The chromosomes which determine sex in the different organisms are called sex chromosomes. ‘Fhe term ^–sex^– refers to the processes that enable species to exchange materials between homologous chromosomes. Ii causes recombination. Recombination is essential to their mechanism for reproduction.

Work on inheritance of sex started after discovery of Menders ork in 1900. The discovery of sex chromosomes revealed the genetic basis of sex determination.

SEX DETERMINATION

Discovery of Sex chromosomes in drosophila

The fruit fly. Drosophila inelanogaster has eight chromosomes. These chromosomes are present in the form of four homologous pairs. T.H. Morgan in 1911 found difference in the chromosomes of male and female Drosophila. The chromosomes of the three homologous pairs were similar in both of the sexes. But the fourth was heteromorphic pair and it had different structures. The female has two similar rod shaped X-chromosomes in the fourth pair. But male has one rod shaped X-chromosome but the other a morphologically different. J-shaped Y chromosome in the ‘Mirth pair.

Sex chromosomes: X and V chromosomes are called sex-chromosomes. These chromosomes have genes for determination of sex.

Autosomes: Chromosomes of the other three pairs are autosomes. All chromosomes other than sex-chromosomes are called autosomes. Autosomes do not carry any sex determining gene.

Sex chromosomes in humans

I Ionians have 46 chromosomes. These are present in 23 pairs. 22 pairs are of autosomes and one pair is of sex-chromosomes. Autosome pairs are common in both the -sexes. But the 23 rd sex chromosome pair is different in males and females. A woman has two similar X chromosomes in her 23′ pair. But a man has an X chromosome and a much shorter Y chromosome. The 23rd pair in man is heteromorphic.

(a)      The female in human is XX

(b)      The male is XY

Sex chromosomes in grasshopper

Males and females have different number of chromosomes in some grasshoppers.

(a)         The female has 24 chromosomes. These chromosomes are present in the form of I I pairs of autosomes and a pair of X chromosomes. Female is XX.

(b)         The male grasshopper has 23 chromosomes. The male grasshopper has I I pairs of autosomes and only one X chromosome. The other sex^–chromosome is entirely missing in male. Thus male is X0.