The male and female individuals possess different structural attributes in terms of chromosomes to decide the sex of the newborn. Genetic determination of sex is based on the chromosomes deciding the sex of the offspring. In these individuals the chromosomes are of two categories – Autosomes and sex chromosomes.
Autosomes are responsible for all the somatic characters, while the sex chromosomes decide the sex besides carrying genes for some somatic characters also In sexually dimorphic individuals (including man) there is a pair of chromosomes meant to govern the sex, these are called X I and Y or Z and W chromosomes.
Whatever may be their nomenclature their role is same. The chromosomal mechanism of sex determination varies in different individuals, although essentially in all these cases, sex chromosomes decide the sex of the offspring. The following table summarises the variations in chromosomal determination of sex.
Structure of the sex chromosome:
ADVERTISEMENTS:
The X and Y sex chromosomes exhibit certain structural differences as revealed by cytological studies. In most of the organisms the X chromosome is straight, rod like and larger than the Y chromosome. The Y chromosome is smaller in size with one end slightly hooked or curved as in Drosophila.
In man however no such curve is found in the Y chromosome. X chromosomes have a large euchromatic area and hence carry many sex linked somatic characters. The Y chromosome has less of euchromatic region and carries but few sex linked somatic characters. In dioeciously organisms chromosomal sex determination is of basically two categories – Heterogametic males and heterogametic females.
Heterogametic males: The male individual produces two types of sperms. 50% of the sperms carry one type of sex chromosome (X) the rest carries another type (Y) of sex chromosome. All the females carry only one type of sex chromosome (X). Hence the females are said to be homogametic. The sex of the new born depends upon the type of sperm fertilized the egg.
The female gamete has no role in deciding the sex in the sense it always carries only one type of a chromosome. Sex determination in heterogametic males belongs to any one of the following types.
ADVERTISEMENTS:
1. XX – XY Type:
In man, Drosophila etc., where male is the heterogametic sex, the male and female have the following types of sex chromosome (this type was first studied in milkweed bug Lygaeus turcicus, hence it is also called as the Lygeus type.
In the XX – XY type, the male produces two type of sperms. 50% sperms would carry X chromosome and the rest carry Y chromosome. All the eggs are of only type in that all of them carry a single X chromosome. The sex of the new born depends on the type of sperm (carrying X or Y) fertilizing the eggs.