There were two special chromosomes in you when you were just beginning life as a fertilized egg. These are called sex chromosomes. Scientist have named them the X chromosome and Y chromosome.
A woman’s egg cell contains only an X chromosome, while a man’s sperm cell Y contains either an X or a Y. If the woman’s egg cell is fertilized by a sperm cell with an X chromosome, the fertilized egg will have two X’s and the baby will be a girl. But if the sperm cell has a Y chromosome, creating both an X and a Y in the fertilized egg, then the egg will develop into a boy.