This document summarizes the chromosome theory of sex determination. It discusses the discovery of sex chromosomes and their different structures in males and females. There are two main types of sex determination mechanisms: XX-XY systems where females are XX and males are XY, and ZW-ZZ systems where females are ZW and males are ZZ. In both systems, the sex is determined by which sex chromosomes are present in the fertilized egg - an XX, XY, ZW, or ZZ combination will develop into a female or male. The document provides examples of different species that use each type of sex determination system.

2. A presentation
on
chromosome theory of sex
determination.
Submitted to:
Mrs. Levenya
Department of life science
Genetics

3. SEX
DETERMINATION
CONTENTS
 INTRODUCTION
1 discovery of sex chromosomes
2 types of sex chromosomes
3 structure of sex chromosomes
 Chromosomal theory of sex
determination.
(a) Heterogametic males ( XX-XY, XX-XO)
(b) Heterogamatic females (ZO-ZZ, ZW- ZZ)

4. What is sex determination?
 The method by which distinction between male and female is
established in an organism under genetic control.
 The chromosomes are responsible for determination of separate
sexes.
 the phenomenon of molecular, morphological, physiological,
behavioral differentiation between male and female sexes is called
sex determination/sexual dimorphism.
 Traditionally, the symbol ♂ designate male
 And the symbol ♀ designates females.
 Sexes is a latin word ‘sexus’ which in english means section or
separation
INTRODUCTION

5.  An animal possessing both male and female reproductive
organs is usually referred to as hermaphrodites.
 in plants where staminate (male) and pistilate (female)
flowers occur on the same plant, the term of preference
is monoecious.
 The organisms in which both male and female gametes
are produced by different individuals are called as
diecious.
 The sex cells and reproductive organs form the primary
sexual characters of male and female sexes.
 Besides the primary sexual characters, the male and
female sexes differ from each other in many somatic
characters known as secondary sexual characters.

6. Mechanism of sex
determination
Most of the mechanisms of the determination of
the sex are under genetic control and they may
be classified into following categories :
1. Sex chromosome mechanism or
heterogamesis;
2. genic balance mechanism;
3. male haploidy or haplodiploidy mechanism;
4. Single gene effects.

7. Sex chromosomal mechanism
(heterogamesis)
01. Discovery of sex chromosomes.
Clarance McClung
(1870-1946)
 A German biologist, Henking in 1891 while Studying
spermatogenesis of the squash bug,Pyrrhocoris, noted
that meiotic nuclie contained 11 pairs of chromosomes
and an unpaired element is moved to one of the poles
during the first meiotic division. Henking called this
unpaired element a “X body” and interpreted
it as a nucleolus.
 The significance of X body was immediately
understood, but in 1902 an American geneticist
Clarance McClung who had made extensive
observations of gametogenesis in grasshoppers,
suggested that the X body was involved in some
way with the Determination of sex

8. 02. Types of sex chromosomes
In diecious organisms, thus, two types of
chromosomes were recognised which are as
follows:
(ⅰ) Autosomes : The chromosomes which have no
relation with the sex and contain the genes which
determine the somatic characters of the individuals are
known as autosomes .
(ⅱ) Sex chromosomes : The chromosomes which are
responsible for the determination of sex are known as
sex chromosomes, e.g., X and Y chromosomes.

9. DIFFERENCE BETWEEN AUTOSOMES AND
ALLOSOMES
AUTOSOMES ALLOSOMES OR SEX
CHROMOSOMES
Refer to other than sex
chromosomes/ chromosomes that
do not determine the sex of
individuals
These are sex chromosomes./
chromosomes that determine the
sex of individuals
Chromosome no. = 2n-1 Chromosome no. = 2
Morphology is similar in male and
female.
Morphology is different in male and
female.
Number differs from species to
species.
Each diploid organism usually has
two allosomes.
Do not exhibit sex linkage. Exhibit sex linkage.

10. 03. Structure of sex
chromosomes:
The X and Y chromosomes exhibit structural differences.
the cytological studies have shown that the X chromosomes
of most organisms are straight, rod-like and comparatively large
than Y chromosomes.
 The Y chromosomes is smaller in size with one end slightly
Curved or bent to one side in drosophila; in man and
Melandrium no such curvature of Y chromosomes occurs.
 The X chromosomes have large amount of euchromatin and
Small amount of heterochromatin.
 The Y chromosome contains small amount of euchromatin
and large amount of heterochromatin.

11. Types of sex chromosomal
mechanism of sex
determination
In diecious diploidic organisms following two
system of sex chromosomal determination of sex
have been recognized:

12. Sex determination in various
organisms.
ORGANISMS GENOTYPE OF
FEMALE
GENOTYPE OF
MALE
01. Insects. XX XO
02. Drosophila,
humans and other
mammals.
XX XY
03. Moths,
butterflies and
insects.
ZO ZZ
04. Birds fishes
and butterflies.
ZW ZZ

13. The heterogametic males:
(ⅰ) XX-XO type: In certain plants (e.g., vallisneria spiralis,
Dioscorea Sinuate, etc.) and in insects specially those of the
orders hemiptera (true bugs) and orthoptera (grasshoppers and
roaches)the female having two X chromosomes (XX) and are,
thus, homogametic, while the male having only one X
chromosome(XO).
 Female is homogametic and produces all the eggs with X
Chromosome.
 The male is heterogametic, which produces half of which hav
X chromosome and the other half have none.
In this type of sex chromosomal determination of sex, the fema
sex has two X chromosomes, while the male sex has only one
X chromosome.

14. XX-XO type of determination of sex

15.  The sex of the offspring depends upon the sperm that
fertilizes the egg (each of which carries single
X chromosome)
 Union of egg with sperm having X chromosome will give
rise to female sex and union of egg with sperm having none
results in development of make sex.
Haploid male and diploid female type

16. (ⅱ) XX-XY type: Seen in man, other mammals, certain insects
including drosophila and lygaeus turicicus and in certain
angiospermic plant such as Melandrium album, humulus
lupulus, coccinia indica.
 In this system, female has two X chromosomes, is
homogametic and produces only one type of gamete, i.e., X.
 The male has one X and one Y chromosome, is
Heterogametic and produces two types of gametes, viz.,
X and Y.
 Union of X ovum with X sperm leads to development of
Female (XX) sex. If X ovum units with Y sperm, it
produces male (XY) sex.

17. XX-XY type of sex determination

18. Male and female drosophila melanogaster

19. The Heterogametic females:
(ⅰ) ZO-ZZ type:
( in moths and butterflies,very low species of insects like fumea
In this system, female has only one Z chromosome and hence
Is heterogametic.
As a result of meiosis, 50% eggs of such female carry an Z
Chromosome and remaining 50% have none.
On the other hand, male has two Z chromosomes and produces
All the sperms with one Z chromosome. Thus, male sex is
Homogametic.
The union of Z sperm with ovum having Z chromosome gives
Rise to male sex and union of Z sperm with ovum having none
Leads to development of female.

20. ZO-ZZ type of sex determination

21. (ⅱ) ZW-ZZ type:
(occurs in insects (gyspy moth), vertebrates such as fishes,
reptiles and birds and plants such as fragaris elatior)
Female is heterogametic and produces two types of gametes
Z and W types.
Male is homogametic and produces all the sperms of same type
Carrying one Z chromosome.
Union of Z sperm with ovum having Z chromosome gives rise
To male and union of Z sperm with ovum carrying W
Chromosome leads to the development of female sex.

22. ZW-ZZ type of sex determination

23. CONCLUSION:
After the gametes fuse with each other
during fertilization, what factors
determine whether the resulting zygote
developed into male or female?
Researchers have studied the process of
sex determination in a wide range of
species and discovered several mechanism
of sex determination. They conclude that
the difference in chromosome constitution
is a mechanism for determining sex at the
time of fertilization.

24. Submitted by:
Jennifer joy.I
B.Sc BGB
Ⅱsem