In humans, the sex of the baby is determined by the sperm. The sperm gamete is heterogamatic because approximately half of them contain the X chromosome which will result in a girl and approximately half of them contain the Y chromosome which will result in a boy.
All of the gametes in the mother's eggs possess X chromosomes. The father's sperm contains about half X and half Y chromosomes. The sperm is the variable factor in determining the gender of the baby. If the sperm carries an X chromosome, it will combine with the egg's X chromosome to form a female zygote.
