  in
Class XI, can you identify cases where self-fertilisation is observed? Of
course, citing such examples in plants is easy.

Plants may have both male and female reproductive structures in the
same plant (bisexual) (Figure 1.6 c, e) or on different plants  (unisexual)
(Figure 1.6 d). In several fungi and plants, terms such as homothallic
and  monoecious  are  used  to  denote  the  bisexual  condition  and
heterothallic and dioecious are the terms used to describe unisexual
condition. In flowering plants, the unisexual male flower is staminate,
i.e., bearing stamens, while the female is pistillate or bearing pistils. In
some flowering plants, both male and female flowers  may be present on
the same individual (monoecious) or on separate individuals (dioecious).
Some examples of monoecious plants are  cucurbits and coconuts and of
dioecious plants are papaya and date palm. Name the type of gametes
that are formed in staminate and pistillate flowers.

But what about animals?  Are individuals of all species either male or
female  (unisexual)?  Or  are  there  species  which  possess  both  the
reproductive  organs  (bisexual)?  You  probably  can  make  a  list
of several unisexual animal species. Earthworms, (Figure 1.6 a) sponge,
tapeworm and leech, typical examples of bisexual animals that possess
both  male  and  female  reproductive  organs,  are  hermaphrodites.
Cockroach (Figure 1.6b) is an example of a unisexual species.
Cell division during gamete formation : Gametes in all heterogametic
species are of two types namely, male and female. Gametes are haploid
though the parent plant body from which they arise may be either haploid
or diploid. A haploid parent produces gametes by mitotic division.  Does
this  mean  that  meiosis  never  occurs  in  organisms  that  are  haploid?
Carefully examine the flow charts of life cycles of algae that you have
studied in Class XI (Chapter 3) to get a suitable answer.

Several organisms belonging to monera, fungi, algae and bryophytes
have  haploid  plant  body,  but  organisms  belonging  to  pteridophytes,
gymnosperms, angiosperms and most of the animals including human
beings, the parental body is diploid.  It is obvious that meiosis, the reduction
division, has to occur if a diploid body has to produce haploid gametes.

In diploid organisms, specialised cells called meiocytes (gamete mother
cell) undergo meiosis.  At the end of meiosis, only one set of chromosomes

11

Clitellum

Testis  sac
with  testis

Ovary

(a)

(c)

BIOLOGY

Male

Testis

Female

Ovary

Archegoniophore

Antheridiophore

(b)

12

Female thallus

(d)

Male thallus

(e)

Figure  1.6 Diversity  of  sexuality  in  organisms  (a)  Bisexual  animal  (Earthworm);  (b)  Unisexual
animal  (Cockroach);  (c)  Monoecious  plant  (Chara);  (d)  Dioecious  plant  (Marchantia);
(e)  Bisexual  flower  (sweet  potato)

REPRODUCTION  IN  ORGANISMS

Table  1.1: Chromosome  Numbers  in  Meiocytes  (diploid,  2n)  and  Gametes

(haploid, n) of Some Organisms. Fill in the Blank Spaces

Name of organism

Chromosome number
in meiocyte (2n)

Chromosome number
in gamete (n)

Human  beings

House fly

Rat

Dog

Cat

Fruit fly

Ophioglossum  (a  fern)

Apple

Rice

Maize

Potato

Butterfly

Onion

46

12

—

78

—

8

—

34

—

20

—

380

—

23

—

21

—

19

—

630

—

12

—

24

—

16

gets incorporated into each gamete.  Carefully study Table 1.1 and fill in
the diploid and haploid chromosome numbers of organisms.  Is there any
relationship in the number of chromosomes of meiocytes and gametes?

1.2.1.2 Gamete Transfer

After  their  formation,  male  and  female  gametes  must  be  physically
brought  together  to  facilitate  fusion  (fertilisation).  Have  you  ever
wondered  how  the  gametes  meet?  In  a  majority  of  organisms,  male
gamete is motile and the female gamete is stationary. Exceptions are a
few  fungi  and  algae  in  which  both  types  of  gametes  are  motile
(Figure1.7a). There is a need for a medium through which the male
gametes  move.  In  several  simple  plants  like  algae,  bryophytes  and
pteridophytes, water is the medium through which this gamete transfer
takes place. A large number of the male gametes, however, fail to reach
the female gametes. To compensate this loss of male gametes during
transport, the number of male gametes produced is several thousand
times the number of female gametes produced.

In seed plants, pollen grains are the carriers of male gametes and
ovule have the egg. Pollen grains produced in anthers therefore, have to

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BIOLOGY

be  transferred  to  the  stigma  before  it  can  lead  to
fertilisation (Figure 1.7b). In bisexual, self-fertilising
plants, e.g., peas, transfer of pollen grains to the stigma
is relatively easy as anthers and stigma are located close
to each other; pollen grains soon after they are shed,
come  in  contact  with  the  stigma.  But  in  cross
pollinating  plants  (including  dioecious  plants),  a
specialised event called pollination facilitates transfer
of pollen grains to the stigma. Pollen grains germinate
on the stigma and the pollen tubes carrying the male
gametes reach the ovule and discharge male gametes
near the egg.  In dioecious animals, since male and
female gametes are formed in different individuals, the
organism must evolve a special mechanism for gamete
transfer.  Successful transfer and coming together of
gametes is essential for the most critical event in sexual
reproduction, the fertilisation.

1.2.2 Fertilisation

The most vital event of sexual reproduction is perhaps
the fusion of gametes. This process called syngamy
results in the formation of a diploid zygote. The term
fertilisation is also often used for this process. The
terms  syngamy  and  fertilisation  are  frequently  used
though , interchangeably.

What would happen if syngamy does not occur?
However, it has to be mentioned here that in some
organisms like rotifers, honeybees and even some lizards
and  birds  (turkey),  the  female  gamete  undergoes
development to form new organisms without fertilisation.
This phenomenon is called parthenogenesis.
Where  does  syngamy  occur?    In  most  aquatic
organisms, such  as a majority of algae and fishes as well as amphibians,
syngamy occurs in the external medium (water), i.e., outside the body of
the organism.  This type of gametic fusion is called external fertilisation.
Organisms exhibiting external fertilisation show great synchrony between
the sexes and release a large number of gametes into the surrounding
medium (water) in order to enhance the chances of syngamy.  This happens
in  the  bony  fishes  and  frogs  where  a  large  number  of  offspring  are
produced.  A  major  disadvantage  is  that  the  offspring  are  extremely
vulnerable to predators threatening their survival up to adulthood.

In many terrestrial organisms, belonging to fungi, higher animals such
as  reptiles  birds,  mammals  and  in  a  majority  of  plants  (bryophytes,
pteridophytes, gymnosperms and angiosperms), syngamy occurs inside

(a)

(b)

Figure  1.7 (a)  Homogametic  contact  in
alga;  (b)  Germinating  pollen
grains on the stigma of  a flower

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REPRODUCTION  IN  ORGANISMS

the body of the organism, hence the process is called internal fertilisation.
In all these organisms, egg is formed inside the female body where they
fuse with the male gamete. In organisms exhibiting internal fertilisation,
the male gamete is motile and has to reach the egg in order to fuse with it.
In these even though the number of sperms produced is very large, there
is a significant reduction in the number of eggs produced. In seed plants,
however, the non-motile male gametes are carried to female gamete by
pollen tubes.

1.2.3 Post-fertilisation Events

Events in sexual reproduction after the formation of zygote are called
post-fertilisation events.

1.2.3.1 The Zygote

Formation of the diploid zygote is universal in all sexually reproducing
organisms. In organisms with external fertilisation, zygote is formed in
the external medium (usually water), whereas in those exhibiting internal
fertilisation, zygote is formed inside the body of the organism.

 Further development of the zygote depends on the type of life cycle
the organism has and the environment it is exposed to. In organisms
belonging to fungi and algae, zygote develops a thick wall that is resistant
to  dessication  and  damage.  It  undergoes  a  period  of  rest  before
germination.  In organisms with haplontic life cycle (As you have read
in  Class XI), zygote divides by meiosis to form haploid spores that grow
into haploid individuals. Consult your Class XI book and find out what
kind of development takes place in the zygote in organisms with diplontic
and haplo-diplontic life cycles.

Zygote  is  the  vital  link  that  ensures  continuity  of  species
between  organisms  of  one  generation  and  the  next.  Every  sexually
reproducing  organism,  including  human  beings  begin  life  as  a  single
cell–the zygote.

1.2.3.2  Embryogenesis

Embryogenesis refers to the process of development of embryo from the
zygote.  During embryogenesis, zygote undergoes cell division (mitosis)
and cell differentiation. While cell divisions increase the number of cells
in the developing embryo; cell differentiation helps groups of cells to
undergo certain modifications to form specialised tissues and organs to
form an organism. You have studied about the process of cell division
and differentiation in the previous class.

Animals are categorised into oviparous and viviparous based on
whether the development of the zygote take place outside the body of the
female parent or inside, i.e., whether they lay fertilised/unfertilised eggs
or give birth to young ones.  In oviparous animals like reptiles and birds,

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BIOLOGY

the fertilised eggs covered by hard calcareous shell are laid in a safe
place in the environment; after a period of incubation young ones hatch
out. On the other hand, in viviparous animals (majority of mammals
including human beings), the zygote develops into a young one inside
the body of the female organism. After attaining a certain stage of growth,
the young ones are delivered out of the body of the female organism.
Because of proper embryonic care and protection, the chances of survival
of young ones is greater in viviparous organisms.

In  flowering  plants,  the  zygote  is  formed  inside  the  ovule.  After
fertilisation the sepals, petals and stamens of the flower wither and fall
off.  Can  you  name  a  plant  in  which  the  sepals  remain  attached? The
pistil however, remains attached to the plant.  The zygote develops  into
the embryo and the ovules develop into the seed. The ovary develops into
the fruit which develops a thick wall called pericarp that is protective in
function (Figure 1.8). After dispersal, seeds germinate under favourable
conditions to produce new plants.

Figure  1.8 A  few  kinds  of  fruit  showing  seeds  (S)  and

protective  pericarp(P)

SUMMARY

Reproduction  enables  a  species  to  live  generation  after  generation.
Reproduction  in  organisms  can  be  broadly  classified  into  asexual  and
sexual  reproduction.  Asexual  reproduction  does  not  involve  the
formation or fusion of gametes. It is common in organisms that have a
relatively  simple  organisation  such  as  the  fungi,  algae  and  some
invertebrate  animals.  The  offspring  formed  by  asexual  reproduction
are identical and are referred to as clones. Zoospores, conidia, etc., are
the most common asexual structures formed in several algae and fungi.
Budding  and  gemmule  formation  are  the  common  asexual  methods
seen  in  animals.

Prokaryotes  and  unicellular  organisms  reproduce  asexually  by
cell division or binary fission of the parent cell. In several aquatic and
terrestrial  species  of  angiosperms,  structures  such  as  runners,

16

REPRODUCTION  IN  ORGANISMS

rhizomes,  suckers,  tubers,  offsets,  etc.,  are  capable  of  giving  rise  to
new  offspring.    This  method  of  asexual  reproduction  is  generally
referred  to  as  vegetative  propagation.

Sexual  reproduction  involves  the  formation  and  fusion  of  gametes.
It is a complex and slower process as compared to asexual reproduction.
Most of the higher animals reproduce almost entirely by sexual method.
Events of sexual reproduction may be categorised into pre-fertilisation,
fertilisation and post-fertilisation events.  Pre-fertilisation events include
gametogenesis  and  gamete  transfer  while  post-fertilisation  events
include  the  formation  of  zygote  and  embryogenesis.

Organisms  may  be  bisexual  or  unisexual.    Sexuality  in  plants  is
varied,  particularly  in  angiosperms,  due  to  the  production  of  diverse
types  of  flowers.    Plants  are  defined  as  monoecious  and  dioecious.
Flowers  may  be  bisexual  or  unisexual  flowers.

Gametes  are  haploid  in  nature  and  usually  a  direct  product  of  meiotic

division except in haploid organisms where gametes are formed by mitosis.

Transfer of male gametes is an essential event in sexual reproduction.
It is relatively easy in bisexual organisms. In unisexual animals it occurs
by copulation or simultaneous release. In angiosperms, a special process
called pollination ensures transfer of pollen grains which carry the pollen
grains  to  the  stigma.

Syngamy (fertilisation) occurs between the male and female gametes.
Syngamy may occur either externally, outside the body of organisms or
internally, inside the body. Syngamy leads to formation of a specialised
cell  called  zygote.

  The  process  of  development  of  embryo  from  the  zygote  is  called
embryogenesis.  In animals, the zygote starts developing soon after its
formation. Animals may be either oviparous or viviparous.  Embryonal
protection  and  care  are  better  in  viviparous  organisms.

 In flowering plants, after fertilisation, ovary develops into fruit and
ovules  mature  into  seeds.  Inside  the  mature  seed  is  the  progenitor  of
the  next  generation,  the  embryo.

EXERCISES

1. Why  is  reproduction  essential  for  organisms?

2. Which is a better mode of reproduction sexual or asexual? Why?

3. Why is the offspring formed by asexual reproduction referred to as clone?
4. Offspring  formed  due  to  sexual  reproduction  have  better  chances  of

survival.  Why?  Is  this  statement  always  true?

5. How  does  the  progeny    formed  from  asexual  reproduction  differ  from

those  formed  by  sexual  reproduction?

6. Distinguish between asexual and sexual reproduction. Why is vegetative

reproduction  also  considered  as  a  type  of  asexual  reproduction?

17

7. What  is  vegetative  propagation?    Give  two  suitable  examples.

BIOLOGY

8. Define

(a)  Juvenile  phase,
(b)  Reproductive  phase,

(c)  Senescent  phase.

9. Higher  organisms  have  resorted  to  sexual  reproduction  in  spite  of  its

complexity.  Why?

10. Explain  why  meiosis  and  gametogenesis  are  always  interlinked?
11.

Identify  each  part  in  a  flowering  plant  and  write  whether  it  is  haploid
(n) or diploid (2n).
(a) Ovary
(b) Anther

———————————
———————————

———————————
(c) Egg
———————————
(d) Pollen
(e) Male  gamete ———————————

(f ) Zygote

———————————

12. Define  external  fertilisation.  Mention  its  disadvantages.
13. Differentiate between a zoospore and a  zygote.

14. Differentiate  between  gametogenesis  from  embryogenesis.
15. Describe  the  post-fertilisation  changes  in  a  flower.
16. What  is  a  bisexual  flower?  Collect  five  bisexual  flowers  from  your
neighbourhood and with the help of your teacher find out their common
and  scientific  names.

17. Examine  a  few  flowers  of  any  cucurbit  plant  and  try  to  identify  the
staminate  and  pistillate  flowers.  Do  you  know  any  other  plant  that
bears  unisexual  flowers?

18. Why  are  offspring  of  oviparous  animals  at  a  greater  risk  as  compared

to  offspring  of  viviparous  animals?

18



CHAPTER 2 SEXUAL REPRODUCTION IN
FLOWERING PLANTS


Are we not lucky that plants reproduce sexually? The
myriads of flowers that we enjoy gazing at, the scents and
the perfumes that we swoon over, the rich colours that
attract us, are all there as an aid to sexual reproduction.
Flowers do not exist only for us to be used for our own
selfishness. All flowering plants show sexual reproduction.
A look at the diversity of structures of the inflorescences,
flowers  and  floral  parts,  shows  an  amazing  range  of
adaptations to ensure formation of the end products of
sexual reproduction, the fruits and seeds.  In this chapter,
let  us  understand  the  morphology,  structure  and  the
processes  of  sexual  reproduction  in  flowering  plants
(angiosperms).

2.1 FLOWER – A FASCINATING ORGAN OF

ANGIOSPERMS

Human  beings  have  had  an  intimate  relationship  with
flowers  since  time  immemorial.    Flowers  are  objects  of
aesthetic, ornamental, social, religious and cultural value
– they have always been used as symbols for conveying
important  human  feelings  such  as  love,  affection,
happiness, grief, mourning, etc.  List at least five flowers
of  ornamental  value  that  are  commonly  cultivated  at

BIOLOGY

Figure  2.1  A  diagrammatic  representation  of  L.S.  of  a  flower

homes and in gardens. Find out the names of five more flowers that are
used in social and cultural celebrations in your family.  Have you heard
of floriculture – what does it refer to?

To a biologist, flowers are morphological and embryological marvels
and the sites of sexual reproduction.  In class XI, you have read the various
parts of a flower.  Figure 2.1 will help you recall the parts of a typical
flower. Can you name the two parts in a flower in which the two most
important units of sexual reproduction develop?

2.2 PRE-FERTILISATION: STRUCTURES AND EVENTS
Much before the actual flower is seen on a plant, the decision that the plant
is going to flower has taken place. Several hormonal and structural changes
are initiated which lead to the differentiation and further development of
the floral primordium. Inflorescences are formed which bear the floral buds
and  then  the  flowers.  In  the  flower  the  male  and  female  reproductive
structures, the androecium and the gynoecium differentiate and develop.
You would recollect that the androecium consists of a whorl of stamens
representing the male reproductive organ and the gynoecium represents
the female reproductive organ.

20

SEXUAL  REPRODUCTION  IN  FLOWERING  PLANTS
HUMAN REPRODUCTION

2.2.1 Stamen, Microsporangium and Pollen Grain

Figure 2.2a shows the two parts of a typical stamen – the long and slender
stalk called the filament, and the terminal generally bilobed structure
called the anther. The proximal end of the filament
is attached to the thalamus or the petal of the
flower.  The  number  and  length  of  stamens  are
variable in flowers of different species.  If you were
to collect a stamen each from ten flowers (each from
different species) and arrange them on a slide, you
would be able to appreciate the large variation in
size seen in nature.  Careful observation of each
stamen under a dissecting microscope and making
neat diagrams would elucidate the range in shape
and attachment of anthers in different flowers.

A typical angiosperm anther is bilobed with
each lobe having two theca, i.e., they are dithecous
(Figure  2.2).  Often  a  longitudinal  groove  runs
lengthwise separating the theca. Let us understand
the various types of tissues and their organisation
in the transverse section of an anther (Figure 2.3 a).
The bilobed nature of an anther is very distinct in
the transverse section of the anther. The anther is
a four-sided (tetragonal) structure consisting of
four microsporangia located at the corners, two
in each lobe.

(b)

(a)

The  microsporangia  develop  further  and
become pollen sacs.  They extend longitudinally
all through the length of an anther and are packed
with pollen grains.
Structure of microsporangium: In a transverse
section, a typical microsporangium appears near
circular  in  outline.    It  is  generally  surrounded  by  four  wall  layers
(Figure 2.3 b)– the epidermis, endothecium, middle layers and the tapetum.
The outer three wall layers perform the function of protection and help in
dehiscence of anther to release the pollen.  The innermost wall layer is the
tapetum.  It nourishes the developing pollen grains. Cells of the tapetum
possess dense cytoplasm and generally have more than one nucleus.  Can
you think of how tapetal cells could become bi-nucleate?

When the anther is young, a group of compactly arranged homogenous
cells  called  the  sporogenous  tissue  occupies  the  centre  of  each
microsporangium.
Microsporogenesis : As the anther develops, the cells of the sporogenous
tissue undergo meiotic divisions to form microspore tetrads.  What would
be the ploidy of the cells of the tetrad?

Figure  2.2 (a) A  typical  stamen;

(b) three–dimensional  cut  section
 of an anther

21

BIOLOGY

(a)

(b)

Figure 2.3 (a)  Transverse  section  of  a  mature  anther;  (b)  Enlarged  view  of  one  microsporangium

showing wall layers; (c) A dehisced anther

(c)

22

As each cell of the sporogenous tissue is capable of giving rise to a
microspore tetrad. Each one is a potential pollen or microspore mother
cell (PMC). The process of formation of microspores from a pollen mother
cell through meiosis is called microsporogenesis. The microspores, as
they are formed, are arranged in a cluster of four cells–the microspore
tetrad (Figure 2.3 a).  As the anthers mature and dehydrate, the microspores
dissociate from each other and develop into pollen grains (Figure 2.3 b).
Inside each microsporangium several thousands of microspores or pollen
grains  are  formed  that  are  released  with  the  dehiscence  of  anther
(Figure 2.3 c).
Pollen grain: The pollen grains represent the male gametophytes.  If you
touch the opened anthers of Hibiscus or any other flower you would find
deposition of yellowish powdery pollen grains on your fingers. Sprinkle
these grains on a drop of water taken on a glass slide and observe under

SEXUAL  REPRODUCTION  IN  FLOWERING  PLANTS
HUMAN REPRODUCTION

Figure  2.4  Scanning  electron  micrographs  of  a  few  pollen  grains

(a)

a microscope.  You will really be amazed at the variety of architecture –
sizes, shapes, colours, designs – seen on the pollen grains
from different species (Figure 2.4).

Pollen  grains  are  generally  spherical  measuring  about
25-50 micrometers in diameter. It has a prominent two-layered
wall.  The hard outer layer called the exine  is made up of
sporopollenin which is one of the most resistant organic material
known. It can withstand high temperatures and strong acids
and alkali. No enzyme that degrades sporopollenin is so far
known. Pollen grain exine has prominent apertures called germ
pores where sporopollenin is absent.  Pollen grains are well-
preserved as fossils because of the presence of sporopollenin.
The exine exhibits a fascinating array of patterns and designs.
Why  do  you  think  the  exine  should  be  hard?    What  is  the
function of germ pore?  The inner wall of the pollen grain is
called the intine.  It is a thin and continuous layer made up of
cellulose  and  pectin.    The  cytoplasm  of  pollen  grain  is
surrounded by a plasma membrane.  When the pollen grain is
mature it contains two cells, the vegetative cell and generative
cell (Figure 2.5b).  The vegetative cell is bigger, has abundant
food  reserve  and  a  large  irregularly  shaped  nucleus.  The
generative cell is small and floats in the cytoplasm of the
vegetative cell.  It is spindle shaped with dense cytoplasm and
a nucleus.  In over 60 per cent of angiosperms, pollen grains
are shed at this 2-celled stage. In the remaining species, the
generative cell divides mitotically to give rise to the two male
gametes before pollen grains are shed (3-celled stage).

(b)

Figure  2.5  (a)  Enlarged  view  of
a  pollen  grain  tetrad;  (b)  stages
of  a  microspore  maturing  into  a
pollen  grain

23

Pollen grains of many species cause severe allergies and bronchial
afflictions  in  some  people  often  leading  to  chronic  respiratory
disorders – asthma, bronchitis, etc. It may be mentioned that Parthenium
or carrot grass that came into India as a contaminant with imported wheat,
has become ubiquitous in occurrence and causes pollen allergy.

BIOLOGY

Pollen grains are rich in nutrients. It has become a fashion in recent
years to use pollen tablets as food supplements. In western countries, a
large number of  pollen products in the form of tablets and syrups are
available in the market. Pollen consumption has been claimed to increase
the performance of athletes and race horses (Figure 2.6).

Figure  2.6  Pollen  products

When once they are shed, pollen grains have to land on the stigma
before they lose viability if they have to bring about fertilisation. How long
do you think the pollen grains retain viability? The period for which pollen
grains remain viable is highly variable and to some extent depends on the
prevailing temperature and humidity. In some cereals such as rice and
wheat, pollen grains lose viability within 30 minutes of their release, and
in  some  members  of  Rosaceae,  Leguminoseae  and  Solanaceae,  they
maintain viability for months. You may have heard of storing semen/
sperms of many animals including humans for artificial insemination. It
is possible to store pollen grains of a large number of species for years in
liquid nitrogen (-1960C). Such stored pollen can be used as pollen banks,
similar to seed banks, in crop breeding programmes.

2.2.2 The Pistil, Megasporangium (ovule) and Embryo sac

The gynoecium represents the female reproductive part of the flower.  The
gynoecium may consist of a single pistil (monocarpellary) or may have
more than one pistil (multicarpellary).  When there are more than one,
the pistils may be fused together (syncarpous) (Figure 2.7b) or may be
free (apocarpous) (Figure 2.7c). Each pistil has three parts (Figure 2.7a),
the stigma, style and ovary. The stigma serves as a landing platform
for pollen grains.  The style is the elongated slender part beneath the
stigma.  The basal bulged part of the pistil is the ovary.  Inside the ovary
is the ovarian cavity ( ( ( ( (locule).  The placenta is located inside the ovarian
cavity.  Recall the definition and types of placentation that you studied in

24

SEXUAL  REPRODUCTION  IN  FLOWERING  PLANTS
HUMAN REPRODUCTION

Stigma

Style

Ovary
Thalamus

(a)

(b)

(c)

(d)

Figure  2.7 (a) A dissected flower of Hibiscus showing pistil (other floral parts have been removed);
(b)  Multicarpellary,  syncarpous  pistil  of  Papaver ;  (c)  A  multicarpellary,  apocarpous
gynoecium  of Michelia;  (d)  A  diagrammatic  view  of  a  typical  anatropous  ovule

Class XI.  Arising from the placenta are the megasporangia, commonly
called ovules.  The number of ovules in an ovary may be one (wheat,
paddy, mango) to many (papaya, water melon, orchids).
The Megasporangium (Ovule) : Let us familiarise ourselves with the
structure of a typical angiosperm ovule (Figure 2.7d). The ovule is a small
structure attached to the placenta by means of a stalk called funicle.
The body of the ovule fuses with funicle in the region called hilum. Thus,
hilum represents the junction between ovule and funicle. Each ovule has
one or two protective envelopes called integuments. Integuments encircle
the ovule except at the tip where a small opening called the micropyle is
organised. Opposite the micropylar end, is the chalaza, representing the
basal part of the ovule.

Enclosed within the integuments is a mass of cells called the nucellus.
Cells of the nucellus have abundant reserve food materials.  Located in the
nucellus is the embryo sac or female gametophyte. An ovule generally has
a single embryo sac formed from a megaspore through reduction division.
Megasporogenesis : The process of formation of megaspores from the
megaspore mother cell is called megasporogenesis. Ovules generally
differentiate a single megaspore mother cell (MMC) in the micropylar region

25

BIOLOGY

(a)

(b)

(c)

Figure 2.8 (a)  Parts  of  the  ovule  showing  a  large  megaspore  mother  cell,  a  dyad  and  a  tetrad  of
megaspores; (b) 1,2, 4, and 8-nucleate stages of embryo sac and a mature embryo sac;
(c)  A  diagrammatic  representation  of  the  mature  embryo  sac.

26

of  the  nucellus.  It  is  a  large  cell  containing  dense  cytoplasm  and  a
prominent nucleus. The MMC undergoes meiotic division. What is the
importance  of  the  MMC  undergoing  meiosis?  Meiosis  results  in  the
production of four megaspores (Figure 2.8a).
Female  gametophyte  :  In  a  majority  of  flowering  plants,  one  of  the
megaspores  is  functional  while  the  other  three  degenerate.  Only  the
functional megaspore develops into the female gametophyte (embryo
sac). This method of embryo sac formation from a single megaspore is termed
monosporic development. What will be the ploidy of the cells of the nucellus,
MMC, the functional megaspore and female gametophyte?

SEXUAL  REPRODUCTION  IN  FLOWERING  PLANTS
HUMAN REPRODUCTION

Let  us  study  formation  of  the  embryo  sac  in  a  little  more  detail.
(Figure 2.8b). The nucleus of the functional megaspore divides mitotically
to  form  two  nuclei  which  move  to  the  opposite  poles,  forming  the
2-nucleate embryo sac. Two more sequential mitotic nuclear divisions
result in the formation of the 4-nucleate and later the 8-nucleate stages
of the embryo sac. It is of interest to note that these mitotic divisions are
strictly free nuclear, that is, nuclear divisions are not followed immediately
by cell wall formation. After the 8-nucleate stage, cell walls are laid down
leading  to  the  organisation  of  the  typical  female  gametophyte
or embryo sac. Observe the distribution of cells inside the embryo sac
(Figure 2.8b, c).  Six of the eight nuclei are surrounded by cell walls and
organised into cells; the remaining two nuclei, called polar nuclei are
situated below the egg apparatus in the large central cell.

There is a characteristic distribution of the cells within the embryo
sac. Three cells are grouped together at the micropylar end and constitute
the egg apparatus.  The egg apparatus, in turn, consists of two synergids
and one egg cell. The synergids have special cellular thickenings at the
micropylar tip called filiform apparatus, which play an important role in
guiding the pollen tubes into the synergid. Three cells are at the chalazal
end and are called the antipodals. The large central cell, as mentioned
earlier, has two polar nuclei. Thus, a typical angiosperm embryo sac, at
maturity, though 8-nucleate is 7-celled.

2.2.3 Pollination

In the preceding sections you have learnt that the male and female gametes
in flowering plants are produced in the pollen grain and embryo sac,
respectively. As both types of gametes are non-motile, they have to be
brought together for fertilisation to occur. How is this achieved?

Pollination is the mechanism to achieve this objective. Transfer
of pollen grains (shed from the anther) to the stigma of a pistil is
termed pollination. Flowering plants have evolved an amazing array
of  adaptations  to  achieve  pollination. They  make  use  of    external
agents  to  achieve  pollination.  Can  you  list  the  possible  external
agents?
Kinds of Pollination : Depending on the source of pollen, pollination
can be divided into three types.
(i)

Autogamy : In this type, pollination is achieved within the same
flower. Transfer of pollen grains from the anther to the stigma of the
same  flower  (Figure  2.9a).  In  a  normal  flower  which  opens  and
exposes the anthers and the stigma, complete autogamy is rather
rare. Autogamy in such flowers requires synchrony in pollen release
and stigma receptivity and also, the anthers and the stigma should

27

BIOLOGY

two 

types  of 

lie close to each other so that self-pollination
can  occur.  Some  plants  such  as  Viola
(common  pansy),  Oxalis,  and  Commelina
produce 
flowers  –
chasmogamous flowers which are similar to
flowers of other species with exposed anthers
and stigma, and cleistogamous flowers which
do not open at all (Figure 2.9c). In such flowers,
the anthers and stigma lie close to each other.
When  anthers  dehisce  in  the  flower  buds,
pollen grains come in contact with the stigma
to  effect  pollination.  Thus,  cleistogamous
flowers are invariably autogamous as there is
no  chance  of  cross-pollen  landing  on  the
stigma.  Cleistogamous  flowers  produce
assured  seed-set  even  in  the  absence  of
pollinators. Do you think that cleistogamy is
advantageous  or  disadvantageous  to  the
plant? Why?

(ii) Geitonogamy – Transfer of pollen grains from
the anther to the stigma of another flower of
the  same  plant.  Although  geitonogamy  is
functionally  cross-pollination  involving  a
pollinating agent, genetically it is similar to
autogamy since the pollen grains come from
the same plant.

(iii) Xenogamy – Transfer of pollen grains from
anther to the stigma of a different plant (Figure
2.9b). This is the only type of pollination which
during pollination brings genetically different
types of pollen grains to the stigma.

Agents of Pollination : Plants use two abiotic (wind
and water) and one biotic (animals) agents to achieve
pollination. Majority of plants use biotic agents for
pollination. Only a small proportion of plants use
abiotic agents. Pollen grains coming in contact with
the stigma is a chance factor in both wind and water
pollination. To compensate for this uncertainties and
associated loss of pollen grains, the flowers produce
enormous amount of pollen when compared to the
number of ovules available for pollination.

(a)

(b)

28

(c)

Figure  2.9 (a)  Self-pollinated  flowers;

(b) Cross  pollinated  flowers;
(c) Cleistogamous  flowers

SEXUAL  REPRODUCTION  IN  FLOWERING  PLANTS
HUMAN REPRODUCTION

Pollination  by  wind  is  more  common
amongst abiotic pollinations. Wind pollination
also requires that the pollen grains are light
and  non-sticky  so  that  they  can  be
transported  in  wind  currents.  They  often
possess  well-exposed  stamens  (so  that  the
pollens are easily dispersed into wind currents,
Figure 2.10) and large often-feathery stigma
to easily trap air-borne pollen grains. Wind-
pollinated flowers often have a single ovule in
each ovary and numerous flowers packed into
an inflorescence; a familiar example is the corn
cob – the tassels you see are nothing but the
stigma and style which wave in the wind to
trap pollen grains.  Wind-pollination is quite
common in grasses.

Pollination  by  water  is  quite  rare  in
flowering plants and is limited to about 30
genera, mostly monocotyledons. As against
this, you would recall that water is a regular
mode of transport for the male gametes among
the  lower  plant  groups  such  as  algae,
bryophytes and pteridophytes. It is believed,
particularly  for  some  bryophytes  and
pteridophytes, that their distribution is limited
because of the need for water for the transport
of  male  gametes  and  fertilisation.  Some
examples of water pollinated plants are  Vallisneria and Hydrilla which
grow  in  fresh water and several marine sea-grasses such as Zostera. Not
all aquatic plants use water for pollination. In a majority of aquatic plants
such as water hyacinth and water lily, the flowers emerge above the level
of water and are pollinated by insects or wind as in most of the land
plants. In Vallisneria, the female flower reach the surface of water by the
long stalk and the male flowers or pollen grains are released on to the
surface of water. They are carried passively by water currents (Figure
2.11a);  some of them eventually reach the female flowers and the stigma.
In another group of water pollinated plants such as seagrasses, female
flowers remain submerged in water and the pollen grains are released
inside the water. Pollen grains in many such species are long, ribbon like
and they are carried passively inside the water; some of them reach the
stigma and achieve pollination. In most of the water-pollinated species,
pollen grains are protected from wetting by a mucilaginous covering.

Both wind and water pollinated flowers are not very colourful and do

not produce nectar. What would be the reason for this?

Figure  2.10 A  wind-pollinated  plant  showing
compact  inflorecence  and  well-
exposed  stamens

29

BIOLOGY

Majority of flowering plants use
a  range  of  animals  as  pollinating
agents.  Bees,  butterflies,  flies,
beetles, wasps, ants, moths, birds
(sunbirds and humming birds)  and
bats  are  the  common  pollinating
agents. (Figure 2.11b). Among the
animals, insects, particularly bees
are the dominant biotic pollinating
agents.  Even larger animals such
as some primates (lemurs), arboreal
(tree-dwelling)  rodents,  or  even
reptiles  (gecko  lizard  and  garden
lizard) have also been reported as
pollinators in some species.

Often  flowers  of  animal-
pollinated  plants  are  specifically
adapted for a particular species of
animal.

(a)

Majority  of  insect-pollinated
flowers are large, colourful, fragrant
and rich in nectar. When the flowers
are small, a number of flowers are
clustered into an inflorescence to
make them conspicuous. Animals
are attracted to flowers by colour
and/or  fragrance.  The  flowers
pollinated  by  flies  and  beetles
secrete foul odours to attract these
animals. To sustain animal visits,
the flowers have to provide rewards
to the animals. Nectar and pollen
grains are the usual floral rewards.
For harvesting the reward(s) from
the flower the animal visitor comes
in contact with the anthers and the
stigma. The body of the animal gets
a coating of pollen grains, which are
generally sticky in animal pollinated flowers. When the animal carrying
pollen on its body comes in contact with the stigma, it brings about
pollination.

(b)

In some species floral rewards are in providing safe places to lay eggs;
an example is that of the tallest flower of Amorphophallus (the flower
itself is about 6 feet in height). A similar relationship exists between a
species of moth and the plant Yucca where both species – moth and the

Figure  2.1 (a)  Pollination  by  water  in  Vallisneria;

(b)  Insect  pollination

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SEXUAL  REPRODUCTION  IN  FLOWERING  PLANTS
HUMAN REPRODUCTION

plant – cannot complete their life cycles without each other. The moth
deposits its eggs in the locule of the ovary and the flower, in turn, gets
pollinated by the moth.  The larvae of the moth come out of the eggs as
the seeds start developing.

Why don’t you observe some flowers of the following plants (or any
others  available  to  you):  Cucumber,  Mango,  Peepal,  Coriander,  Papaya,
Onion, Lobia, Cotton, Tobacco, Rose, Lemon, Eucalyptus, Banana? Try to
find  out  which  animals  visit  them  and  whether  they  could  be
pollinators.You’ll have to patiently observe the flowers over a few days
and at different times of the day.  You could also try to see whether there
is  any  correlation  in  the  characteristics  of  a  flower  to  the  animal  that
visits it. Carefully observe if any of the visitors come in contact with the
anthers and the stigma as only such visitors can bring about pollination.
Many insects may consume pollen or the nectar without bringing about
pollination. Such floral visitors are referred to as pollen/nectar robbers.
You may or may not be able to identify the pollinators, but you will surely
enjoy your efforts!

Outbreeding Devices : Majority of flowering plants produce hermaphrodite
flowers and pollen grains are likely to come in contact with the stigma of
the same flower. Continued self-pollination result in inbreeding depression.
Flowering  plants  have  developed  many  devices  to  discourage  self-
pollination and to encourage cross-pollination. In some species, pollen
release and stigma receptivity are not synchronised. Either the pollen is
released before the stigma becomes receptive or stigma becomes receptive
much before the release of pollen. In some other species, the anther and
stigma are placed at different positions so that the pollen cannot come in
contact with the stigma of the same flower. Both these devices prevent
autogamy. The third device to prevent inbreeding is self-incompatibility.
This is a genetic mechanism and prevents self-pollen (from the same flower
or other flowers of the same plant) from fertilising the ovules by inhibiting
pollen germination or pollen tube growth in the pistil.  Another device to
prevent self-pollination is the production of unisexual flowers. If both male
and female flowers are present on the same plant such as castor and maize
(monoecious), it prevents autogamy but not geitonogamy. In several species
such as papaya, male and female flowers are present on different plants,
that is each plant is either male or female (dioecy). This condition prevents
both autogamy and geitonogamy.

Pollen-pistil Interaction : Pollination does not guarantee the transfer
of the right type of pollen (compatible pollen of the same species as the
stigma). Often, pollen of the wrong type, either from other species or from
the same plant (if it is self-incompatible), also land on the stigma. The
pistil has the ability to recognise the pollen, whether it is of the right type
(compatible) or of the wrong type (incompatible). If it is of the right type,
the pistil accepts the pollen and promotes post-pollination events that

31

BIOLOGY

(a)

(b)

(d)

(c)

(e)

Figure  2.12

(a)  Pollen  grains  germinating  on  the  stigma;  (b)  Pollen  tubes  growing  through  the
style;  (c)  L.S.  of  pistil  showing  path  of  pollen  tube  growth;  (d)  enlarged  view  of  an
egg  apparatus  showing  entry  of  pollen  tube  into  a  synergid;  (e)  Discharge  of  male
gametes  into  a  synergid  and  the  movements  of  the  sperms,  one  into  the  egg  and
the  other  into  the  central  cell

32

leads to fertilisation. If the pollen is of the wrong type, the pistil rejects the
pollen by preventing pollen germination on the stigma or the pollen tube
growth in the style. The ability of the pistil to recognise the pollen followed
by  its  acceptance  or  rejection  is  the  result  of  a  continuous  dialogue
between pollen grain and the pistil. This dialogue is mediated by chemical
components of the pollen interacting with those of the pistil. It is only in
recent years that botanists have been able to identify some of the pollen
and pistil components and the interactions leading to the recognition,
followed by acceptance or rejection.

As mentioned earlier, following compatible pollination, the pollen grain
germinates on the stigma to produce a pollen tube through one of the
germ pores (Figure 2.12a). The contents of the pollen grain move into the

SEXUAL  REPRODUCTION  IN  FLOWERING  PLANTS
HUMAN REPRODUCTION

pollen tube. Pollen tube grows through the tissues of the stigma and
style and reaches the ovary (Figure 2.12b, c). You would recall that in
some plants, pollen grains are shed at two-celled condition (a vegetative
cell and a generate cell).  In such plants, the generative cell divides and
forms the two male gametes during the growth of pollen tube in the stigma.
In plants which shed pollen in the three-celled condition, pollen tubes
carry the two male gametes from the beginning. Pollen tube, after reaching
the ovary, enters the ovule through the micropyle and then enters one of
the synergids through the filiform apparatus (Figure 2.12d, e). Many recent
studies have shown that filiform apparatus present at the micropylar part
of the synergids guides the entry of pollen tube.  All these events–from
pollen deposition on the stigma until pollen tubes enter the ovule–are
together referred to as pollen-pistil interaction. As pointed out earlier,
pollen-pistil interaction is a dynamic process involving pollen recognition
followed by promotion or inhibition of the pollen. The knowledge gained
in this area would help the plant breeder in manipulating pollen-pistil
interaction, even in incompatible pollinations, to get desired hybrids.

You can easily study pollen germination by dusting some pollen from
flowers such as  pea, chickpea, Crotalaria, balsam and Vinca on a glass slide
containing a drop of sugar solution (about 10 per cent). After about 15–30
minutes, observe the slide under the low power lens of the microscope.  You
are likely to see pollen tubes coming out of the pollen grains.

As you shall learn in the chapter on plant breeding (Chapter 9), a
breeder is interested in crossing different species and often genera to
combine desirable characters to produce commercially ‘superior’ varieties.
Artificial  hybridisation  is  one  of  the  major  approaches  of  crop
improvement programme. In such crossing experiments it is important
to make sure that only the desired pollen grains are used for pollination
and the stigma is protected from contamination (from unwanted pollen).
This is achieved by emasculation and bagging techniques.

If the female parent bears bisexual flowers, removal of anthers from
the  flower  bud  before  the  anther  dehisces  using  a  pair  of  forceps  is
necessary. This step is referred to as emasculation. Emasculated flowers
have to be covered with a bag of suitable size, generally made up of butter
paper, to prevent contamination of its stigma with unwanted pollen. This
process  is  called  bagging.  When  the  stigma  of  bagged  flower  attains
receptivity, mature pollen grains collected from anthers of the male parent
are dusted on the stigma, and the flowers are rebagged, and the fruits
allowed to develop.

If the female parent produces unisexual flowers, there is no need for
emasculation. The female flower buds are bagged before the flowers open.
When the stigma becomes receptive, pollination is carried out using the
desired pollen and the flower rebagged.

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BIOLOGY

2.3 DOUBLE FERTILISATION

After entering one of the synergids, the pollen tube releases the two male
gametes into the cytoplasm of the synergid.  One of the male gametes
moves towards the egg cell and fuses with its nucleus thus completing the
syngamy.  This results in the formation of a diploid cell, the zygote. The
other male gamete moves towards the two polar nuclei located in the central
cell and fuses with them to produce a triploid primary endosperm nucleus
(PEN) (Figure 2.13a).  As this involves the fusion of three haploid nuclei it
is termed triple fusion. Since two types of fusions, syngamy and triple
fusion take place in an embryo sac the phenomenon is termed double
fertilisation, an event unique to flowering plants. The central cell after
triple fusion becomes the primary endosperm cell (PEC) and develops
into the endosperm while the zygote develops into an embryo.

(a)

(b)

Figure  2.13 (a)  Fertilised  embryo  sac  showing  zygote  and  Primary  Endosperm  Nucleus  (PEN);
(b) Stages in embryo development in a dicot [shown in reduced size as compared to (a)]

34

2.4  POST-FERTILISATION : STRUCTURES AND EVENTS
Following  double  fertilisation,  events  of    endosperm  and  embryo
development, maturation of ovule(s) into seed(s) and ovary into fruit, are
collectively termed post-fertilisation events.

2.4.1 Endosperm

Endosperm development precedes embryo development. Why? The
primary  endosperm  cell  divides  repeatedly  and  forms  a  triploid

SEXUAL  REPRODUCTION  IN  FLOWERING  PLANTS
HUMAN REPRODUCTION

endosperm tissue. The cells of this tissue are filled with
reserve food materials and are used for the nutrition of
the  developing  embryo.  In  the  most  common  type  of
endosperm development, the PEN undergoes successive
nuclear divisions to give rise to free nuclei. This stage of
endosperm development is called free-nuclear endosperm.
Subsequently  cell  wall  formation  occurs  and  the
endosperm becomes cellular. The number of free nuclei
formed before cellularisation varies greatly. The coconut
water from tender coconut that you are familiar with, is
nothing  but  free-nuclear  endosperm  (made  up  of
thousands of nuclei) and the surrounding white kernel is
the cellular endosperm.

Endosperm may either be completely consumed by the
developing embryo (e.g., pea, groundnut, beans) before seed
maturation or it may persist in the mature seed (e.g. castor
and coconut) and be used up during seed germination. Split
open some seeds of castor, peas, beans, groundnut, fruit of
coconut and look for the endosperm in each case.  Find out
whether the endosperm is persistent in cereals – wheat, rice
and maize.

2.4.2 Embryo

Embryo develops at the micropylar end of the embryo sac where
the zygote is situated. Most zygotes divide only after certain
amount  of  endosperm  is  formed.  This  is  an  adaptation  to
provide assured nutrition to the developing embryo. Though
the seeds differ greatly, the early stages of embryo development
(embryogeny)  are  similar  in  both  monocotyledons  and
dicotyledons. Figure 2.13 depicts the stages of embryogeny in
a  dicotyledonous  embryo.    The  zygote  gives  rise  to  the
proembryo and subsequently to the globular, heart-shaped
and mature embryo.

A typical dicotyledonous embryo (Figure 2.14a), consists
of an embryonal axis and two cotyledons. The portion of
embryonal axis above the level of cotyledons is the epicotyl,
which terminates with the plumule or stem tip. The cylindrical
portion  below  the  level  of  cotyledons  is  hypocotyl  that
terminates at its lower end in the radical or root tip.  The root
tip is covered with a root cap.

Embryos of monocotyledons (Figure 2.14 b) possess only
one  cotyledon.  In  the  grass  family  the  cotyledon  is  called
scutellum  that  is  situated  towards  one  side  (lateral)  of  the
embryonal axis. At its lower end, the embryonal axis has the

(a)

(b)

Figure  2.14 (a)  A  typical  dicot
embryo; (b)  L.S. of an
embryo of grass

35

BIOLOGY

radical  and  root  cap  enclosed  in  an  undifferentiated  sheath  called
coleorrhiza.  The  portion  of  the  embryonal  axis  above  the  level  of
attachment of scutellum is the epicotyl. Epicotyl has a shoot apex and a
few leaf primordia enclosed in a hollow foliar structure, the coleoptile.
Soak a few seeds in water (say of wheat, maize, peas, chickpeas,
ground  nut)  overnight.    Then  split  the  seeds  and  observe  the  various
parts of the embryo and the seed.

2.4.3 Seed

In angiosperms, the seed is the final product of sexual reproduction.  It is
often described as a fertilised ovule. Seeds are formed inside fruits. A
seed typically consists of seed coat(s), cotyledon(s) and an embryo axis.
The  cotyledons  (Figure  2.15a)  of  the  embryo  are  simple  structures,
generally thick and swollen due to storage of food reserves (as in legumes).
Mature seeds may be non-albuminous or albuminous. Non-albuminous
seeds have no residual endosperm as it is completely consumed during
embryo development (e.g., pea, groundnut).  Albuminous seeds retain a
part  of  endosperm  as  it  is  not  completely  used  up  during  embryo
development (e.g., wheat, maize, barley, castor, sunflower). Occasionally,
in some seeds such as black pepper and beet, remnants of nucellus are
also persistent.  This residual, persistent nucellus is the perisperm.

Integuments  of  ovules  harden  as  tough  protective  seed  coats
(Figure 2.15a). The micropyle remains as a small pore in the seed coat.
This facilitates entry of oxygen and water into the seed during germination.
As the seed matures, its water content is reduced and seeds become
relatively dry (10-15 per cent moisture by mass). The general metabolic
activity of the embryo slows down.  The embryo may  enter a state of
inactivity  called  dormancy,  or  if  favourable  conditions  are  available
(adequate moisture, oxygen and suitable temperature), they germinate.
As ovules mature into seeds, the ovary develops into a fruit, i.e., the
transformation  of  ovules  into  seeds  and  ovary  into  fruit  proceeds
simultaneously.  The wall of the ovary develops into the wall of fruit called
pericarp.  The fruits may be fleshy as in guava, orange, mango, etc., or
may be dry, as in groundnut, and  mustard, etc.  Many fruits have evolved
mechanisms for dispersal of seeds.  Recall the classification of fruits and
their dispersal mechanisms that you have studied in an earlier class. Is
there any relationship between number of ovules in an ovary and the
number of seeds present in a fruit?

In most plants, by the time the fruit develops from the ovary, other
floral parts degenerate and fall off. However, in a few species such as apple,
strawberry, cashew, etc., the thalamus  also contributes to fruit formation.
Such fruits are called false fruits (Figure 2.15b).  Most fruits however
develop only from the ovary and are called true fruits. Although in most
of the species, fruits are the results of fertilisation, there are a few species

36

SEXUAL  REPRODUCTION  IN  FLOWERING  PLANTS
HUMAN REPRODUCTION

(a)

(b)

Figure  2.15  (a)  Structure  of  some  seeds.  (b)  False  fruits  of  apple  and  strawberry

in  which  fruits  develop  without  fertilisation.  Such  fruits  are  called
parthenocarpic fruits. Banana is one such example. Parthenocarpy can
be induced through the application of growth hormones and such fruits
are seedless.

Seeds  offer  several  advantages  to  angiosperms.  Firstly,  since
reproductive  processes  such  as  pollination  and  fertilisation  are
independent of water, seed formation is more dependable. Also seeds have
better adaptive strategies for dispersal to new habitats and help the species

37

BIOLOGY

to colonise in other areas. As they have  sufficient food reserves, young
seedlings are nourished until they are capable of photosynthesis on their
own. The hard seed coat provides protection to the young embryo. Being
products of sexual reproduction, they generate new genetic combinations
leading to variations.

Seed is the basis of our agriculture. Dehydration and dormancy of
mature seeds are crucial for storage of seeds which can be used as food
through out the year and also to raise crop in the next season. Can you
imagine agriculture in the absence of seeds, or in the presence of seeds
which germinate straight away soon after formation and cannot be stored?
How long do the seeds remain alive after they are dispersed? This
period again varies greatly. In a few species the seeds lose viability within
a few months. Seeds of a large number of species live for several years.
Some seeds can remain alive for hundreds of years. There are several
records of very old yet viable seeds. The oldest is that of a lupine, Lupinus
arcticus excavated from Arctic Tundra. The seed germinated and flowered
after an estimated record of 10,000 years of dormancy. A recent record of
2000  years  old  viable  seed  is  of  the  date  palm,  Phoenix  dactylifera
discovered during the archeological excavation at King Herod’s palace
near the Dead Sea.

After completing a brief account of sexual reproduction of flowering
plants  it  would  be  worth  attempting  to  comprehend  the  enormous
reproductive capacity of some flowering plants by asking  the following
questions:  How many eggs are present in an embryo sac? How many
embryo sacs are present in an ovule? How many ovules are present in
an ovary? How many ovaries are present in a typical flower? How many
flowers are present on a tree? And so on...

Can  you  think  of  some  plants  in  which  fruits  contain  very  large
number  of  seeds.  Orchid  fruits  are  one  such  category  and  each  fruit
contain  thousands  of  tiny  seeds.  Similar  is  the  case  in  fruits  of  some
parasitic species such as Orobanche and Striga.  Have you seen a tiny
seed of Ficus? How large is the tree of Ficus developed from that tiny
seed. How many billions of seeds does each Ficus tree produce? Can
you  imagine  any  other  example  in  which  such  a  tiny  structure  can
produce such a large biomass over the years?

2.5  APOMIXIS AND POLYEMBRYONY
Although seeds, in general are the products of fertilisation, a few flowering
plants such as some species of Asteraceae and grasses, have evolved a
special mechanism, to produce seeds without fertilisation, called apomixis.
What is fruit production without fertilisation called? Thus, apomixis is a
form of asexual reproduction that mimics sexual reproduction. There are
several ways of development of apomictic seeds. In some species, the
diploid egg cell is formed without reduction division and develops into
the embryo without fertilisation. More often, as in many Citrus and Mango

38

SEXUAL  REPRODUCTION  IN  FLOWERING  PLANTS
HUMAN REPRODUCTION

varieties some of the nucellar cells surrounding the embryo sac start
dividing, protrude into the embryo sac and develop into the embryos. In
such species each ovule contains many embryos. Occurrence of more
than one embryo in a seed is referred as polyembryony. Take out some
seeds  of  orange  and  squeeze  them.  Observe  the  many  embryos  of
different sizes and shapes from each seed. Count the number of embryos
in each seed. What would be the genetic nature of apomictic embryos?
Can they be called clones?

Hybrid varieties of several of our food and vegetable crops are being
extensively cultivated. Cultivation of hybrids has tremendously increased
productivity. One of the problems of hybrids is that hybrid seeds have
to be produced every year. If the seeds collected from hybrids are sown,
the plants in the progeny will segregate and do not maintain hybrid
characters. Production of hybrid seeds is costly and hence the cost of
hybrid seeds become too expensive for the farmers. If  these hybrids are
made into apomicts, there is no segregation of characters in the hybrid
progeny. Then the farmers can keep on using the hybrid seeds to raise
new crop year after year and he does not have to buy hybrid seeds every
year. Because of the importance of apomixis in hybrid seed industry,
active research is going on in many laboratories around the world to
understand the genetics of apomixis and to transfer apomictic genes
into hybrid varieties.

SUMMARY

Flowers are the seat of sexual reproduction in angiosperms.  In the flower,
androecium  consisting  of  stamens  represents  the  male  reproductive
organs  and  gynoecium  consisting  of  pistils  represents  the  female
reproductive  organs.

A  typical  anther  is  bilobed,  dithecous  and  tetrasporangiate.    Pollen
grains  develop  inside  the  microsporangia.    Four  wall  layers,  the
epidermis,  endothecium,  middle  layers  and  the  tapetum  surround  the
microsporangium. Cells of the  sporogenous tissue lying in the centre of
the  microsporangium,  undergo  meiosis  (microsporogenesis)  to  form
tetrads of microspores.  Individual microspores mature into pollen grains.
Pollen  grains  represents  the  male  gametophytic  generation.    The
pollen  grains  have  a  two-layered  wall,  the  outer  exine  and  inner  intine.
The exine is made up of sporopollenin and has germ pores.  Pollen grains
may  have  two-celled  (a  vegetative  cell  and  generative  cell)  or  three  cells
(a vegetative cell and two male gametes) at the time of shedding.

The pistil has three parts – the stigma, style and the ovary.  Ovules
are present in the ovary. The ovules have a stalk called funicle, protective
integument(s),  and  an  opening  called  micropyle.  The  central  tissue  is
the  nucellus  in  which  the  archesporium  differentiates.  A  cell  of  the
archesporium, the megaspore mother cell divides meiotically and one of
the  megaspores  forms  the  embryo  sac  (the  female  gametophyte).  The
mature embryo sac is 7-celled and 8-nucleate.  At the micropylar end is

39

BIOLOGY

the  egg  apparatus  consisting  of  two  synergids  and  an  egg  cell.    At  the
chalazal end are three antipodals.  At the centre is a large central cell
with  two  polar  nuclei.

Pollination  is  the  mechanism  to  transfer  pollen  grains  from  the
anther  to  the  stigma.    Pollinating  agents  are  either  abiotic  (wind  and
water)  or  biotic  (animals).

Pollen-pistil interaction involves all events from the landing of pollen
grains on the stigma until the pollen tube enters the embryo sac (when
the  pollen  is  compatible)  or  pollen  inhibition  (when  the  pollen  is
incompatible).  Following compatible pollination, pollen grain germinates
on  the  stigma  and  the  resulting    pollen  tube  grow  through  the  style,
enter the ovules and finally discharges two male gametes in one of the
synergids.  Angiosperms exhibit double fertilisation because two fusion
events  occur  in  each  embryo  sac,  namely  syngamy  and  triple  fusion.
The  products  of  these  fusions  are  the  diploid  zygote  and  the  triploid
primary  endosperm  nucleus  (in  the  primary  endosperm  cell).  Zygote
develops  into  the  embryo  and  the  primary  endosperm  cell  forms  the
endosperm  tissue.  Formation  of  endosperm  always  precedes
development  of  the  embryo.

The  developing  embryo  passes  through  different  stages  such  as
the  proembryo,  globular  and  heart-shaped  stages  before  maturation.
Mature  dicotyledonous  embryo  has  two  cotyledons  and  an  embryonal
axis  with  epicotyl  and  hypocotyl.    Embryos  of  monocotyledons  have  a
single cotyledon.  After fertilisation, ovary develops into fruit and ovules
develop  into  seeds.

A  phenomenon  called  apomixis  is  found  in  some  angiosperms,
particularly  in  grasses.  It  results  in  the  formation  of  seeds  without
fertilisation.    Apomicts  have  several  advantages  in  horticulture  and
agriculture.

Some  angiosperms  produce  more  than  one  embryo  in  their  seed.

This  phenomenon  is  called  polyembryony.

EXERCISES

1. Name the parts of an angiosperm flower in which development of male

and  female  gametophyte  take  place.

2. Differentiate  between  microsporogenesis  and  megasporogenesis.  Which
type  of  cell  division  occurs  during  these  events?  Name  the  structures
formed at the end of these two events.

3. Arrange  the  following  terms  in  the  correct  developmental  sequence:

Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell,
male  gametes.

4. With a neat, labelled diagram, describe the parts of a typical angiosperm

ovule.

5. What  is  meant  by  monosporic  development  of  female  gametophyte?
6. With a neat diagram explain the 7-celled, 8-nucleate nature of the female

gametophyte.

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SEXUAL  REPRODUCTION  IN  FLOWERING  PLANTS
HUMAN REPRODUCTION

7. What  are  chasmogamous  flowers?  Can  cross-pollination  occur  in

cleistogamous  flowers?    Give  reasons  for  your  answer.

8. Mention  two  strategies  evolved  to  prevent  self-pollination  in  flowers.
9. What is self-incompatibility? Why does self-pollination not lead to seed

formation  in  self-incompatible  species?

10. What  is  bagging  technique?    How  is  it  useful  in  a  plant  breeding

programme?

11. What  is  triple  fusion?  Where  and  how  does  it  take  place?  Name  the

nuclei  involved  in  triple  fusion.

12. Why  do  you  think  the  zygote  is  dormant  for  sometime  in  a  fertilised

ovule?

13. Differentiate  between:

(a)  hypocotyl  and  epicotyl;
(b)  coleoptile  and  coleorrhiza;
(c)  integument  and  testa;
(d)  perisperm  and  pericarp.

14. Why is apple called a false fruit?  Which part(s) of the flower forms the

fruit?

15. What  is  meant  by  emasculation?  When  and  why  does  a  plant  breeder

16.

employ  this  technique?
If  one  can  induce  parthenocarpy  through  the  application  of  growth
substances,  which  fruits  would  you  select  to  induce  parthenocarpy
and  why?

17. Explain  the  role  of  tapetum  in  the  formation  pollen-grain  wall.
18. What  is  apomixis  and  what  is  its  importance?

41




CHAPTER 3 HUMAN REPRODUCTION

n

As you are aware, humans are sexually reproducing and
viviparous. The reproductive events in humans include
formation of gametes (gametogenesis), i.e., sperms in males
and ovum in females, transfer of sperms into the female
genital tract (insemination) and fusion of male and female
gametes (fertilisation) leading to formation of zygote. This
is followed by formation and development of blastocyst
and its attachment to the uterine wall (implantation),
embryonic development (gestation) and delivery of the
baby (parturition). You have learnt that these reproductive
events  occur  after  puberty.  There  are  remarkable
differences between the reproductive events in the male
and in the female, for example, sperm formation continues
even in old men, but formation of ovum ceases in women
around the age of fifty years. Let us examine the male and
female reproductive systems in human.

3.1 THE MALE REPRODUCTIVE SYSTEM

The male reproductive system is located in the pelvis region
(Figure  3.1a).  It  includes  a  pair  of  testes  alongwith
accessory ducts, glands and the external genitalia.

HUMAN  REPRODUCTION

The testes are situated outside the
abdominal  cavity  within  a  pouch
called scrotum. The scrotum helps
in maintaining the low temperature
of the testes (2–2.5o C lower than
the  normal 
internal  body
temperature)  necessary  for
spermatogenesis. In adults, each
testis is oval in shape, with a length
of about 4 to 5 cm and a width of
about  2  to  3  cm.  The  testis  is
covered by a dense covering. Each
testis has about 250 compartments
lobules
called 
(Figure 3.1b).

testicular 

Each  lobule  contains  one  to
three  highly  coiled  seminiferous
tubules  in  which  sperms  are
produced. Each seminiferous tubule
is lined on its inside by two types
of  cells  called  male  germ  cells
(spermatogonia) and Sertoli cells
(Figure 3.2 ). The male germ cells
undergo  meiotic  divisions  finally
leading to sperm formation, while
Sertoli cells provide nutrition to the
germ cells. The regions outside the
seminiferous  tubules  called
interstitial  spaces,  contain  small
blood vessels and interstitial cells
or Leydig cells (Figure 3.2). Leydig
cells  synthesise  and  secrete
testicular  hormones  called
androgens. Other  immunologically
competent cells are also present.

Figure  3.1(a) Diagrammatic sectional view of  male  pelvis

showing  reproductive  system

Figure  3.1(b) Diagrammatic view of male reproductive system

(part of testis is open to show inner details)

The male sex accessory ducts include rete testis, vasa efferentia,
epididymis and vas deferens (Figure 3.1b). The seminiferous tubules of
the testis open into the vasa efferentia through rete testis. The vasa efferentia
leave the testis and open into epididymis located along the posterior surface
of each testis. The epididymis leads to vas deferens that ascends to the
abdomen and loops over the urinary bladder. It receives a duct from seminal
vesicle and opens into urethra as the ejaculatory duct (Figure 3.1a). These
ducts store and transport the sperms from the testis to the outside through
urethra. The urethra originates from the urinary bladder and extends
through the penis to its external opening called urethral meatus.

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BIOLOGY

Figure 3.2 Diagrammatic sectional view of seminiferous tubule

The penis is the male external genitalia (Figure 3.1a, b). It is made up
of special tissue that helps in erection of the penis to facilitate insemination.
The enlarged end of penis called the glans penis is covered by a loose fold
of skin called foreskin.

The male accessory glands (Figure 3.1a, b) include paired seminal
vesicles, a prostate and paired bulbourethral glands. Secretions of these
glands constitute the seminal plasma which is rich in fructose, calcium
and certain enzymes. The secretions of bulbourethral glands also helps
in the lubrication of the penis.

3.2 THE FEMALE REPRODUCTIVE SYSTEM
The female reproductive system consists of a pair of ovaries alongwith a pair
of oviducts, uterus, cervix, vagina and the external genitalia located in
pelvic region (Figure 3.3a). These parts of the system alongwith a pair of the
mammary glands are integrated structurally and functionally to support
the processes of ovulation, fertilisation, pregnancy, birth and child care.

Ovaries are the primary female sex organs that produce the female
gamete  (ovum)  and  several  steroid  hormones  (ovarian  hormones).
The  ovaries  are  located  one  on  each  side  of  the  lower  abdomen
(Figure 3.3b). Each ovary is about 2 to 4 cm in length and is connected to
the pelvic wall and uterus by ligaments. Each ovary is covered by a thin
epithelium which encloses the ovarian stroma. The stroma is divided into
two zones – a peripheral cortex and an inner medulla.

44

HUMAN  REPRODUCTION

Figure  3.3  (a) Diagrammatic  sectional  view  of  female  pelvis  showing

reproductive  system

The oviducts (fallopian tubes), uterus and vagina constitute the female
accessory ducts. Each fallopian tube is about 10-12 cm long and extends
from the periphery of each ovary to the uterus (Figure 3.3b), the part closer
to  the  ovary  is  the  funnel-shaped  infundibulum.  The  edges  of  the
infundibulum possess finger-like projections called fimbriae, which help in
collection of the ovum after ovulation. The infundibulum leads to a wider

Figure 3.3  (b) Diagrammatic sectional view of the female reproductive  system

45

BIOLOGY

part of the oviduct called ampulla. The last part of the oviduct, isthmus has
a narrow lumen and it joins the uterus.

The uterus is single and it is also called womb. The shape of the uterus
is like an inverted pear. It is supported by ligaments attached to the pelvic
wall. The uterus opens into vagina through a narrow cervix. The cavity of
the cervix is called cervical canal (Figure 3.3b) which alongwith vagina
forms the birth canal. The wall of the uterus has three layers of tissue. The
external thin membranous perimetrium, middle  thick layer of smooth
muscle, myometrium and inner glandular layer called endometrium that
lines the uterine cavity. The endometrium undergoes cyclical changes during
menstrual cycle while the myometrium exhibits strong contraction during
delivery of the baby.

The female external genitalia include mons pubis, labia majora, labia
minora, hymen and clitoris (Figure 3.3a). Mons pubis is a cushion of fatty
tissue covered by skin and pubic hair. The labia majora are fleshy folds of
tissue, which extend down from the mons pubis and surround the vaginal
opening. The labia minora are paired folds of tissue under the labia majora.
The opening of the vagina is often covered partially by a membrane called
hymen. The clitoris is a tiny finger-like structure which lies at the upper
junction of the two labia minora above the urethral opening.  The hymen is
often torn during the first coitus (intercourse). However, it can also be broken
by a sudden fall or jolt, insertion of a vaginal tampon, active participation
in some sports like horseback riding, cycling, etc. In some women the hymen
persists even after coitus. In fact, the presence or absence of hymen is not
a reliable indicator of virginity or sexual experience.

46

Figure  3.4  A  diagrammatic  sectional  view  of  Mammary  gland

HUMAN  REPRODUCTION

A functional mammary gland is characteristic of all female mammals.
The  mammary  glands  are  paired  structures  (breasts)  that  contain
glandular tissue and variable amount of fat. The glandular tissue of each
breast is divided into 15-20 mammary lobes containing clusters of cells
called alveoli (Figure 3.4). The cells of alveoli secrete milk, which is stored
in the cavities (lumens) of alveoli. The alveoli open into mammary tubules.
The tubules of each lobe join to form a mammary duct. Several mammary
ducts join to form a wider mammary ampulla  which is connected to
lactiferous duct through which milk is sucked out.

3.3 GAMETOGENESIS
The primary sex organs – the testis in the males and the ovaries in the
females – produce gametes, i.e, sperms and ovum, respectively, by the
process called gametogenesis. In testis, the immature male germ cells
(spermatogonia) produce sperms by spermatogenesis that begins at
puberty.  The spermatogonia (sing. spermatogonium) present on the
inside wall of seminiferous tubules multiply by mitotic division and
increase in numbers. Each spermatogonium is diploid and contains 46
chromosomes.  Some  of  the  spermatogonia  called  primary
spermatocytes periodically undergo meiosis. A primary spermatocyte
completes  the  first  meiotic  division  (reduction  division)  leading  to
formation of two equal,  haploid cells called
secondary spermatocytes, which have only
23  chromosomes  each.  The  secondary
spermatocytes undergo the second meiotic
division  to  produce  four  equal,  haploid
spermatids (Figure 3.5). What would be the
number of chromosome in the spermatids?
The  spermatids  are  transformed  into
spermatozoa (sperms) by the process called
spermiogenesis.  After  spermiogenesis,
sperm  heads  become  embedded  in  the
Sertoli cells, and are finally released from
the  seminiferous  tubules  by  the  process
called spermiation.

Spermatogenesis  starts  at  the  age  of
puberty  due  to  significant  increase  in  the
secretion of gonadotropin releasing hormone
(GnRH). This, if you recall, is a hypothalamic hormone. The increased
levels of GnRH then acts at the anterior pituitary gland and stimulates
secretion of two gonadotropins – luteinising hormone (LH) and follicle
stimulating hormone (FSH).  LH acts at the Leydig cells and stimulates
synthesis and secretion of androgens. Androgens, in turn, stimulate the
process of spermatogenesis. FSH acts on the Sertoli cells and stimulates

Figure  3.5 Diagrammatic  sectional  view  of  a

seminiferous  tubule  (enlarged)

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BIOLOGY

secretion  of  some  factors  which  help  in  the
process of spermiogenesis.

Let us examine the structure of a sperm. It
is a microscopic structure composed of a head,
neck, a middle piece and a tail (Figure 3.6). A
plasma membrane envelops the whole body of
sperm. The sperm head contains an elongated
haploid nucleus, the anterior portion of which
is covered by a cap-like structure, acrosome.
The acrosome is filled with enzymes that help
fertilisation  of  the  ovum.  The  middle  piece
possesses  numerous  mitochondria,  which
produce energy for the movement of  tail that
facilitate sperm motility essential for fertilisation.
The human male ejaculates about 200 to 300
million sperms during a coitus of which, for
normal fertility, at least 60 per cent sperms
must have normal shape and size and for at
least 40 per cent of them must show vigorous
motility.

Figure  3.6  Structure  of  a  sperm

Sperms  released  from  the  seminiferous
tubules,  are  transported  by  the  accessory
ducts.  Secretions  of  epididymis,  vas  deferens,  seminal  vesicle  and
prostate are essential for maturation and motility of sperms. The seminal
plasma along with the sperms constitute the semen. The functions of
male sex accessory ducts and glands are maintained by the testicular
hormones (androgens).

The process of formation of a mature female gamete is called oogenesis
which is markedly different from spermatogenesis. Oogenesis is initiated
during the embryonic development stage when a couple of million gamete
mother cells (oogonia) are formed within each fetal ovary; no more oogonia
are formed and added after birth. These cells start division and enter into
prophase-I of the meiotic division and get temporarily arrested at that stage,
called primary oocytes. Each primary oocyte then gets surrounded by a
layer of granulosa cells and then called the primary follicle (Figure 3.7).
A large number of these follicles degenerate during the phase from birth to
puberty. Therefore, at puberty only 60,000-80,000 primary follicles are
left in each ovary. The primary follicles get surrounded by more layers of
granulosa cells and a new theca and called secondary follicles.

The secondary follicle soon transforms into a tertiary follicle which is
characterised by a fluid filled cavity called antrum. The theca layer is
organised into an inner theca interna and an outer theca externa. It is
important to draw your attention that it is at this stage that the primary
oocyte within the tertiary follicle grows in size and completes its first meiotic
division. It is an unequal division resulting in the formation of a large
haploid secondary oocyte and a tiny first polar body (Figure 3.8b). The

48

HUMAN  REPRODUCTION

secondary  oocyte  retains  bulk  of  the
nutrient rich cytoplasm of the primary
oocyte. Can you think of any advantage
for this? Does the first polar body born
out of first meiotic division divide further
or degenerate?  At present we are not
very  certain  about  this.  The  tertiary
follicle further changes into the mature
follicle or Graafian follicle (Figure 3.7).
The  secondary  oocyte  forms  a  new
membrane  called  zona  pellucida
surrounding it. The Graafian follicle now
ruptures to release the secondary oocyte
(ovum)  from  the  ovary  by  the
process  called  ovulation.  Can  you
identify  major  differences  between
spermatogenesis  and  oogenesis?    A  diagrammatic  representation  of
spermatogenesis and oogenesis is given below (Figure 3.8).

Figure  3.7  Diagrammatic  Section  view  of  ovary

(a)

(b)

Figure  3.8  Schematic  representation  of  (a)  Spermatogenesis;  (b)  Oogenesis

3.4 MENSTRUAL CYCLE

The reproductive cycle in the female primates (e.g. monkeys, apes and
human beings) is called menstrual cycle. The first menstruation begins
at puberty and is called menarche. In human females, menstruation
is repeated at an average interval of about 28/29 days, and the cycle of
events starting from one menstruation till the next one is called the
menstrual cycle. One ovum is released (ovulation) during the middle

49

BIOLOGY

Figure  3.9  Diagrammatic  presenation  of  various  events  during  a  menstrual  cycle

of each menstrual cycle. The major events of the menstrual cycle are
shown in Figure 3.9. The cycle starts with the menstrual phase, when
menstrual flow occurs and it lasts for 3-5 days. The menstrual flow
results due to breakdown of endometrial lining of the uterus and its
blood  vessels  which  forms  liquid  that  comes  out  through  vagina.
Menstruation only occurs if the released ouvm is not fertilised. Lack of
menstruation may be indicative of pregnancy. However, it may also be
caused due to some other underlying causes like stress, poor health etc.
The  menstrual  phase  is  followed  by  the  follicular  phase.  During
this  phase,  the  primary  follicles  in  the  ovary  grow  to  become  a
fully  mature  Graafian  follicle  and  simultaneously  the  endometrium
of  uterus  regenerates  through  proliferation.  These  changes  in  the
ovary  and  the  uterus  are  induced  by  changes  in  the  levels  of
pituitary  and  ovarian  hormones  (Figure  3.9).  The  secretion  of

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HUMAN  REPRODUCTION

gonadotropins (LH and FSH) increases gradually during the follicular
phase, and stimulates follicular development as well as secretion of
estrogens by the growing follicles. Both LH and FSH attain a peak level
in the middle of cycle (about 14th day). Rapid secretion of LH leading to
its maximum level during the mid-cycle called LH surge induces rupture
of Graafian follicle and thereby the release of ovum (ovulation). The
ovulation (ovulatory phase) is followed by the luteal phase during which
the remaining parts of the Graafian follicle transform as the corpus
luteum  (Figure  3.9).  The  corpus  luteum  secretes  large  amounts  of
progesterone which is essential for maintenance of the endometrium.
Such an endometrium is necessary for implantation of the fertilised
ovum and other events of pregnancy. During pregnanacy all events of
the menstrual cycle stop and there is no menstruation.  In the absence
of fertilisation, the corpus luteum degenerates. This causes disintegration
of the endometrium leading to menstruation, marking a new cycle.  In
human beings, menstrual cycles ceases around 50 years of age; that is
termed as menopause. Cyclic menstruation is an indicator of normal
reproductive phase and extends between menarche and menopause.

3.5 FERTILISATION AND IMPLANTATION
During copulation (coitus) semen is released by the penis into the vagina
(insemination). The motile sperms swim rapidly, pass through the cervix,
enter into the uterus and finally reach the junction of the isthmus and
ampulla (ampullary-isthmic junction) of the fallopian tube (Figure 3.11b).
The ovum released by the ovary is also
transported to the ampullary-isthmic
junction  where  fertilisation  takes
place.  Fertilisation can only occur if
the ovum and sperms are transported
simultaneously  to  the  ampullary-
isthmic junction.  This is the reason
why  not  all  copulations  lead  to
fertilisation and pregnancy.

The process of fusion of a sperm
with an ovum is called fertilisation.
During fertilisation, a sperm comes in
contact with the zona pellucida layer
of the ovum  (Figure 3.10) and induces
changes in the membrane  that block
the entry of additional sperms.  Thus,
it  ensures  that  only  one  sperm  can
fertilise an ovum.  The secretions of the
acrosome help the sperm enter into the
cytoplasm  of  the  ovum  through  the
zona  pellucida  and  the  plasma

51

51

Figure  3.10  Ovum  surrounded  by  few  sperms

BIOLOGY

membrane. This induces the completion of the meiotic division of the
secondary oocyte. The second meiotic division is also unequal and results
in the formation of a second polar body and a haploid ovum (ootid). Soon
the haploid nucleus of the sperms and that of the ovum fuse together to
form a diploid zygote.  How many chromosomes will be there in the zygote?
One has to remember that the sex of the baby has been decided at this
stage itself. Let us see how? As you know the chromosome pattern in the
human female is XX and that in the male is XY. Therefore, all the haploid
gametes produced by the female (ova) have the sex chromosome X whereas
in the male gametes (sperms) the sex chromosome could be either X or Y,
hence, 50 per cent of sperms carry the X chromosome while the other 50 per
cent carry the Y. After fusion of the male and female gametes the zygote
would carry either XX or XY depending on whether the sperm carrying X
or Y fertilised the ovum. The zygote carrying XX would develop into a female
baby and XY would form a male (you will learn more about the chromosomal
patterns in Chapter 5). That is why, scientifically it is correct to say that the
sex of the baby is determined by the father and not by the mother!

The mitotic division  starts as the zygote moves through the isthmus
of the oviduct called cleavage towards the uterus (Figure 3.11) and forms
2, 4, 8, 16 daughter cells called blastomeres. The embryo with 8 to 16

52

Figure  3.11  Transport  of  ovum,  fertilisation  and  passage  of  growing  embryo  through  fallopian  tube

HUMAN  REPRODUCTION

blastomeres is called a morula (Figure 3.11e). The morula continues to
divide and transforms into blastocyst (Figure 3.11g) as it moves further
into the uterus. The blastomeres in the blastocyst are arranged into an
outer layer called trophoblast and an inner group of cells attached to
trophoblast called the inner cell mass. The trophoblast layer then gets
attached to the endometrium and the inner cell mass gets differentiated
as the embryo. After attachment, the uterine cells divide rapidly and covers
the  blastocyst.  As  a  result,  the  blastocyst  becomes  embedded  in  the
endometrium of the uterus (Figure 3.11h). This is called implantation
and it leads to pregnancy.

3.6 PREGNANCY AND EMBRYONIC DEVELOPMENT
After implantation, finger-like projections appear on the trophoblast called
chorionic villi which are surrounded by the uterine tissue and maternal
blood. The chorionic villi and uterine tissue become interdigitated with
each  other  and  jointly  form  a  structural  and  functional  unit  between
developing embryo (foetus) and maternal body called placenta (Figure 3.12).
The  placenta  facilitate  the  supply  of  oxygen  and  nutrients  to  the
embryo and also removal of carbon dioxide and excretory/waste materials
produced by the embryo. The placenta is connected to the embryo through
an umbilical cord which helps in the transport of substances to and from
the embryo. Placenta also acts as an endocrine tissue and produces
several hormones like human chorionic gonadotropin (hCG), human
placental lactogen (hPL), estrogens, progestogens, etc. In the later
phase  of  pregnancy,  a  hormone  called  relaxin  is  also  secreted  by
the  ovary.  Let  us  remember
that  hCG,  hPL  and  relaxin
are  produced  in  women
only  during  pregnancy.  In
addition,  during  pregnancy
the levels of other hormones
like estrogens, progestogens,
cortisol, prolactin, thyroxine,
etc.,  are  increased  several-
folds in the maternal blood.
Increased production of these
hormones  is  essential  for
supporting the fetal growth,
metabolic  changes  in  the
mother  and  maintenance  of
pregnancy.

Immediately 

after
implantation,  the  inner  cell
mass  (embryo)  differentiates

Figure  3.12  The  human  foetus  within  the  uterus

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BIOLOGY

into an outer layer called ectoderm and an inner layer called endoderm. A
mesoderm soon appears between the ectoderm and the endoderm. These
three layers give rise to all tissues (organs) in adults. It needs to be mentioned
here that the inner cell mass contains certain cells called stem cells which
have the potency to give rise to all the tissues and organs.

What  are  the  major  features  of  embryonic  development  at  various
months of pregnancy?  The human pregnancy lasts 9 months. Do you
know many months pregnancy last in dogs, elephants, cats? Find out.
In human beings, after one month of pregnancy, the embryo’s heart is
formed. The first sign of growing foetus may be noticed by listening to the
heart sound carefully through the stethoscope. By the end of the second
month of pregnancy, the foetus develops limbs and digits. By the end of
12 weeks (first trimester), most of the major organ systems are formed,
for example, the limbs and external genital organs are well-developed.
The first movements of the foetus and appearance of hair on the head are
usually observed during the fifth month. By the end of 24 weeks (second
trimester),  the  body  is  covered  with  fine  hair,  eye-lids  separate,  and
eyelashes are formed.  By the end of nine months of pregnancy, the foetus
is fully developed and is ready for delivery.

3.7 PARTURITION AND LACTATION
The  average  duration  of  human  pregnancy  is  about  9  months
which is called the gestation period. Vigorous contraction of the uterus at
the end of pregnancy causes expulsion/delivery of the foetus. This process
of delivery of the foetus (childbirth) is called parturition. Parturition is
induced  by  a  complex  neuroendocrine  mechanism.  The  signals  for
parturition originate from the fully developed fetus and the placenta which
induce mild uterine contractions called foetal ejection reflex. This triggers
release of oxytocin from the maternal pituitary. Oxytocin acts on the uterine
muscle and causes stronger uterine contractions, which in turn stimulates
further secretion of oxytocin. The stimulatory reflex between the uterine
contraction  and  oxytocin  secretion  continues  resulting  in  stronger  and
stronger contractions. This leads to expulsion of the baby out of the uterus
through the birth canal – parturition. Soon after the infant is delivered, the
placenta is also expelled out of the uterus. What do you think the doctors
inject  to  induce  delivery?

The mammary glands of the female undergo differentiation during
pregnancy and starts producing milk towards the end of pregnancy by
the process called lactation. This helps the mother in feeding the new-
born. The milk produced during the initial few days of lactation is called
colostrum  which  contains  several  antibodies  absolutely  essential  to
develop resistance for the new-born babies. Breast-feeding during the
initial period of infant growth is recommended by doctors for bringing up
a healthy baby.

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HUMAN  REPRODUCTION

SUMMARY

Humans  are  sexually  reproducing  and  viviparous.  The  male
reproductive  system  is  composed  of  a  pair  of  testes,  the  male  sex
accessory ducts and the accessory glands and external genitalia. Each
testis has about 250 compartments called testicular lobules, and each
lobule  contains  one  to  three  highly  coiled  seminiferous  tubules.  Each
seminiferous tubule is lined inside by spermatogonia and Sertoli cells.
The spermatogonia undergo meiotic divisions leading to sperm formation,
while Sertoli cells provide nutrition to the dividing germ cells. The Leydig
cells outside the seminiferous tubules, synthesise and secrete testicular
hormones called androgens. The male external genitalia is called penis.
The female reproductive system consists of a pair of ovaries, a pair
of  oviducts,  a  uterus,  a  vagina,  external  genitalia,  and  a  pair  of
mammary  glands.  The  ovaries  produce  the  female  gamete  (ovum)  and
some steroid hormones (ovarian hormones). Ovarian follicles in different
stages of development are embedded in the stroma. The oviducts, uterus
and  vagina  are  female  accessory  ducts.  The  uterus  has  three  layers
namely  perimetrium,  myometrium  and  endometrium.  The  female
external  genitalia  includes  mons  pubis,  labia  majora,  labia  minora,
hymen  and  clitoris.  The  mammary  glands  are  one  of  the  female
secondary  sexual  characteristics.

Spermatogenesis  results  in  the  formation  of  sperms  that  are
transported  by  the  male  sex  accessory  ducts.  A  normal  human  sperm
is  composed  of  a  head,  neck,  a  middle  piece  and  tail.  The  process  of
formation  of  mature  female  gametes  is  called  oogenesis.  The
reproductive  cycle  of  female  primates  is  called  menstrual  cycle.
Menstrual cycle starts only after attaining sexual maturation (puberty).
During  ovulation  only  one  ovum  is  released  per  menstrual  cycle.  The
cyclical  changes  in  the  ovary  and  the  uterus  during  menstrual  cycle
are induced by changes in the levels of pituitary and ovarian hormones.
After coitus, sperms are transported to the junction of the isthmus and
ampulla, where the sperm fertilises the ovum leading to formation of a
diploid  zygote.  The  presence  of  X  or  Y  chromosome  in  the  sperm
determines the sex of the embryo. The zygote undergoes repeated mitotic
division to form a blastocyst, which is implanted in the uterus resulting
in pregnancy. After nine months of pregnancy, the fully developed  foetus
is ready for delivery. The process of childbirth is called parturition which
is induced by a complex neuroendocrine mechanism involving cortisol,
estrogens and oxytocin. Mammary glands differentiate during pregnancy
and secrete milk after child-birth. The new-born baby is fed milk by the
mother  (lactation)  during  the  initial  few  months  of  growth.

EXERCISES

1. Fill  in  the  blanks:

(a) Humans  reproduce  _____________  (asexually/sexually)
(b) Humans  are  _____________  (oviparous,  viviparous,  ovoviviparous)
(c) Fertilisation  is  _____________  in  humans  (external/internal)
(d) Male  and  female  gametes  are  _____________  (diploid/haploid)
(e) Zygote  is  _____________  (diploid/haploid)

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BIOLOGY

(f) The  process  of  release  of  ovum  from  a  mature  follicle  is  called

_____________

(g) Ovulation  is  induced  by  a  hormone  called  _____________
(h) The  fusion  of  male  and  female  gametes  is  called  _____________
(i) Fertilisation  takes  place  in  _____________
(j) Zygote  divides  to  form  _____________which  is  implanted  in  uterus.
(k) The  structure  which  provides  vascular  connection  between  fetus

and  uterus  is  called  _____________

2. Draw  a  labelled  diagram  of  male  reproductive  system.
3. Draw  a  labelled  diagram  of  female  reproductive  system.
4. Write two major functions each of testis and ovary.
5. Describe  the  structure  of  a  seminiferous  tubule.
6. What is spermatogenesis? Briefly describe the process of spermatogenesis.
7. Name  the  hormones  involved  in  regulation  of  spermatogenesis.
8. Define  spermiogenesis  and  spermiation.
9. Draw a labelled diagram of sperm.
10. What  are  the  major  components  of  seminal  plasma?
11. What  are  the  major  functions  of  male  accessory  ducts  and  glands?
12. What is oogenesis? Give a brief account of oogenesis.
13. Draw  a  labelled  diagram  of  a  section  through  ovary.
14. Draw  a  labelled  diagram  of  a  Graafian  follicle?
15. Name  the  functions  of  the  following:

16.

(b)  Endometrium
(d)  Sperm  tail

(a)  Corpus  luteum
(c)  Acrosome
(e)  Fimbriae
Identify  True/False  statements.  Correct  each  false  statement  to  make
it  true.
(a) Androgens  are  produced  by  Sertoli  cells.  (True/False)
(b) Spermatozoa  get  nutrition  from  Sertoli  cells.  (True/False)
(c) Leydig  cells  are  found  in  ovary.  (True/False)
(d) Leydig  cells  synthesise  androgens.  (True/False)
(e) Oogenesis  takes  place  in  corpus  luteum.  (True/False)
(f) Menstrual  cycle  ceases  during  pregnancy.  (True/False)
(g) Presence or absence of hymen is not a reliable indicator of virginity

or  sexual  experience.  (True/False)

17. What  is  menstrual  cycle?  Which  hormones  regulate  menstrual  cycle?
18. What is parturition? Which hormones are involved in induction of parturition?
In our society the women are often blamed for giving birth to daughters.
19.
Can you explain why this is not correct?

20. How many eggs are released by a human ovary in a month? How many
eggs do you think would have been released if the mother gave birth to
identical  twins?  Would  your  answer  change  if  the  twins  born  were
fraternal?

21. How many eggs do you think were released by the ovary of a female dog

which gave birth to 6 puppies?

56



CHAPTER 4 REPRODUCTIVE HEALTH
You have learnt about human reproductive system and its
functions in Chapter 3. Now, let’s discuss a closely related
topic – reproductive health. What do we understand by
this term?  The term simply refers to healthy reproductive
organs with normal functions. However, it has a broader
perspective and includes the emotional and social aspects
of  reproduction  also.  According  to  the  World  Health
Organisation (WHO), reproductive health means a total
well-being in all aspects of reproduction, i.e., physical,
emotional, behavioural and social. Therefore, a society with
people  having  physically  and  functionally  normal
reproductive organs and normal emotional and behavioural
interactions among them in all sex-related aspects might
be called reproductively healthy. Why is it significant to
maintain reproductive health and what are the methods
taken up to achieve it? Let us examine them.

4.1 REPRODUCTIVE HEALTH – PROBLEMS AND STRATEGIES

India was amongst the first countries in the world to
initiate action plans and programmes at a national level
to  attain  total  reproductive  health  as  a  social  goal.
These  programmes  called  ‘family  planning’  were
initiated in 1951 and were periodically assessed over
the past decades. Improved programmes covering wider

BIOLOGY

reproduction-related  areas  are  currently  in  operation  under  the
popular name ‘Reproductive and Child Health Care (RCH) programmes’.
Creating awareness among people about various reproduction related
aspects  and  providing  facilities  and  support  for  building  up  a
reproductively  healthy  society  are  the  major  tasks  under  these
programmes.

With the help of audio-visual and the print-media governmental and
non-governmental agencies have taken various steps to create awareness
among the people about reproduction-related aspects. Parents, other
close  relatives,  teachers  and  friends,  also  have  a  major  role  in  the
dissemination of the above information. Introduction of sex education
in schools should also be encouraged to provide right information to
the young so as to discourage children from believing in myths and
having misconceptions about sex-related aspects. Proper information
about reproductive organs, adolescence and related changes, safe and
hygienic sexual practices, sexually transmitted diseases (STD), AIDS,
etc., would help people, especially those in the adolescent age group to
lead a reproductively healthy life. Educating people, especially fertile
couples and those in marriageable age group, about available birth
control options, care of pregnant mothers, post-natal care of the mother
and child, importance of breast feeding, equal opportunities for the male
and the female child, etc., would address the importance of bringing up
socially conscious healthy families of desired size. Awareness of problems
due to uncontrolled population growth, social evils like sex-abuse and
sex-related crimes, etc., need to be created to enable people to think
and take up necessary steps to prevent them and thereby build up a
socially responsible and healthy society.

Successful  implementation  of  various  action  plans  to  attain
reproductive health requires strong infrastructural facilities, professional
expertise and material support. These are essential to provide medical
assistance  and  care  to  people  in  reproduction-related  problems  like
pregnancy, delivery, STDs, abortions, contraception, menstrual problems,
infertility, etc. Implementation of better techniques and new strategies
from time to time are also required to provide more efficient care and
assistance  to  people.  Statutory  ban  on  amniocentesis  (a  foetal  sex
determination test based on the chromosomal pattern in the amniotic
fluid surrounding the developing embryo) for sex-determination to legally
check increasing female foeticides, massive child immunisation, etc., are
some programmes that merit mention in this connection.

Research on various reproduction-related areas are encouraged and
supported by governmental and non-governmental agencies to find out
new methods and/or to improve upon the existing ones. Do you know
that  ‘Saheli’–a  new  oral  contraceptive  for  the  females–was  developed
by scientists at Central Drug Research Institute (CDRI) in Lucknow, India?
Better awareness about sex related matters, increased number of medically
assisted deliveries and better post-natal care leading to decreased maternal

58

REPRODUCTIVE  HEALTH

and  infant  mortality  rates,  increased  number  of  couples  with  small
families, better detection and cure of STDs and overall increased medical
facilities for all sex-related problems, etc. all indicate improved reproductive
health of the society.

4.2 POPULATION EXPLOSION AND BIRTH CONTROL
In the last century an all-round development in various fields significantly
improved the quality of life of the people. However, increased health facilities
along with better living conditions had an explosive impact on the growth
of  population.  The  world  population  which  was  around  2  billion
(2000 million) in 1900 rocketed to about 6 billions  by 2000. A similar
trend was observed in India too. Our population which was approximately
350 million at the time of our independence reached close to the billion
mark by 2000 and crossed 1 billion in May 2000. That means, every
sixth person in the world is an Indian. A rapid decline in death rate,
maternal mortality rate (MMR) and infant mortality rate (IMR) as
well as an increase in number of people in reproducible age are probable
reasons for this. Through our RCH programmes, though we could bring
down the population growth rate, it was only marginal. According to the
2001 census report, the population growth rate was still around  1.7 per
cent, i.e., 17/1000/year, a rate at which our population could double in
33 years. Such an alarming growth rate could lead to an absolute scarcity
of even the basic requirements, i.e., food, shelter and clothing, in spite of
significant progress made in those areas. Therefore, the government was
forced to take up serious measures to check this population growth rate.
The most important step to overcome this problem is to motivate smaller
families by using various contraceptive methods. You might have seen
advertisements in the media as well as posters/bills, etc., showing a happy
couple with two children with a slogan Hum Do Hamare Do (we two, our
two). Many couples, mostly the young, urban, working ones have even
adopted an ‘one child norm’. Statutory raising of marriageable age of the
female to 18 years and that of males to 21 years, and incentives given to
couples with small families are two of the other measures taken to tackle
this problem. Let us describe some of the commonly used contraceptive
methods, which help prevent unwanted pregnancies.

An  ideal  contraceptive  should  be  user-friendly,  easily  available,
effective and reversible with no or least side-effects. It also should in no
way interfere with the sexual drive, desire and/or the sexual act of the
user. A wide range of contraceptive methods are presently available which
could  be  broadly  grouped  into  the  following  categories,  namely
Natural/Traditional,  Barrier,  IUDs,  Oral  contraceptives,  Injectables,
Implants and Surgical methods.

Natural methods work on the principle of avoiding chances of ovum
and sperms meeting. Periodic abstinence is one such method in which
the couples avoid or abstain from coitus from day 10 to 17 of the menstrual
cycle when ovulation could be expected. As chances of fertilisation are

59

BIOLOGY

very high during this period, it is called the fertile period. Therefore, by
abstaining from coitus during this period, conception could be prevented.
Withdrawal or coitus interruptus is another method in which the male
partner withdraws his penis from the vagina just before ejaculation so as
to  avoid  insemination.  Lactational  amenorrhea  (absence  of
menstruation) method is based on the fact that ovulation
and therefore the cycle do not occur during the period of
intense lactation following parturition. Therefore, as long
as  the  mother  breast-feeds  the  child  fully,  chances  of
conception are almost nil. However, this method has been
reported to be effective only upto a maximum period of six
months following parturition. As no medicines or devices
are  used  in  these  methods,  side  effects  are  almost  nil.
Chances of failure, though, of this method are also high.

Figure  4.1(a)  Condom  for  male

Figure  4.1(b)  Condom  for  female

In barrier methods, ovum and sperms are prevented
from physically meeting with the help of barriers. Such
methods    are  available  for  both  males  and  females.
Condoms (Figure 4.1 a, b) are barriers made of thin rubber/
latex sheath that are used to cover the penis in the male or
vagina and cervix in the female, just before coitus so that
the  ejaculated  semen  would  not  enter  into  the  female
reproductive tract. This can prevent conception. ‘Nirodh’ is
a popular brand of condom for the male. Use of condoms
has increased in recent years due to its additional benefit of
protecting the user from contracting STDs and AIDS. Both
the male and the female condoms are disposable, can be
self-inserted  and  thereby  gives  privacy  to  the  user.
Diaphragms, cervical caps and vaults are also barriers
made of rubber that are inserted into the female reproductive
tract  to  cover  the  cervix  during  coitus.  They  prevent
conception by blocking the entry of sperms through the
cervix. They are reusable. Spermicidal creams, jellies and
foams are usually used alongwith these barriers to increase
their contraceptive efficiency.

Figure  4.2.  Copper  T  (CuT)

60

Another effective and popular method is the use of Intra
Uterine  Devices  (IUDs).  These  devices  are  inserted  by
doctors  or  expert  nurses  in  the  uterus  through  vagina.
These Intra Uterine Devices are presently available as the
non-medicated IUDs (e.g., Lippes loop), copper releasing
IUDs  (CuT,  Cu7,  Multiload  375)  and  the  hormone  releasing  IUDs
(Progestasert, LNG-20) (Figure 4.2). IUDs increase phagocytosis of sperms
within the uterus and the Cu ions released suppress sperm motility
and  the  fertilising  capacity  of  sperms.  The  hormone  releasing  IUDs,
in  addition,  make  the  uterus  unsuitable  for  implantation  and  the
cervix hostile to the sperms. IUDs are ideal contraceptives for the females

REPRODUCTIVE  HEALTH

who want to delay pregnancy and/or space children. It is
one of most widely accepted methods of contraception in
India.

Oral administration of small doses of either progestogens
or  progestogen –estrogen  combinations  is  another
contraceptive method used by the females. They are used
in the form of tablets and hence are popularly called the
pills. Pills have to be taken daily for a period of 21 days
starting preferably within the first five days of menstrual
cycle. After a gap of 7 days (during which menstruation
occurs) it has to be repeated in the same pattern till the
female desires to prevent conception. They inhibit ovulation
and implantation as well as alter the quality of cervical mucus to prevent/
retard entry of sperms. Pills are very effective with lesser side effects and
are well accepted by the females. Saheli –the new oral contraceptive for
the females contains a non-steroidal preparation. It is a ‘once a week’ pill
with very few side effects and high contraceptive value.

Figure  4.3  Implants

Progestogens alone or in combination with estrogen can also be used
by females as injections or implants under the skin (Figure 4.3). Their
mode of action is similar to that of pills and their effective periods are
much longer. Administration of progestogens or progestogen-estrogen
combinations or IUDs within 72 hours of coitus have been found to be
very effective as emergency contraceptives as they could be used to avoid
possible pregnancy due to rape or casual unprotected intercourse.

Surgical methods, also called sterilisation, are generally advised for
the  male/female  partner  as  a  terminal  method  to  prevent  any  more
pregnancies. Surgical intervention blocks gamete transport and thereby
prevent conception. Sterilisation procedure in the male is called ‘vasectomy’

61

Figure  4.4a  Vasectomy

Figure  4.4  (b)  Tubectomy

BIOLOGY

and that in the female, ‘tubectomy’. In vasectomy, a small part of the vas
deferens is removed or tied up through a small incision on the scrotum
(Figure 4.4a) whereas in tubectomy, a small part of the fallopian tube is
removed (Figure 4.4b) or tied up through a small incision in the abdomen
or  through  vagina.  These  techniques  are  highly  effective  but  their
reversibility is very poor.

It needs to be emphasised that the selection of a suitable contraceptive
method and its use should always be undertaken in consultation with
qualified  medical  professionals.  One  must  also  remember  that
contraceptives  are  not  regular  requirements  for  the  maintenance  of
reproductive  health.  In  fact,  they  are  practiced  against  a  natural
reproductive event, i.e., conception/pregnancy. One is forced to use these
methods either to prevent pregnancy or to delay or space pregnancy due
to personal reasons. No doubt, the widespread use of these methods have
a significant role in checking uncontrolled growth of population. However,
their  possible  ill-effects  like  nausea,  abdominal  pain,  breakthrough
bleeding, irregular menstrual bleeding or even breast cancer, though not
very significant, should not be totally ignored.

4.3 MEDICAL TERMINATION OF PREGNANCY (MTP)
Intentional or voluntary termination of pregnancy before full term is called
medical termination of pregnancy (MTP) or induced abortion. Nearly
45 to 50 million MTPs are performed in a year all over the world which
accounts to 1/5th of the total number of conceived pregnancies in a year.
Obviously, MTP has a significant role in decreasing the population though
it is not meant for that purpose. Whether to accept / legalise MTP or not is
being debated upon in many countries due to emotional, ethical, religious
and social issues involved in it. Government of India legalised MTP in
1971 with some strict conditions to avoid its misuse. Such restrictions
are all the more important to check indiscriminate and illegal female
foeticides which are reported to be high in India.

Why  MTP ?  Obviously  the  answer  is – to  get  rid  of  unwanted
pregnancies either due to casual unprotected intercourse or failure of the
contraceptive used during coitus or rapes. MTPs are also essential in
certain cases where continuation of the pregnancy could be harmful or
even fatal either to the mother or to the foetus or both.

MTPs are considered relatively safe during the first trimester, i.e., upto
12 weeks of pregnancy. Second trimester abortions are much more riskier.
One disturbing trend observed is that a majority of the MTPs are performed
illegally by unqualified quacks which are not only unsafe but could be
fatal too. Another dangerous trend is the misuse of amniocentesis to
determine the sex of the unborn child.  Frequently, if the foetus is found
to be female, it is followed by MTP- this is totally against what is legal.
Such practices should be avoided because these are dangerous both for
the young mother and the foetus. Effective counselling on the need to

62

REPRODUCTIVE  HEALTH

avoid unprotected coitus and the risk factors involved in illegal abortions
as well as providing more health care facilities could reverse the mentioned
unhealthy trend.

4.4 SEXUALLY TRANSMITTED DISEASES (STDS)
Diseases or infections which are transmitted through sexual intercourse
are collectively called sexually transmitted diseases (STD) or venereal
diseases (VD) or reproductive tract infections (RTI). Gonorrhoea, syphilis,
genital herpes, chlamydiasis, genital warts, trichomoniasis, hepatitis-B
and of course, the most discussed infection in the recent years, HIV leading
to AIDS are some of the common STDs. Among these, HIV infection is
most dangerous and is discussed in detail in Chapter 8.

Some  of  these  infections  like  hepatitis–B  and  HIV  can  also  be
transmitted by sharing of injection needles, surgical instruments, etc.,
with infected persons, transfusion of blood, or from an infected mother to
the foetus too. Except for hepatitis-B, genital herpes and HIV infections,
other  diseases  are  completely  curable  if  detected  early  and  treated
properly. Early symptoms of most of these are minor and include itching,
fluid discharge, slight pain, swellings, etc., in the genital region. Infected
females may often be asymptomatic and hence, may remain undetected
for long. Absence or less significant symptoms in the early stages of
infection and the social stigma attached to the STDs, deter the infected
persons from going for timely detection and proper treatment. This could
lead to complications later, which include pelvic inflammatory diseases
(PID), abortions, still births, ectopic pregnancies, infertility or even cancer
of the reproductive tract. STDs are a major threat to a healthy society.
Therefore, prevention or early detection and cure of these diseases are
given  prime  consideration  under  the  reproductive  health-care
programmes. Though all persons are vulnerable to these infections, their
incidences are reported to be very high among persons in the age group
of 15-24 years – the age group to which you also belong. Don’t panic.
Prevention is in your hands. You could be free of these infections if you
follow the simple principles given below:

(i) Avoid sex with unknown partners/multiple partners.
(ii) Always use condoms during coitus.
(iii)

In case of doubt, go to a qualified doctor for early detection and
get complete treatment if diagnosed with disease.

4.5 INFERTILITY
A discussion on reproductive health is incomplete without a mention of
infertility. A large number of couples all over the world including India
are infertile, i.e., they are unable to produce children inspite of unprotected
sexual  co-habitation.  The  reasons  for  this  could  be  many–physical,
congenital,  diseases,  drugs,  immunological  or  even  psychological.

63

BIOLOGY

In India, often the female is blamed for the couple being childless, but
more often than not, the problem lies in the male partner. Specialised
health care units (infertility clinics, etc.) could help in diagnosis and
corrective treatment of some of these disorders and enable these couples to
have children. However, where such corrections are not possible, the couples
could be assisted to have children through certain special techniques
commonly known as assisted reproductive technologies (ART).

In vitro fertilisation (IVF–fertilisation outside the body in almost
similar conditions as that in the body) followed by embryo transfer (ET)
is one of such methods. In this method, popularly known as test tube
baby programme, ova from the wife/donor (female) and sperms from the
husband/donor (male) are collected and are induced to form zygote under
simulated conditions in the laboratory. The zygote or early embryos (with
upto 8 blastomeres) could then be transferred into the fallopian tube
(ZIFT–zygote intra fallopian transfer) and embryos with more than
8 blastomeres, into the uterus (IUT – intra uterine transfer), to complete
its further development. Embryos formed by in-vivo fertilisation (fusion
of gametes within the female) also could be used for such transfer to assist
those females who cannot conceive.

Transfer of an ovum collected from a donor into the fallopian tube
(GIFT – gamete intra fallopian transfer) of another female who cannot
produce one, but can provide suitable environment for fertilisation and
further development is another method attempted. Intra cytoplasmic
sperm injection (ICSI) is another specialised procedure to form an embryo
in the laboratory in which a sperm is directly injected into the ovum.
Infertility cases either due to inability of the male partner to inseminate
the female or due to very low sperm counts in the ejaculates, could be
corrected by artificial insemination (AI) technique. In this technique,
the semen collected either from the husband or a healthy donor is artificially
introduced either into the vagina or into the uterus (IUI – intra-uterine
insemination) of the female.

Though options are many, all these techniques require extremely
high  precision  handling  by  specialised  professionals  and  expensive
instrumentation. Therefore, these facilities are presently available only
in very few centres in the country. Obviously their benefits is affordable
to only a limited number of people.  Emotional, religious and social
factors are also deterrents in the adoption of these methods.  Since the
ultimate aim of all these procedures is to have children, in India we have
so  many  orphaned  and  destitute  children,  who  would  probably  not
survive till maturity, unless taken care of. Our laws permit legal adoption
and  it  is  as  yet,  one  of  the  best  methods  for  couples  looking  for
parenthood.

64

REPRODUCTIVE  HEALTH

SUMMARY

Reproductive  health  refers  to  a  total  well-being  in  all  aspects  of
reproduction,  i.e.,  physical,  emotional,  behavioural  and  social.  Our
nation was the first nation in the world to initiate various action plans
at  national  level  towards  attaining  a  reproductively  healthy  society.

Counselling  and  creating  awareness  among  people  about
reproductive  organs,  adolescence  and  associated  changes,  safe  and
hygienic  sexual  practices,  sexually  transmitted  diseases  (STDs)
including  AIDS,  etc.,  is  the  primary  step  towards  reproductive  health.
Providing  medical  facilities  and  care  to  the  problems  like  menstrual
irregularities,  pregnancy  related  aspects,  delivery,  medical  termination
of  pregnancy,  STDs,  birth  control,  infertility,  post  natal  child  and
maternal management is another important aspect of the Reproductive
and  Child  Health  Care  programmes.

An  overall  improvement  in  reproductive  health  has  taken  place  in
our  country  as  indicated  by  reduced  maternal  and  infant  mortality
rates, early detection and cure of STDs, assistance to infertile couples,
etc. Improved health facilities and better living conditions promoted an
explosive  growth  of  population.  Such  a  growth  necessitated  intense
propagation  of  contraceptive  methods.  Various  contraceptive  options
are  available  now  such  as  natural,  traditional,  barrier,  IUDs,  pills,
injectables, implants and surgical methods. Though contraceptives are
not  regular  requirements  for  reproductive  health,  one  is  forced  to  use
them  to  avoid  pregnancy  or  to  delay  or  space  pregnancy.

Medical termination of pregnancy is legalised in our country.  MTP is
generally performed to get rid of unwanted pregnancy due to rapes, casual
relationship,  etc.,  as  also  in  cases  when  the  continuation  of  pregnancy
could be harmful or even fatal to either the mother, or the foetus or both.

Diseases  or  infections  transmitted  through  sexual  intercourse  are
called  Sexually  Transmitted  Diseases  (STDs).  Pelvic  Inflammatory
Diseases  (PIDs),  still  birth,  infertility  are  some  of  the  complications  of
them.  Early detection facilitate better cure of these diseases.  Avoiding
sexual  intercourse  with  unknown/multiple  partners,  use  of  condoms
during coitus are some of the simple precautions to avoid  contracting
STDs.

Inability  to  conceive  or  produce  children  even  after  2  years  of
unprotected  sexual  cohabitation  is  called  infertility.  Various  methods
are now available to help such couples. In Vitro fertilisation followed by
transfer of embryo into the female genital tract is one such method and
is commonly known as the ‘Test Tube Baby’ Programme.

65

EXERCISES

BIOLOGY

1. What do you think is the significance of reproductive health in a society?
2. Suggest  the  aspects  of  reproductive  health  which  need  to  be  given

special  attention  in  the  present  scenario.
Is  sex  education  necessary  in  schools?    Why?

3.
4. Do you think that reproductive health in our country has improved in
the past 50 years?  If yes, mention some such areas of improvement.

Is  the  use  of  contraceptives  justified?    Give  reasons.

5. What  are  the  suggested  reasons  for  population  explosion?
6.
7. Removal of gonads cannot be considered as a contraceptive option.  Why?
8. Amniocentesis  for  sex  determination  is  banned  in  our  country.  Is  this

ban  necessary?  Comment.

9. Suggest  some  methods  to  assist  infertile  couples  to  have  children.
10. What are the measures one has to take to prevent from contracting STDs?
11. State  True/False  with  explanation

(a) Abortions  could  happen  spontaneously  too.  (True/False)
(b)

Infertility  is  defined  as  the  inability  to  produce  a  viable  offspring
and is always due to abnormalities/defects in the female partner.
(True/False)

(c) Complete  lactation  could  help  as  a  natural  method  of

contraception.  (True/False)

(d) Creating  awareness  about  sex  related  aspects  is  an  effective
method to improve reproductive health of the people. (True/False)

12. Correct  the  following  statements  :

(a) Surgical  methods  of  contraception  prevent  gamete  formation.
(b) All  sexually  transmitted  diseases  are  completely  curable.
(c) Oral pills are very popular contraceptives among the rural women.
(d) In E. T. techniques, embryos are always transferred into the uterus.

66



The  work  of  Mendel  and  others  who  followed  him  gave  us  an
idea of inheritance patterns. However the nature of those ‘factors’
which  determine  the  phenotype  was  not  very  clear.  As  these
‘factors’ represent the genetic basis of inheritance, understanding
the  structure  of  genetic  material  and  the  structural  basis  of
genotype  and  phenotype  conversion  became  the  focus  of
attention  in  biology  for  the  next  century.  The  entire  body  of
molecular  biology  was  a  consequent  development  with  major
contributions from Watson, Crick, Nirenberg, Khorana, Kornbergs
(father and son), Benzer, Monod, Brenner, etc. A parallel problem
being tackled was the mechanism of evolution. Awareness in the
areas of molecular genetics, structural biology and bio informatics
have  enriched  our  understanding  of  the  molecular  basis  of
evolution. In this unit the structure and function of DNA and the
story and theory of evolution have been examined and explained.

James Dewey Watson was born in Chicago on 6 April 1928. In 1947, he
received  B.Sc.  degree  in  Zoology.  During  these  years  his  interest  in
bird-watching had matured into a serious desire to learn genetics. This
became possible when he received a Fellowship for graduate study in
Zoology at Indiana University, Bloomington, where he received his Ph.D.
degree in 1950 on a study of the effect of hard X-rays on bacteriophage
multiplication.

He met Crick and discovered their common interest in solving the
DNA structure. Their first serious effort, was unsatisfactory. Their second effort
based upon more experimental evidence and better appreciation of
the nucleic acid literature, resulted, early in March 1953, in the proposal
of the complementary double-helical configuration.
Francis Harry Compton Crick was born on 8 June 1916, at Northampton,
England. He studied physics at University College, London and obtained
a B.Sc. in 1937. He completed Ph.D. in 1954 on a thesis entitled “X-ray
Diffraction: Polypeptides and Proteins”.

A  critical  influence  in  Crick’s  career  was  his  friendship  with  J.  D.
Watson, then a young man of 23, leading in 1953 to the proposal of
the double-helical structure for DNA and the replication scheme. Crick
was made an F.R.S. in 1959.

The honours to Watson with Crick include: the John Collins Warren
Prize of the Massachusetts General Hospital, in 1959; the Lasker Award,
in  1960;  the  Research  Corporation  Prize,  in  1962  and  above  all,  the
Nobel Prize in 1962.

JAMES WATSON
FRANCIS CRICK

CHAPTER 5 PRINCIPLES OF INHERITANCE
AND VARIATION


Have you ever wondered why an elephant always gives
birth only to a baby elephant and not some other animal?
Or why a mango seed forms only a mango plant and not
any other plant?

Given that they do, are the offspring identical to their
parents? Or do they show differences in some of their
characteristics? Have you ever wondered why siblings
sometimes look so similar to each other? Or sometimes
even so different?

These and several related questions are dealt with,
scientifically, in a branch of biology known as Genetics.
This subject deals with the inheritance, as well as the
variation  of  characters  from  parents  to  offspring.
Inheritance is the process by which characters are passed
on  from  parent  to  progeny;  it  is  the  basis  of  heredity.
Variation is the degree by which progeny differ from their
parents.

Humans knew from as early as 8000-1000 B.C. that
one  of  the  causes  of  variation  was  hidden  in  sexual
reproduction.  They  exploited  the  variations  that  were
naturally present in the wild populations of plants and
animals to selectively breed and select for organisms that
possessed  desirable  characters.  For  example,  through
artificial selection and domestication from ancestral

BIOLOGY

wild  cows,  we  have  well-known  Indian
breeds, e.g., Sahiwal cows in Punjab. We
must, however, recognise that though our
ancestors knew about the inheritance of
characters  and  variation,  they  had  very
little idea about the scientific basis of these
phenomena.

5.1 MENDEL’S LAWS OF INHERITANCE
It was during the mid-nineteenth century that
headway was made in the understanding of
inheritance.  Gregor  Mendel,  conducted
hybridisation experiments on garden peas for
seven  years  (1856-1863)  and  proposed  the
laws of inheritance in living organisms. During
Mendel’s  investigations  into  inheritance
patterns it was for the first time that statistical
analysis and mathematical logic were applied
to problems in biology. His experiments had a
large  sampling  size,  which  gave  greater
credibility to the data that he collected. Also,
the  confirmation  of  his  inferences  from
experiments on successive generations of his
test plants, proved that his results pointed to
general rules of inheritance rather than being
unsubstantiated  ideas.  Mendel  investigated
characters in the garden pea plant that were
manifested as two opposing traits, e.g., tall or
dwarf  plants,  yellow  or  green  seeds.  This
allowed him to set up a basic framework of
rules  governing  inheritance,  which  was
expanded on by later scientists to account for
all the diverse natural observations and the
complexity inherent in them.

Figure 5.1 Seven  pairs  of  contrasting  traits  in

pea  plant  studied  by  Mendel

70

Mendel  conducted  such  artificial
pollination/cross  pollination  experiments
using several  true-breeding pea lines. A true-
breeding line is one that, having undergone
continuous  self-pollination,  shows  the  stable  trait  inheritance  and
expression for several generations. Mendel selected 14 true-breeding pea
plant varieties, as pairs which were similar except for one character with
contrasting traits. Some of the contrasting traits selected were smooth or
wrinkled seeds, yellow or green seeds, smooth or inflated pods, green or
yellow pods and tall or dwarf plants (Figure 5.1, Table 5.1).

PRINCIPLES  OF  INHERITANCE  AND  VARIATION

Table  5.1: Contrasting  Traits  Studied  by

Mendel  in  Pea

S.No.

Characters

Contrasting  Traits

1.

2.

3.

4.

5.

6.

7.

Stem height

Tall/dwarf

Flower colour

Violet/white

Flower  position Axial/terminal

Pod shape

Pod colour

Inflated/constricted

Green/yellow

Seed shape

Round/wrinkled

Seed colour

Yellow/green

5.2 INHERITANCE OF ONE GENE
Let  us  take  the  example  of  one  such
hybridisation  experiment  carried  out  by
Mendel where he crossed tall and dwarf pea
plants to study the inheritance of one gene
(Figure 5.2). He collected the seeds produced
as a result of this cross and grew them to
generate plants of the first hybrid generation.
This  generation  is  also  called  the  Filial1
progeny or the F1. Mendel observed that all
the F1  progeny  plants were tall, like one of
its parents; none were dwarf (Figure 15.3).
He made similar observations for the other
pairs of traits – he found that the F1  always
resembled either one of the parents, and that
the trait of the other parent was not seen in
them.

Mendel  then  self-pollinated  the  tall  F1
plants and to his surprise found  that in the
Filial2 generation some of the offspring were
‘dwarf ’; the character that was not seen in
the F1 generation was now expressed. The
proportion of plants that were dwarf were
1/4th of the F2 plants while 3/4th of the F2 plants were tall. The tall and
dwarf traits were identical to their parental type and did not show any
blending, that is all the offspring were either tall or dwarf, none were of in-
between height (Figure 5.3).

Similar results were obtained with the other traits that he studied:
only one of the parental traits was expressed in the F1 generation while at
the F2 stage both the traits were expressed in the proportion 3:1. The
contrasting traits did not show any blending at either F1 or F2 stage.

Figure 5.2 Steps in making a cross in pea

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BIOLOGY

Based  on  these  observations,
Mendel  proposed  that  something
was  being  stably  passed  down,
unchanged, from parent to offspring
through 
the  gametes,  over
successive  generations.  He  called
these things as ‘factors’. Nowadays,
we  call  them  as  genes.  Genes,
therefore,  are  the  units  of
inheritance.  They  contain  the
information  that  is  required  to
express  a  particular  trait,  in  an
organism. Genes which code for a
pair of contrasting traits are known
as  alleles,  i.e.,  they  are  slightly
different forms of the same gene.

If we use  alphabetical symbols
for each gene, then the capital letter
is used for the trait expressed at the
F1 stage and the small alphabet for
the other trait. For example, in case
of the character of height, T is used
for the Tall trait and t for the ‘dwarf’,
and T and t are alleles of each other.
Hence, in plants the pair of alleles
for  height  would  be  TT, Tt  or  tt.
Mendel also proposed that in a true
breeding, tall or dwarf pea variety
the allelic pair of genes for height are
identical or homozygous, TT and tt, respectively. TT and tt are called
the genotype of the plant while the descriptive terms tall and dwarf are
the phenotype. What then would be the phenotype of a plant that had
a genotype Tt?

of  monohybrid  cross

Figure 5.3 Diagrammatic representation

72

As Mendel found the phenotype of the F1 heterozygote Tt to be exactly
like the TT parent in appearance, he proposed that in a pair of dissimilar
factors, one dominates the other (as in the F1 ) and hence is called the
dominant factor while the other factor is recessive . In this case T (for
tallness) is dominant over t (for dwarfness), that is recessive. He observed
identical behaviour for all the other characters/trait-pairs that he studied.
It is convenient (and logical) to use the capital and lower case of an
alphabetical  symbol  to  remember  this  concept  of  dominance  and
recessiveness. (Do not use T for tall and d for dwarf because you will
find it difficult to remember whether T and d are alleles of the same
gene/character or not). Alleles can be similar as in the case of homozygotes
TT and tt or can be dissimilar as in the case of the heterozygote Tt. Since

PRINCIPLES  OF  INHERITANCE  AND  VARIATION

the Tt plant is heterozygous for genes controlling
one character (height), it is a monohybrid and the
cross between TT and tt is a monohybrid cross.
From the observation that the recessive parental
trait is expressed without any blending in the F2
generation, we can infer that, when the tall and
dwarf plant produce gametes, by the process of
meiosis, the alleles of the parental pair separate or
segregate from each other and only one allele is
transmitted to a gamete. This segregation of alleles
is a random process and so there is a 50 per cent
chance of a gamete containing either allele, as has
been verified by the results of the crossings. In this
way the gametes of the tall TT plants have the allele
T and the gametes of the dwarf tt plants have the
allele t. During fertilisation the two alleles, T from
one parent say, through the pollen, and t from the
other parent, then through the egg, are united to
produce zygotes that have one T allele and one t
allele.  In other words the hybrids have Tt. Since
these  hybrids  contain  alleles  which  express
contrasting traits, the plants are heterozygous. The
production of gametes by the parents, the formation
of  the  zygotes,  the  F1  and  F2  plants  can  be
understood from a diagram called Punnett Square
as shown in Figure 5.4. It was developed by a British
geneticist, Reginald C. Punnett. It is a graphical
representation to calculate the probability of all
possible genotypes of offspring in a genetic cross.
The  possible  gametes  are  written  on  two  sides,
usually the top row and left columns. All  possible
combinations are represented in boxes below in the
squares, which generates a square output form.

Figure  5.4 A  Punnett  square  used  to
understand a typical monohybrid
cross  conducted  by  Mendel
between  true-breeding  tall  plants
and  true-breeding  dwarf  plants

The Punnett Square shows the parental tall TT
(male) and dwarf tt (female) plants, the gametes
produced by them and, the F1 Tt  progeny. The F1
plants  of  genotype  Tt  are  self-pollinated.  The
symbols &&&&& and %%%%% are used to denote the female
(eggs) and male (pollen) of the F1 generation, respectively. The F1 plant of
the genotype Tt when self-pollinated, produces gametes of the genotype
T and t in equal proportion. When fertilisation takes place, the pollen
grains of genotype T have a 50 per cent chance to pollinate eggs of the
genotype T, as well as of genotype t. Also pollen grains of genotype t have
a 50 per cent chance of pollinating eggs of genotype T, as well as of

73

BIOLOGY

genotype t. As a result of random fertilisation, the resultant zygotes can
be of the genotypes TT, Tt or tt.

From the Punnet square it is easily seen that 1/4th of the random
fertilisations lead to TT, 1/2 lead to Tt and 1/4th to tt. Though the F1
have a genotype of Tt, but the phenotypic  character seen is ‘tall’. At F2,
3/4th of the plants are tall, where some of them are TT while others are
Tt. Externally it is not possible to distinguish between  the plants with
the genotypes TT and Tt. Hence, within the genopytic pair Tt only one
character ‘T’ tall is expressed. Hence the character T or ‘tall’ is said to
dominate over the other allele t or ‘dwarf’ character. It is thus due to this
dominance of one character over the other that all the F1 are tall (though
the genotype is Tt) and in the F2 3/4th of  the  plants  are  tall  (though
genotypically 1/2  are Tt and only 1/4th are TT).  This leads to a phenotypic
ratio of 3/4th tall : (1/4 TT + 1/2 Tt) and 1/4th tt, i.e., a 3:1 ratio,  but a
genotypic ratio of 1:2:1.

The 1/4 : 1/2 : 1/4 ratio of TT: Tt: tt is mathematically condensable
to the form of the binomial expression (ax +by)2, that has the gametes
bearing genes T or t in equal frequency of ½. The expression is expanded
as given below :

(1/2T + 1/2 t)2  = (1/2T + 1/2t) X (1/2T + 1/2t) = 1/4 TT + 1/2Tt + 1/4 tt

Mendel self-pollinated the F2 plants and found that dwarf F2 plants
continued to generate dwarf plants in F3 and F4 generations. He concluded
that the genotype of the dwarfs was homozygous – tt. What do you think
he would have got had he self-pollinated a tall F2 plant?

From the preceeding paragraphs it is clear that though the genotypic
ratios  can  be  calculated  using  mathematical  probability,  but  simply
looking at the phenotype of a dominant trait, it  is not possible to know
the genotypic composition. That is, for example, whether a tall plant from
F1 or F2 has TT or Tt composition, cannot be predicted. Therefore, to
determine the genotype of a tall plant at F2, Mendel crossed the tall plant
from F2 with a dwarf plant. This he called a test cross. In a typical test
cross an organism (pea plants here) showing a dominant phenotype (and
whose genotype is to be determined) is crossed with the recessive parent
instead  of  self-crossing.  The  progenies  of  such  a  cross  can  easily  be
analysed to predict the genotype of the test organism. Figure 5.5 shows
the results of typical test cross where violet colour flower (W) is dominant
over white colour flower (w).

Using Punnett square, try to find out the nature of offspring of a test cross.

What ratio did you get?

Using the genotypes of this cross, can you give a general definition for
a  test  cross?

74

PRINCIPLES  OF  INHERITANCE  AND  VARIATION

Figure  5.5  Diagrammatic  representation  of  a  test  cross

Based on his observations on monohybrid crosses Mendel proposed
two  general  rules  to  consolidate  his  understanding  of  inheritance  in
monohybrid crosses. Today these rules are called the Principles or Laws
of Inheritance: the First Law or Law of Dominance and the Second
Law or Law of Segregation.

5.2.1 Law of Dominance

(i) Characters are controlled by discrete units called factors.
(ii) Factors occur in pairs.
(iii)

In a dissimilar pair of factors one member of the pair dominates
(dominant) the other (recessive).

The law of dominance is used to explain the expression of only one of
the parental characters in a monohybrid cross in the F1 and the expression
of both in the F2. It also explains the proportion of 3:1 obtained at the F2.

5.2.2 Law of Segregation

This law is based on the fact that the alleles do not show any blending
and that both the characters are recovered as such in the F2 generation
though one of these is not seen at the F1 stage. Though the parents contain
two alleles during gamete formation, the factors or alleles of a pair segregate
from each other such that a gamete receives only one of the two factors.
Of course, a homozygous parent produces all gametes that are similar
while a heterozygous one produces two kinds of gametes each having
one allele with equal proportion.

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BIOLOGY

5.2.2.1 Incomplete Dominance

When experiments on peas were repeated using other
traits in other plants, it was found that sometimes
the F1 had a phenotype that did not resemble either
of the two parents and was in between the two.  The
inheritance  of  flower  colour  in  the  dog  flower
(snapdragon or Antirrhinum sp.) is a good example
to understand incomplete dominance. In a cross
between true-breeding red-flowered (RR) and true-
breeding white-flowered plants (rr),  the F1 (Rr) was
pink (Figure 5.6).  When the F1  was self-pollinated
the F2  resulted in the following ratio 1 (RR) Red : 2
(Rr) Pink: 1 (rr) White. Here the genotype ratios were
exactly  as  we  would  expect  in  any  mendelian
monohybrid cross, but the phenotype ratios had
changed from the 3:1 dominant : recessive ratio.
What  happened  was  that  R  was  not  completely
dominant  over  r  and  this    made  it  possible  to
distinguish Rr as pink from  RR (red)  and rr (white) .
Explanation  of  the  concept  of  dominance:
What exactly is dominance? Why are some alleles
dominant and some recessive? To tackle these
questions, we must understand what a gene does.
Every gene, as you know by now, contains the
information  to  express  a  particular  trait.  In  a
diploid organism, there are two copies of each
gene, i.e., as a pair of alleles. Now, these two alleles
need not always be identical, as in a heterozygote.
One of them may be different due to some changes
that it has undergone (about which you will read
further on, and in the next chapter) which modifies
the information that particular allele contains.

Figure  5.6 Results of monohybrid cross in
the  plant  Snapdragon,  where
one  allele  is  incompletely
dominant over the other allele

Let’s take an example of a gene that contains
the information for producing an enzyme. Now
there are two copies of this gene, the two allelic
forms. Let us assume (as is more common) that
the normal allele produces the normal enzyme
that  is  needed  for  the  transformation  of  a
substrate S. Theoretically, the modified allele could be responsible for
production of –

76

(i) the normal/less efficient enzyme, or
(ii) a non-functional enzyme, or
(iii) no enzyme at all

PRINCIPLES  OF  INHERITANCE  AND  VARIATION

In the first case, the modified allele is equivalent to the unmodified allele,
i.e., it will produce the same phenotype/trait, i.e., result in the transformation
of substrate S. Such equivalent allele pairs are very common. But, if the
allele produces a non-functional enzyme or no enzyme, the phenotype may
be effected. The phenotype/trait will only be dependent on the functioning
of the unmodified allele. The unmodified (functioning) allele, which represents
the original phenotype is the dominant allele and the modified allele is
generally the recessive allele. Hence, in the example above the recessive trait
is seen due to non-functional enzyme or because no enzyme is produced.

5.2.2.2  Co-dominance

Till now we were discussing crosses where the F1 resembled either of the
two parents (dominance) or was in-between (incomplete dominance). But,
in the case of co-dominance the F1 generation resembles both parents. A
good example is different types of red blood cells that determine ABO
blood grouping in human beings. ABO blood groups are controlled by
the gene I.  The plasma membrane of the red blood cells has sugar polymers
that protrude from its surface and the kind of sugar is controlled by the
gene. The gene (I) has three alleles IA, I B and i. The alleles I A and IB produce
a slightly different form of the sugar while allele i doesn’t produce any
sugar. Because humans are diploid organisms, each person possesses
any two of the three I gene alleles. IA and I B are completely dominant over
i, in other words when IA and i are present only IA expresses (because
i does not produce any sugar), and when IB and i are present IB expresses.
But when IA and IB are present together they both express their own types
of sugars: this is because of co-dominance. Hence red blood cells have
both A and B types of sugars. Since there are three different alleles, there
are six different combinations of these three alleles that are possible a
total of six different genotypes of the human ABO blood types (Table 5.2).
How many phenotypes are possible?

Table  5.2: Table Showing the Genetic Basis of Blood Groups

in  Human  Population

Allele from
Parent  1

Allele from
Parent  2

Genotype  of
offspring

Blood
types of
offspring

I A

I A

I A

I B

I B

I B

i

I A

I B

i

I A

I B

i

i

I A I A

I A I B

I A i

I A I B

I B I B

I B i

i i

A

AB

A

AB

B

B

O

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BIOLOGY

Do you realise that the example of ABO blood grouping also provides
a good example of multiple alleles? Here you  can see that there are
more than two, i.e.,  three alleles, governing the same character. Since in
an individual only two alleles can be present, multiple alleles can be found
only when population studies are made.

Occasionally, a single gene product may produce more than one effect.
For example, starch synthesis in pea seeds is controlled by one gene. It
has  two  alleles  (B  and  b).  Starch  is  synthesised  effectively  by  BB
homozygotes and therefore, large starch grains are produced. In contrast,
bb homozygotes have lesser efficiency in starch synthesis and produce
smaller starch grains. After maturation of the seeds, BB seeds are round
and the bb seeds are wrinkled. Heterozygotes produce round seeds, and
so B seems to be the dominant allele. But, the starch grains produced are
of intermediate size in Bb seeds. So if starch grain size is considered as
the  phenotype,  then  from  this  angle,  the  alleles  show  incomplete
dominance.

Therefore, dominance is not an autonomous feature of a gene or the
product that it has information for. It depends as much on the gene
product and the production of a particular phenotype from this product
as it does on the particular phenotype that we choose to examine, in case
more than one phenotype is influenced by the same gene.

5.3 INHERITANCE OF TWO GENES

Mendel also worked with and crossed pea plants that differed in two
characters, as is seen in the cross between a pea plant that has seeds with
yellow colour and round shape and one that had seeds of green colour
and wrinkled shape (Figure5.7). Mendel found that the seeds resulting
from the crossing of the parents, had yellow coloured and round shaped
seeds.  Here  can  you  tell  which  of  the  characters  in  the  pairs  yellow/
green colour and round/wrinkled shape was dominant?

Thus,  yellow  colour  was  dominant  over  green  and  round  shape
dominant over wrinkled. These results were identical to those that he got
when he made separate monohybrid crosses between yellow and  green
seeded plants and between round and wrinkled seeded plants.

Let us use the genotypic symbols Y for dominant yellow seed colour
and y for recessive green seed colour, R for round shaped seeds and r  for
wrinkled seed shape. The genotype of the parents can then be written as
RRYY and  rryy. The cross between the two plants can be written down
as in Figure 5.7 showing the genotypes of the parent plants. The gametes
RY and ry unite on fertilisation to produce the F1 hybrid RrYy. When
Mendel self hybridised  the F1 plants he found that 3/4th of F2 plants had
yellow seeds and 1/4th had green. The yellow and green colour segregated
in a 3:1 ratio. Round and wrinkled seed shape also segregated in a 3:1
ratio; just like in a monohybrid cross.

78

PRINCIPLES  OF  INHERITANCE  AND  VARIATION

Figure 5.7 Results of a dihybrid cross where the two parents differed in two pairs of

contrasting  traits:  seed  colour  and  seed  shape

79

BIOLOGY

5.3.1 Law of Independent Assortment

In  the  dihybrid  cross  (Figure  5.7),  the  phenotypes  round, yellow;
wrinkled, yellow; round, green  and wrinkled, green appeared in the
ratio 9:3:3:1. Such a ratio was observed for several pairs of characters
that Mendel studied.

The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow:
1  green,  with  3  round  :  1  wrinkled.  This  derivation  can  be  written
as follows:

(3 Round : 1 Wrinkled)  (3 Yellow : 1 Green) = 9 Round, Yellow : 3

Wrinkled, Yellow: 3 Round, Green : 1 Wrinkled, Green

Based upon such observations on dihybrid crosses (crosses between
plants differing in two traits) Mendel proposed a second set of generalisations
that we call Mendel’s Law of Independent Assortment. The law states that
‘when two pairs of traits are combined in a hybrid, segregation of one pair
of characters is independent of the other pair of characters’.

The  Punnett  square  can  be  effectively  used  to  understand  the
independent segregation of the two pairs of genes during meiosis and
the production of eggs and pollen in the F1 RrYy plant. Consider the
segregation of one pair of genes R and r. Fifty per cent of the gametes
have the gene R and the other 50 per cent have r. Now besides each
gamete having either R or r, it should also have the allele Y or y.  The
important thing to remember here is that segregation of 50 per cent R
and 50 per cent r is independent from the segregation of 50 per cent Y
and 50 per cent y. Therefore, 50 per cent of the r bearing gamete has Y
and the other 50 per cent has y. Similarly, 50 per cent of the R bearing
gamete has Y and the other 50 per cent has y. Thus there are four
genotypes  of  gametes  (four  types  of  pollen  and four  types  of  eggs).
The four types are  RY, Ry, rY and ry each with a frequency of 25 per
cent or ¼th of the total gametes produced. When you write down the
four types of eggs and pollen on the two sides of a Punnett square it is
very easy to derive the composition of the zygotes  that give rise to the
F2 plants    (Figure  5.7).  Although  there  are  16  squares  how  many
different types of genotypes and phenotypes are formed? Note them
down in the format given.

Can you, using the Punnett square data work out the genotypic ratio
at  the  F2  stage  and  fill  in  the  format  given?  Is  the  genotypic  ratio
also 9:3:3:1?

80

S.No. Genotypes found in F2

Their expected Phenotypes

5.3.2 Chromosomal Theory of Inheritance

Mendel published his work on inheritance of characters in 1865 but
for  several  reasons,  it  remained  unrecognised  till  1900.  Firstly,

PRINCIPLES  OF  INHERITANCE  AND  VARIATION

communication was not easy (as it is now) in those days and his work
could not be widely publicised.  Secondly, his concept of genes (or
factors, in Mendel’s words) as stable and discrete units that controlled
the expression of traits and, of the pair of alleles which did not ‘blend’
with  each  other,  was  not  accepted  by  his  contemporaries  as  an
explanation for the apparently continuous variation seen in nature.
Thirdly, Mendel’s approach of using mathematics to explain biological
phenomena  was  totally  new  and  unacceptable  to  many  of  the
biologists of his time. Finally, though Mendel’s work suggested that
factors (genes) were discrete units, he could not provide any physical
proof for the existence of factors or say what they were made of.

In  1900,  three  Scientists  (de  Vries,  Correns  and  von  Tschermak)
independently  rediscovered  Mendel’s  results  on  the  inheritance  of
characters. Also, by this time due to advancements in microscopy that
were taking place, scientists were able to carefully observe cell division.
This led to the discovery of structures in the nucleus that appeared to
double  and  divide  just  before  each  cell  division.  These  were  called
chromosomes (colored bodies, as they were visualised by staining). By
1902, the chromosome movement during meiosis had been worked out.
Walter  Sutton  and  Theodore  Boveri  noted  that  the  behaviour  of
chromosomes  was  parallel  to  the  behaviour  of  genes  and  used
chromosome movement (Figure 5.8) to explain Mendel’s laws (Table 5.3).
Recall that you have studied the behaviour of chromosomes during mitosis
(equational  division)  and  during  meiosis  (reduction  division).  The
important things to remember are that chromosomes as well as genes
occur in pairs. The two alleles of a gene pair are located on homologous
sites on homologous chromosomes.

Figure  5.8 Meiosis and germ cell formation in a cell with four chromosomes.
Can  you  see  how  chromosomes  segregate  when  germ  cells
are  formed?

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BIOLOGY

Table  5.3: A  Comparison  between  the  Behaviour  of  Chromosomes

and  Genes

A

B

Occur in pairs

Occur in pairs

Segregate  at  the  time  of  gamete
formation such that only one of each
pair is transmitted to a gamete

Segregate at gamete formation and only
one  of  each  pair  is  transmitted  to  a
gamete

Independent  pairs  segregate
independently of each other

One  pair  segregates  independently  of
another pair

Can you tell which of these columns A or B represent the chromosome

and which represents the gene? How did you decide?

During Anaphase of meiosis I, the two chromosome pairs can align at
the  metaphase  plate  independently  of  each  other  (Figure  5.9).  To
understand this, compare the chromosomes of four different colour in
the left and right columns. In the left column (Possibility I) orange and
green is segregating together. But in the right hand column (Possibility II)
the orange chromosome is segregating with the red chromosomes.

Possibility I

  Possibility II

One long orange and short green          One long orange and short red
chromosome and long yellow and       chromosome and long yellow and
 short red chromosome at the               short green chromosome at the
               same pole                                              same pole

82

Figure  5.9  Independent  assortment  of  chromosomes

PRINCIPLES  OF  INHERITANCE  AND  VARIATION

Sutton and Boveri argued that the pairing and separation of a
pair of chromosomes would lead to the segregation of a pair of
factors they carried. Sutton united the knowledge of chromosomal
segregation  with  Mendelian  principles  and  called  it  the
chromosomal theory of inheritance.

Following this synthesis of ideas, experimental verification of
the chromosomal theory of inheritance by Thomas Hunt Morgan
and his colleagues, led to discovering the basis for the variation
that sexual reproduction produced. Morgan worked with the tiny
fruit files, Drosophila melanogaster (Figure 5.10), which were
found  very  suitable  for  such  studies.  They  could  be  grown  on
simple synthetic medium in the laboratory. They complete their life
cycle in about two weeks, and a single mating could produce a large
number of progeny flies. Also, there was a clear differentiation of the
sexes – the male and female flies are easily distinguisable. Also, it
has many types of hereditary variations that can be seen with low
power microscopes.

5.3.3 Linkage and Recombination

Morgan carried out several dihybrid crosses in Drosophila to study genes
that were sex-linked. The crosses were similar to the dihybrid crosses carried
out by Mendel in peas. For example Morgan hybridised yellow-bodied,
white-eyed females to brown-bodied, red-eyed males and intercrossed their
F1 progeny.  He observed that the two genes did not segregate independently
of each other and the F2 ratio deviated very significantly from the 9:3:3:1
ratio (expected when the two genes are independent).

Morgan and his group knew that the genes were located on the X
chromosome (Section 5.4) and saw quickly that when the two genes in a
dihybrid cross were situated on the same chromosome, the proportion
of parental gene combinations were much higher than the non-parental
type.  Morgan attributed this due to the physical association or linkage
of the two genes and coined the term linkage to describe this physical
association of genes on a chromosome and the term recombination to
describe the generation of non-parental gene combinations (Figure 5.11).
Morgan and his group also found that even when genes were grouped
on  the same chromosome, some genes were very tightly linked (showed
very low recombination) (Figure 5.11, Cross A) while others were loosely
linked  (showed  higher  recombination)  (Figure  5.11,  Cross  B).  For
example he found that the genes white and yellow were very tightly linked
and showed only 1.3 per cent recombination while white and miniature
wing showed 37.2 per cent recombination. His student Alfred Sturtevant
used the frequency of recombination between gene pairs on the same
chromosome as a measure of the distance between genes and ‘mapped’
their position on the chromosome. Today genetic maps are extensively

(a)

(b)

Figure  5.10  Drosophila
melanogaster  (a)  Male

(b)  Female

83

used as a starting point in the sequencing of whole genomes as was done
in the case of the Human Genome Sequencing Project, described later.

BIOLOGY

84
84

Figure  5.11 Linkage:  Results  of  two  dihybrid  crosses  conducted  by  Morgan.  Cross  A  shows
crossing between gene y and w; Cross B shows crossing between genes w and m.
Here  dominant  wild  type  alleles  are  represented  with  (+)  sign  in  superscript
Note: The strength of linkage between y and w is higher than w and m.

PRINCIPLES  OF  INHERITANCE  AND  VARIATION

(a)

(b)

5.4 SEX DETERMINATION
The mechanism of sex determination has
always been a puzzle before the geneticists.
The  initial  clue  about  the  genetic/
chromosomal  mechanism  of  sex
determination can be traced back to some
of the experiments carried out in insects. In
fact, the cytological observations made in a
number of insects led to the development of
the concept of genetic/chromosomal basis
of sex-determination. Henking (1891) could
trace a specific nuclear structure all through
spermatogenesis in a few insects, and it was
also observed by him that 50 per  cent of the
sperm  received  this  structure  after
spermatogenesis, whereas the other 50 per
cent sperm did not receive it. Henking gave
a name to this structure as the X body but
he could not explain its significance. Further
investigations by other scientists led to the
conclusion that the ‘X body’ of Henking was
in fact a chromosome and that is why it was
given the name X-chromosome. It was also
observed that in a large number of insects
the mechanism of sex determination is of the
XO type, i.e., all eggs bear an additional
X - c h r o m o s o m e   b e s i d e s   t h e   o t h e r
chromosomes (autosomes). On the other
hand,  some  of  the  sperms  bear  the
X-chromosome  whereas  some  do  not.
Eggs  fertilised  by  sperm  having  an
X-chromosome  become  females  and,
those  fertilised  by  sperms  that  do  not
have an X-chromosome become males. Do
you  think  the  number  of  chromosomes  in
the male and female are equal? Due to the
involvement  of  the  X-chromosome  in  the
determination of sex, it was designated to
be the sex chromosome, and the rest of the chromosomes were named
as autosomes.Grasshopper is an example of XO type of sex determination
in which the males have only one X-choromosome besides the autosomes,
whereas females have a pair of X-chromosomes.

These observations led to the investigation of a number of species to
understand the mechanism of sex determination. In a number of other
insects and mammals including man, XY type of sex determination is
seen where both male and female have same number of chromosomes.

(c)

Figure  5.12 Determination  of  sex  by  chromosomal
differences:  (a,b)  Both  in  humans  and
in  Drosophila,  the  female  has  a  pair  of
XX chromosomes (homogametic) and the
male  XY  (heterogametic)  composition;
(c)  In  many  birds,  female  has  a  pair  of
dissimilar  chromosomes  ZW  and  male
two similar ZZ chromosomes

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BIOLOGY

Among the males an X-chromosome is present but its counter part is
distinctly smaller and called the Y-chromosome. Females, however, have
a pair of X-chromosomes. Both males and females bear same number of
autosomes. Hence, the males have autosomes plus XY, while female have
autosomes plus XX. In human beings  and in Drosophila the males have
one  X  and  one  Y  chromosome,  whereas  females  have  a  pair  of  X-
chromosomes besides autosomes (Figure 5.12 a, b).

In the above description you have studied about two types of sex
determining mechanisms, i.e., XO type and XY type. But in both cases
males produce two different types of gametes, (a) either with or without
X-chromosome or (b) some gametes with X-chromosome and some with
Y-chromosome.  Such  types  of  sex  determination  mechanism  is
designated to be the example of male heterogamety. In some other
organisms, e.g., birds a different mechanism of sex determination is
observed (Figure 5.12 c). In this case the total number of chromosome
is same in both males and females. But two different types of gametes in
terms of the sex chromosomes, are produced by females, i.e., female
heterogamety. In order to have a distinction with the mechanism of
sex determination described earlier, the two different sex chromosomes
of a female bird has been designated to be the Z and W chromosomes.
In these organisms the females have one Z and one W chromosome,
whereas males have a pair of Z-chromosomes besides the autosomes.

5.4.1 Sex Determination in Humans

It has already been mentioned that the sex determining mechanism in
case of humans is XY type. Out of 23 pairs of chromosomes present, 22
pairs are exactly same in both males and females; these are the autosomes.
A pair of X-chromosomes are present in the female, whereas the presence
of an X and Y chromosome are determinant of the male characteristic.
During spermatogenesis among males, two types of gametes are produced.
50 per cent of the total sperm produced carry the X-chromosome and
the rest 50 per cent has Y-chromosome besides the autosomes. Females,
however, produce only one type of ovum with an X-chromosome. There
is an equal probability of fertilisation of the ovum with the sperm carrying
either  X  or  Y  chromosome.  In  case  the  ovum  fertilises  with  a  sperm
carrying X-chromosome the zygote develops into a female (XX) and the
fertilisation of ovum with Y-chromosome carrying sperm results into a
male offspring. Thus, it is evident that it is the genetic makeup of the
sperm that determines the sex of the child. It is also evident that in each
pregnancy there is always 50 per cent probability of either a male or a
female child. It is unfortunate that in our society women are blamed for
producing  female  children  and  have  been  ostracised  and  ill-treated
because of this false notion.

How  is  the  sex-determination  mechanism  different  in  the  birds?

Is the sperm or the egg responsible for the sex of the chicks?

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PRINCIPLES  OF  INHERITANCE  AND  VARIATION

5.5 MUTATION

Mutation is a phenomenon which results in alteration of DNA sequences
and consequently results in changes in the genotype and the phenotype
of  an  organism.  In  addition  to  recombination,  mutation  is  another
phenomenon that leads to variation in DNA.

As you will learn in Chapter 6, one DNA helix runs continuously from
one end to the other in each chromatid, in a highly supercoiled form.
Therefore loss (deletions) or gain (insertion/duplication) of a segment of
DNA, result in alteration in chromosomes. Since genes are known to be
located  on  chromosomes,  alteration  in  chromosomes  results  in
abnormalities or aberrations. Chromosomal aberrations are commonly
observed in cancer cells.

In addition to the above, mutation also arise due to change in a single
base pair of DNA. This is known as point mutation. A classical example of
such a mutation is sickle cell anemia. Deletions and insertions of base
pairs of DNA, causes frame-shift mutations (see Chapter 6).

The mechanism of mutation is beyond the scope of this discussion, at
this level. However, there are many chemical and physical factors that
induce mutations. These are referred to as mutagens. UV radiations can
cause mutations in organisms – it is a mutagen.

5.6 GENETIC DISORDERS
5.6.1 Pedigree Analysis

The idea that disorders are inherited has been prevailing in the human
society  since  long.  This  was  based  on  the  heritability  of  certain
characteristic features in families. After the rediscovery of Mendel’s work
the practice of analysing inheritance pattern of traits in human beings
began. Since it is evident that control crosses that can be performed in
pea plant or some other organisms, are not possible in case of human
beings, study of the family history about inheritance of a particular trait
provides  an  alternative.  Such  an  analysis  of  traits  in  a  several  of
generations of a family is called the pedigree analysis. In the pedigree
analysis the inheritance of a particular trait is represented in the family
tree over generations.

In human genetics, pedigree study provides a strong tool, which is
utilised to trace the inheritance of a specific trait, abnormality or disease.
Some of the important standard symbols used in the pedigree analysis
have been shown in Figure 5.13.

As you have studied in this chapter, each and every feature in any
organism is controlled by one or the other gene located on the DNA present
in the chromosome. DNA is the carrier of genetic information. It is hence
transmitted from one generation to the other without any change or

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BIOLOGY

alteration. However, changes or alteration do take
place occasionally. Such an alteration or change in
the  genetic  material  is  referred  to  as  mutation.  A
number  of  disorders  in  human  beings  have  been
found to be associated with the inheritance of changed
or altered genes or chromosomes.

5.6.2 Mendelian Disorders

Broadly, genetic disorders may be grouped into two
categories – Mendelian disorders and Chromosomal
disorders.  Mendelian  disorders  are  mainly
determined by alteration or mutation in the single
gene. These disorders are transmitted to the offspring
on the same lines as we have studied in the principle
of inheritance. The pattern of inheritance of such
Mendelian disorders can be traced in a family by
the  pedigree  analysis.  Most  common  and
prevalent  Mendelian  disorders  are  Haemophilia,
Cystic fibrosis, Sickle-cell anaemia, Colour blindness,
Phenylketonuria, Thalesemia, etc. It is important to
mention here that such Mendelian disorders may be
dominant or recessive. By pedigree analysis one can
easily understand whether the trait in question is
dominant or recessive. Similarly, the trait may also
be  linked  to  the  sex  chromosome  as  in  case  of
haemophilia. It is evident that this X-linked recessive
trait  shows  transmission  from  carrier  female  to  male  progeny.  A
representative pedigree is shown in Figure 5.14 for dominant and recessive
traits, discuss with your teacher and design pedigrees for characters linked
to both autosomes and sex chromosome.

Figure 5.13 Symbols  used  in  the  human

pedigree analysis

88

(a)

(b)

Figure  5.14 Representative  pedigree  analysis  of  (a)  Autosomal  dominant  trait  (for  example:
Myotonic  dystrophy)  (b)  Autosomal  recessive  trait  (for  example:  Sickle-cell  anaemia)

PRINCIPLES  OF  INHERITANCE  AND  VARIATION

Haemophilia  :  This  sex  linked  recessive  disease,  which  shows  its
transmission from unaffected carrier female to some of the male progeny
has been widely studied. In this disease, a single protein that is a part of
the cascade of proteins involved in the clotting of blood is affected. Due to
this, in an affected individual a simple cut will result in non-stop bleeding.
The heterozygous female (carrier) for haemophilia may transmit the disease
to sons. The possibility of a female becoming a haemophilic is extremely
rare because mother of such a female has to be at least carrier and the
father should be haemophilic (unviable in the later stage of life). The family
pedigree of Queen Victoria shows a number of haemophilic descendents
as she was a carrier of the disease.
Sickle-cell anaemia : This is an autosome linked recessive trait that can
be transmitted from parents to the offspring when both the partners are
carrier for the gene (or heterozygous). The disease is controlled by a single
pair of allele, HbA and HbS. Out of the three possible genotypes only
homozygous individuals for HbS (HbSHbS) show the diseased phenotype.
Heterozygous (HbAHbS) individuals appear apparently unaffected but they
are carrier of the disease as there is 50 per cent probability of transmission
of the mutant gene to the progeny, thus exhibiting sickle-cell trait (Figure
5.15). The defect is caused by the substitution of Glutamic acid (Glu) by

Figure 5.15 Micrograph of the red blood cells and the amino acid composition of the relevant portion
of β-chain of haemoglobin: (a) From a normal individual; (b) From an individual with
sickle-cell anaemia

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BIOLOGY

Valine (Val) at the sixth position of the beta globin chain of the haemoglobin
molecule. The substitution of amino acid in the globin protein results
due to the single base substitution at the sixth codon of the beta globin
gene from GAG to GUG. The mutant haemoglobin molecule undergoes
polymerisation under low oxygen tension causing the change in the shape
of  the  RBC  from  biconcave  disc  to  elongated  sickle  like  structure
(Figure 5.15).

Phenylketonuria : This inborn error of metabolism is also inherited as
the autosomal recessive trait. The affected individual lacks an enzyme
that converts the amino acid phenylalanine into tyrosine. As a result of
this phenylalanine is accumulated and converted into phenylpyruvic acid
and other derivatives. Accumulation of these in brain results in mental
retardation. These are also excreted through urine because of its poor
absorption by kidney.

5.6.3 Chromosomal disorders
The chromosomal disorders on the other hand are caused due to absence
or excess or abnormal arrangement of one or more chromosomes.

Failure of segregation of chromatids during cell division cycle results
in the gain or loss of a chromosome(s), called aneuploidy. For example,
Down’s syndrome results in the gain of extra copy of chromosome 21.
Similarly, Turner’s syndrome results due to loss of an X chromosome in
human females. Failure of cytokinesis after telophase stage of cell division
results in an increase in a whole set of chromosomes in an organism and,

Flat back of head
Many “loops” on
finger  tips

Palm  crease

Broad  flat  face

Big  and  wrinkled
tongue

Congenital  heart
disease

90

Figure  5.16 A  representative  figure  showing  an  individual  inflicted  with  Down’s

syndrome and the corresponding chromosomes of the individual

PRINCIPLES  OF  INHERITANCE  AND  VARIATION

this phenomenon is known as polyploidy. This
condition is often seen in plants.

The total number of chromosome of a normal
human being is 46 (23 pairs). Out of these 22
pairs  are  autosomes  and  one  pair  of
chromosomes  are  sex  chromosome.
Sometimes,  though  rarely,  either  an
additional  copy  of  a  chromosome  may  be
included in an individual or an individual may
lack  one  of  any  one  pair  of  chromosomes.
These  situations  are  known  as  trisomy  or
monosomy  of  a  chromosome,  respectively.
Such  a  situation  leads  to  very  serious
consequences  in  the  individual.  Down’s
syndrome,  Turner’s  syndrome,  Klinefelter’s
syndrome  are  common  examples  of
chromosomal disorders.

Down’s Syndrome : The cause of this genetic
disorder is the presence of an additional copy
of the chromosome number 21 (trisomy of 21).
This disorder was first described by Langdon
Down (1866). The affected individual is short
statured  with  small  round  head,  furrowed
tongue and partially open mouth (Figure 5.16).
Palm is broad with characteristic palm crease.
Physical, psychomotor and mental development
is retarded.

Klinefelter’s  Syndrome  :  This  genetic
disorder is also caused due to the presence of
an additional copy of X-chromosome resulting
into  a  karyotype  of  47,  XXY.  Such  an
individual has overall masculine development
,  however,  the  feminine  development
(development of breast, i.e., Gynaecomastia)
is  also  expressed  (Figure  5.17  a).  Such
individuals are sterile.

Turner’s  Syndrome  :  Such  a  disorder  is
caused  due  to  the  absence  of  one  of  the  X
chromosomes, i.e., 45 with  X0, Such females
are sterile as ovaries are rudimentary besides
other features including lack of other secondary
sexual characters (Figure 5.17 b).

(b)

(a)

Tall  stature

with  feminised

character

Short  stature  and
underdeveloped

feminine  character

Figure  5.17  Diagrammatic  represe-
ntation of genetic disorders due to sex
chromosome composition in humans :
(a)  Klinefelter  Syndrome;  (b)  Turner’s
Syndrome

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BIOLOGY

SUMMARY

Genetics is a branch of biology which deals with principles of inheritance
and  its  practices.  Progeny  resembling  the  parents  in  morphological  and
physiologycal  features  has  attracted  the  attention  of  many  biologists.
Mendel  was  the  first  to  study  this  phenomenon  systematically.  While
studying  the  pattern  of  inheritance  in  pea  plants  of  contrasting
characters,  Mendel  proposed  the  principles  of  inheritance,  which  are
today  referred  to  as  ‘Mendel’s  Laws  of  Inheritance’.  He  proposed  that
the ‘factors’ (later named as genes) regulating the characters are found
in  pairs  known  as  alleles.  He  observed  that  the  expression  of  the
characters  in  the  offspring  follow  a  definite  pattern  in  different–first
generations (F1), second (F2) and so on. Some characters are dominant
over  others.  The  dominant  characters  are  expressed  when  factors  are
in heterozygous condition (Law of Dominance). The recessive characters
are  only  expressed  in  homozygous  conditions.  The  characters  never
blend  in  heterozygous  condition.  A  recessive  character  that  was  not
expressed  in  heterozygous  conditon  may  expressed  again  when  it
becomes  homozygous.  Hence,  characters  segregate  while  formation  of
gametes  (Law  of  Segregation).

Not  all  characters  show  true  dominance.  Some  characters  show
incomplete,  and  some  show  co-dominance.  When  Mendel  studied  the
inheritance  of  two  characters  together,  it  was  found  that  the  factors
independently  assort  and  combine  in  all  permutations  and
combinations (Law of Independent Assortment). Different combinations
of gametes are theoretically represented in a square tabular form known
as ‘Punnett Square’.  The factors (now known as gene) on chromosomes
regulating  the  characters  are  called  the  genotype  and  the  physical
expression  of  the  chraracters  is  called  phenotype.

After  knowing  that  the  genes  are  located  on  the  chromosomes,  a
good  correlation  was  drawn  between  Mendal’s  laws  :  segregation  and
assortment  of  chromosomes  during  meiosis.  The  Mendel’s  laws  were
extended  in  the  form  of  ‘Chromosomal  Theory  of  Inheritance’.  Later,  it
was found that Mendel’s law of independent assortment does not hold
true for the genes that were located on the same chromosomes. These
genes  were  called  as  ‘linked  genes’.  Closely  located  genes  assorted
together,  and  distantly  located  genes,  due  to  recombination,  assorted
independently.  Linkage  maps,  therefore,  corresponded  to  arrangement
of genes on a chromosome.

Many  genes  were  linked  to  sexes  also,  and  called  as  sex-linked
genes.  The  two  sexes  (male  and  female)  were  found  to  have  a  set  of
chromosomes  which  were  common,  and  another  set  which  was
different.  The  chromosomes  which  were  different  in  two  sexes  were
named  as  sex  chromosomes.  The  remaining  set  was  named  as
autosomes.  In  humans,  a  normal  female  has  22  pairs  of  autosomes

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PRINCIPLES  OF  INHERITANCE  AND  VARIATION

and a pair of sex chromosomes (XX). A male has 22 pairs of autosomes
and a pair of sex chromosome as XY. In chicken, sex chromosomes in
male are ZZ, and in females are ZW.

Mutation  is  defined  as  change  in  the  genetic  material.  A  point
mutation is a change of a single base pair in DNA. Sickle-cell anemia is
caused  due  to  change  of  one  base  in  the  gene  coding  for  beta-chain  of
hemoglobin.  Inheritable  mutations  can  be  studied  by  generating  a
pedigree  of  a  family.  Some  mutations  involve  changes  in  whole  set  of
chromosomes (polyploidy) or change in a subset of chromosome number
(aneuploidy).  This  helped  in  understanding  the  mutational  basis  of
genetic disorders. Down’s syndrome is due to trisomy of chromosome 21,
where  there  is  an  extra  copy  of  chromosome  21  and  consequently  the
total number of chromosome becomes 47. In Turner’s syndrome, one X
chromosome  is  missing  and  the  sex  chromosome  is  as  XO,  and  in
Klinefelter’s syndrome, the condition is XXY. These can be easily studied
by  analysis  of  Karyotypes.

EXERCISES

1. Mention the advantages of selecting pea plant for experiment by Mendel.

2. Differentiate  between  the  following  –

(a)  Dominance  and  Recessive

(b) Homozygous and Hetrozygous

(c)  Monohybrid  and  Dihybrid.

3. A diploid organism is heterozygous for 4 loci, how many types of gametes

can  be  produced?

4. Explain  the  Law  of  Dominance  using  a  monohybrid  cross.

5. Define  and  design  a  test-cross.

6. Using a Punnett Square, workout the distribution of phenotypic features
in the first filial generation after a cross between a homozygous female
and a heterozygous male for a single locus.

7. When  a  cross  in  made  between  tall  plant  with  yellow  seeds    (TtYy)  and
tall  plant  with  green  seed  (Ttyy),  what  proportions  of  phenotype  in  the
offspring could be expected to be

(a)  tall  and  green.

(b)  dwarf  and  green.

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BIOLOGY

8. Two  heterozygous  parents  are  crossed.  If  the  two  loci  are  linked  what
would be the distribution of phenotypic features in F1 generation for a
dibybrid  cross?

9. Briefly  mention  the  contribution  of  T.H.  Morgan  in  genetics.

10. What is pedigree analysis? Suggest how such an analysis, can be useful.

11. How  is  sex  determined  in  human  beings?

12. A child has blood group O. If the father has blood group A and mother
blood group B, work out the genotypes of the parents and the possible
genotypes  of  the  other  offsprings.

13. Explain  the  following  terms  with  example

(a) Co-dominance

(b) Incomplete  dominance

14. What is point mutation? Give one example.

15. Who  had  proposed  the  chromosomal  theory  of  the  inheritance?

16. Mention  any  two  autosomal  genetic  disorders  with  their  symptoms.

94