Hemophilia A: Classic hemophilia, due to profound deficiency of factor VIII, a blood factor necessary to normal clotting. The hemophilia A gene is on the X chromosome, so females carry the gene. Each son of a female carrier stands a 50% chance of receiving the gene and having hemophilia. Treatment is by blood products that introduce clotting factor and replace lost blood. However, use of contaminated blood products exposed many people with hemophilia to HIV infection in the 1980s. Hemophilia A has affected the Russian royal house and other descendants of Queen Victoria.