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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 gamets one with an X and the other with none. The first gamet 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. Sex .
No. of Sets of
Sex index X Chrosomes Autosomes Ratio X/A Super female XXX
AA
1.5 Normal female XXXX
ΑΑΑΑ
1.00 Normal female XXX
ААА
1.00 Normal female XX AA
1.00 Normal female
1.00 Intersex XX AAA
0.67 Normal Male
AA
0.50 Super Male
AAA
0.33
Х
Α.
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