Both men and women have sex chromosomes. Men usually have one X and one Y chromosome, while women have two X's.
When an egg or sperm is made, it only gets one of the sex chromosomes from the parent. This means that women can only make eggs with an X chromosome. But men can make either X or Y sperm.
During fertilization, the sperm cells race toward the mother-to-be's egg cell. If a sperm with a Y beats all others, then the fetus will be XY. The pregnancy will result in a boy.
However, if a sperm with an X wins the race to the egg, then the fetus will be XX. The parents will have a baby girl.
Nearly everyone's chances are around 50% for having a boy and 50% for having a girl. And yet, we all know families that are all boys or all girls.
