During fertilization, one sperm and one egg fuse to form a diploid embryo.  The fertilized egg contains one of each of the 22 autosomal chromosomes (autosomes) from each parent, one X chromosome from the mother and an X or a Y chromosome from the father.  Each sperm cell has either an X or a Y chromosome.  All eggs have a single X chromosome.

Individuals who receive an X chromosome from their father become girls. Individuals who receive a Y chromosome become boys.  Boys always receive their X chromosome from their mother.