Mechanism of sex determination in birds:
ZW-ZZ type of sex determination is observed in birds. They exhibit female heterogamety, which means that the female individual produces two different types of gametes, while the male individual produces two similar gametes. Thus, the egg determines the sex of the offspring. Both the male and female individual possesses two sex chromosomes, but the female contains heteromorphic sex chromosomes. The male is produced ZZ gametes, and the female produces ZW gametes.
Mechanism of sex determination in humans:
XY-XX type of sex determination is observed in humans. Both male and female have the same number of chromosomes, 44 autosomes and 2 sex chromosomes. Y-chromosome is shorter than the X-chromosome. Humans exhibit male heterogamety, which means that the male individual produces two different types of gametes (XY), while the female individual produces two similar gametes (XX). Thus, the sperm determines the sex of the child. There is always a 50% probability of having either a male or a female child because, during spermatogenesis, 50% of the total sperms produced contain X-chromosome and the other 50% have Y-chromosome. If the sperm is having X-chromosome fuses with the ovum, the zygote develops into a female child (XX). And if the sperm is having Y-chromosome fuses with the ovum, the zygote develops into a male child.
NOTE: The mechanism of sex determination shown by birds and humans differ from each other because birds show female heterogamety and humans show male heterogamety.
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