The determination of the sex in an animal is the complex system for deciding the sex of organism. it is depends on the chromosomes present in the animals. some animals determine the sex of an animal by external environmental factors.

1. Sex determination in animal
Dr. Manoj K. Bangadkar

2. • Two sexes- Male and Female
• Sex in animal is determined by chromosomes
• Chromosomes:
– Autosomes- they are found in all cells. The genes situated
in the autosomes are responsible for determination of
somatic characters
– Sex chromosomes: they are variously named as ‘X’ and ‘Y’
chromosomes, ‘Z’ and ‘W’ chromosomes, idiosomes,
heterosomes or allosomes. The genes which determine sex
are located on these chromosomes.

3. Theories of sex determination
• According to Hicker (1920), embryologically sex-determination
can be classified into three groups
a) Progamic: When sex is determined before fertilization
b) Syngamic: When sex is determined at the time of
fertilization and
c) Epigamic: sex is determined after the formation of the
zygote i.e. after fertilization
The modern theories of sex determination belong to the
syngamic and epigamic type and are mainly the
chromosomal theory, the genic balance theory, the
hormonal theory and the environmental theory

4. Chromosomal theory of sex determination
• Proposed by Miss Stevens (1905) and is supported by Bridges
(1922) and Goldschmidt (1938)
• According to this theory the chromosome is the main factor to
determine the sex
• Two types of chromosomes in an organism- Autosomes and
Allosomes
• Allosomes are sex chromosomes
• Two types of sex chromosomes ‘X’ and ‘Y’
• X chromosome:
– Larger. Straight, contain large amount of euchromatin and
small amount of heterochromatin
• Y chromosome:
– Smaller, bend at one end contain small amount of euchromatin
and large amount of heterochromatin
• Chromosomal theory is the theory of heterogenesis

6. Theory of heterogenesis
• It was proposed by Correns in 1906
• According to this theory, one sex produced two types of gametes
and such individual is called as heterogametic. The other sex
produces the same type of gametes such individual are called
homogametic

7. XX-XY Method
• Male : Heterogametic and has XY chromosome
• Female : Homogametic and has XX chromosome
• Example : Sex determination in human

8. ZW-ZZ Method
• This type of sex determination is found in animals such as
butterflies, moths, birds and some fishes
• It is just reverse of the XX-XY method
• Male- Homogametic (ZZ), Female- Heterogametic (ZW)
• Z and W letters are used instead of X and Y respectively
• In male- 8 pairs of autosomes + ZZ
• In female- 8 pairs of autosomes + ZW

9. XX-XO Method
• In some nematodes, spiders and insects, the Y chromosome is
absent
• It occurs in most insects of the order Hemoptera and Orthoptera
(Cockroaches and Grasshopper)
• The letter ‘O’ means zero and
indicate absence of
chromosomes in one sex i.e.
Male (XO)
• In 1902 C. E. McClung reported
the absence of one
chromosome from grasshopper
testes, he found 11 pairs of
chromosomes with one odd
chromosome (23 chr) in male in
female germ cells number are
24

10. ZO-ZZ Method
• This method derived from the basic ZW-ZZ method, in which male
is homogametic (ZZ) and female is heterogametic (ZW)
• In ZZ-ZO method W chromosome is lost in female
• Here ‘O’ denotes the absence of sex chromosome
• Therefore female is heterogametic produces two types of egg Z
and O

11. Sex determination in Drosophila- Genic balance theory
• Proposed by Calvin B. Bridges in 1921
• Female sex determining genes are located on X chromosomes
while the male sex determining genes are located on autosomes
• Although Y chromosome is essential for the fertility in male
drosophila, but it is inert and has nothing to do with the sex
determination
• Thus the balance between the female sex determining genes of X
chromosome and male sex determining genes autosomes
determine the sex in drosophila
• If ratio between X chromosome and set of autosome is 2:2 then
female sex
• If ratio between X chromosome and set of autosome is 1:2 then
male sex

12. Environmental sex determination in Bonellia
• External environment plays a key role in sex determination
• In Bonellia the males are ciliated and externally small with respect
to the females which are big in size
• Males lives as a parasite inside the body of female