Jain Biology: Sex Determination

In many animals a heteromorphic pair of chromosomes has been identified with sex-determination. In Drosophila for e.g. the female has 2 X chromosomes both alike whereas the male has 1 X chromosome and another which is morphologically very distinct, called a Y chromosomes. In addition there are 3 other pairs of chromosomes whose homologues are alike. Man has 23 pairs of chromosomes, 22 of which are similar, whose homologues are alike and one heteromorphic pair associated with sex-determination. In some animals, particularly moths and birds, the chromosomes in the male are all paired perfectly with no heteromorphic pair such as the X and Y chromosomes. In these cases the female has the heteromorphic pair of chromosomes. In poultry, the female has only one X chromosomes with no mate. Therefore, the female is known as one having an XO condition for the sex-chromosomes, instead of XY. She produces two kinds of garnets one with an X and the other with none. The first garnet with an X chromosomes produces a male when united with an X from the father, while the second produces a bird with XO condition, a female. Consequently 1:1 sex ratio is maintained. 'Balance' theory of sex determination in Drosophila was proposed by Calvin Bridges (1925). His work began with an accidental triploid (3n) Drosophila (with 3X chromosomes and 3 sets of autosomes). He crossed the triploid (3n) female with a normal male and secured a number of interesting individuals which differed in the number of X chromosomes and the number of sets of autosomes. The results of Bridges experiment are shown in the Table below. He concluded that the presence or absence of Y sex chromosomes is not the determining factor in sexual determination in Drosophila, but rather it is the balance of the number of X chromosomes and the number of sects of autosomes present.

Balance of X Chromosomes and Autosomes in Drosophila Chromosomal Constitution.