Hemophilia occurs when there is a problem with the amount or functioning of proteins in the body that affect the ability of the blood to coagulate (clump together to form blood clots). Because of their function, these proteins are called clotting factors.

Most cases are the result of a genetic defect and are present from birth. Hemophilia A and B are usually passed from mother to son. The gene that produces clotting factors is located on X chromosomes. Females have two X chromosomes, while males have an X chromosome and a Y chromosome. Females inherit an X chromosome from each parent, while males inherit an X chromosome from their mothers and a Y chromosome from their fathers.

A woman is a carrier for hemophilia if she has a defective gene for either of two clotting factors on one of her X chromosomes. This means that she can pass this defective chromosome on to her children, with the odds of such a transmission being about 50 percent. A female child who receives the defective X chromosome from her mother will be a carrier for hemophilia, but will not develop the disease because she has another functioning X chromosome from her father. A male child who receives a defective X chromosome from his mother will have hemophilia.

Males who have hemophilia cannot pass it on to their sons, but can pass on the defective gene that causes these disorders to their daughters, who will then become carriers of the disorder. A female can inherit hemophilia only if her father has hemophilia and her mother is a carrier. This is extremely rare.

This genetic origin of hemophilia makes the risk factors clearly identifiable: having a family history of the condition, especially a mother who is a carrier, and being male.

About 30 percent of hemophilia cases occur where there is no traceable family history of the disease. In these cases, a genetic mutation occurs in the male child or in the mother to make her a carrier, who passes it on to her children. In addition, some patients have acquired hemophilia, a disease that does not have a genetic component that occurs when certain antibodies form and interfere with the functioning of clotting factor VIII or clotting factor IX.

Hemophilia C can occur in both boys and girls and can be passed on by either parent. However, this disease is rare and usually mild.