The probability of a baby being a boy or a girl depends on the contribution of the

The probability of a baby being a boy or a girl depends on the contribution of the father's and mother's sex cells. The father contributes a sperm cell, which can be either X (female) or Y (male) chromosome. The mother contributes an egg cell, which is always X chromosome. When the sperm fertilizes the egg, the combination of X and Y chromosomes determine the gender of the baby. If the sperm carries an X chromosome, the baby will be female (XX). If the sperm carries a Y chromosome, the baby will be male (XY).