(a) The mismatched pair of sex chromosome in humans is XY pair of chromosomes.
(b) Human chromosomes have one pair of sex chromosomes. Women have a perfect pair of sex chromosomes, both called X.
(c) Different species use very different strategies for sex determination. In some animals like a few reptiles, the temperature at which fertilised eggs are kept determines whether the animals developing in the eggs will be male or female. In other animals, such as snails, individuals can change sex, during their lifetime indicating that sex is not genetically determined.
OR
In human beings, the females have two X chromosomes and the males have one X and one Y chromosome. Therefore, the females are XX and the males are XY.
The process of fusion of the sperm and ovum is called fertilisation. The sperm has either X or Y chromosome and egg has only X chromosome.
So, if sperm carrying Y chromosome fuses with egg the newly born child will be male and if sperm carrying X chromosome fuses with the egg the newly born child will be female. There is an equal chance of fusion of either X or Y chromosome with the egg so we can say that the sex of new born child is a matter of chance and none of the parent is responsible for it. Thus, sex is determined genetically in humans.
Sex determination in humans is shown below: