How is hemophilia inherited?
The hemophilia gene is passed down from a parent to a child. When the father has hemophilia but the mother does not, none of the sons will have hemophilia. All of the daughters will carry the hemophilia gene. The genes for hemophilia A and B are on the X chromosome. The chromosomes that determine a person’s sex are called X and Y. Men have an X and a Y chromosome and women have two X chromosomes. Since men have only one X chromosome, if they inherit the hemophilia gene, they will have hemophilia. Because women have two X chromosomes, if one X chromosome has the hemophilia gene, the other X chromosome makes up for it. Women who have the hemophilia gene are called carriers, and they can pass it on to their children. When the mother is a carrier and the father is normal, for each child there is a 50% chance that a son will have hemophilia and a 50% chance that a daughter will carry the gene.
