Hemophilia A is an X-linked recessive disease caused by a lack of a coagulant, or blood clotting agent, called factor VIII (factor 8). This is caused by a mutation in a gene on the X chromosome called F8. If a father is affected, his daughters will be carriers of hemophilia A and his sons will be unaffected. If a mother is an unaffected carrier, each daughter has a 1 in 2 chance (i.e., 50%) of being an unaffected carrier and each son has a 1 in 2 chance (i.e., 50%) of being affected with hemophilia A.
