The genetic factors that cause hemophilia are passed down through the X chromosome. Every male has one X chromosome and one Y chromosome. Every female has two X chromosomes. Males inherit a Y chromosome from their father and an X chromosome from their mother; females inherit an X chromosome from each parent.

The chance that a child will inherit hemophilia depends upon whether he or she inherits an affected gene from one or both parents. Hemophilia affects men almost exclusively. Females may be a carrier of the gene that causes hemophilia but will not usually have any signs or symptoms of the disease. A female who is a carrier usually has one defective gene on the X chromosome and one normal gene on the X chromosome. When she has a normal working chromosome, she won't have symptoms. She has a 50% chance of passing on the defective gene to her children. If a woman who is a carrier is affected by hemophilia, the symptoms are usually very mild. In rare cases, a woman gets a defective gene from both her mother and her father and is born with hemophilia.

Hemophilia can also develop when the gene that is responsible for producing clotting factors changes (mutates). In this case, hemophilia is not inherited; the person in whom the gene changes will be the first person in a family to have hemophilia or will be a carrier. The person may pass this gene down to any children that he or she has.

See an illustration of the hemophilia inheritance pattern.