What causes hemophilia?

As mentioned above, hemophilia is caused by a genetic mutation. The mutations involve genes that code for proteins that are essential in the blood clotting process. The bleeding symptoms arise because blood clotting is impaired.
The process of blood clotting involves a series of complex mechanisms, usually involving 13 different proteins classically termed factors I through XIII and written with Roman numerals. If the lining of the blood vessels becomes damaged, platelets are recruited to the injured area to form an initial plug. These activated platelets release chemicals that start the clotting cascade, activating a series of 13 proteins known as clotting factors. Ultimately, fibrin is formed, the protein that crosslinks with itself to form a mesh that makes up the final blood clot. The protein involved with hemophilia A is factor VIII (factor 8) and with hemophilia B is factor IX (factor 9).

Picture of the blood clotting process

Hemophilia A is caused by a mutation in the gene for factor VIII, so there is deficiency of this clotting factor. Hemophilia B (also called Christmas disease) results from a deficiency of factor IX due to a mutation in the corresponding gene.
A condition referred to as hemophilia C involves a deficiency of clotting factor XI. This condition is much rarer than hemophilia A and B and typically leads to mild symptoms. It is also not inherited in an X-linked manner and affects persons of both sexes.
Hemophilia A is more common than hemophilia B. About 80% of people with hemophilia have hemophilia A. Hemophilia B occurs in about 1 out of every 20,000 to 30,000 people. A subgroup of those with hemophilia B has the so-called Leyden phenotype, which is characterized by a severe hemophilia in childhood that improves at puberty.