a) Explain the mechanism of sex-determination in humans.
b) Differentiate between male heterogamety and female heterogamety with the help of an example of each.

Explanation

(a) Human female has 44 autosomes + XX sex chromosomes while male has 44 autosomes + XY sex chromosomes. The males produce two types of sperms. 50% of the sperms carry X-chromosome and the other 50% carry Y chromosome. The females produce only one type of gametes (eggs or ovum). All the eggs bear only one, Le. X type of chromosome. When the sperm bearing X-chromosome fuses with egg (ovum during fertilisation to form zygote, such zygote develops into a female baby having XX sex-chromosomes sperm bearing Y-chromosome fuses with egg to form zygote, this zygote gives rise to a male baby having XY sex-chromosome. Thus, in humans, 50 percent chances of a baby boy and 50 percent chances of a baby girl to be bone. In human. it is the sperm which determines the sex of the child
(b) In humans we see male heterogamety as human males produce two types of sperms X and Y types. Female heterogamety is seen in birds where female produces two types of egg Z and W while males are homogametic which produce only Z type of sperms