Each human cell contains 23 pairs of chromosomes out of which 22 pairs of chromosomes are autosomes.
1 pair of chromosomes are called allosomes or sex chromosomes.
Gametes produced by women will be with only ‘X’ chromosomes.
Gametes produced by man will be of two types one with X chromosomes and the other with Y chromosomes.
If the sperm carries the X chromosome and fertilizes the ovum with the X chromosome, the resultant baby will have XX condition. So the baby will be a girl.
If the sperm carries the Y chromosome and fertilizes the ovum with the X chromosome, the resultant baby will have XY condition. So the baby will be a boy.