Sex determination in animals

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 Progamic : When sex is determined before fertilization Syngamic : When sex is determined at the time of fertilization and 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

5. 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

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

7. 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 M ale- Homogametic (ZZ), Female- H eterogametic (ZW) Z and W letters are used instead of X and Y respectively In male- 8 pairs of autosome s + ZZ In female- 8 pairs of autosomes + ZW

8. 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

9. 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

10. 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

11. 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