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 gametes, as we know, receive half of the chromosomes. The male gametes have 22 autosomes and either X or Y sex chromosome. Type of male gametes: 22+X OR 22+ Y. However, since the females have XX sex chromosomes, their gametes can only have X sex chromosome. Type of female gamete: 22+X
Thus, the mother provides only X chromosomes. The sex of the baby is determined by the
type of male gamete (X or Y) that fuses with the X chromosome of the female.