The haemophilia gene is passed down from a parent to a child.

When the father has haemophilia but the mother does not, none of the sons will have haemophilia. All of the daughters will carry the haemophilia gene.

The genes for haemophilia 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 haemophilia gene, they will have haemophilia. Because women have two X chromosomes, if one X chromosome has the haemophilia gene, the other X chromosome makes up for it.

Women who have the haemophilia 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 haemophilia and a 50% chance that a daughter will carry the gene.