Hemophilia is a hereditary bleeding disorder caused by a deficiency in one of two blood clotting factors: factor VIII or factor IX.
Several different gene abnormalities can cause the disorder.
People can bleed unexpectedly or after minor injuries.
Blood tests are needed for diagnosis.
Transfusions are given to replace missing clotting factors.
Clotting factors are proteins in the blood that interact to help the blood clot, stopping bleeding. There are many different clotting factors (see also How Blood Clots).
There are two forms of hemophilia.
Hemophilia A, which accounts for about 80% of all cases, is a deficiency in clotting factor VIII.
Hemophilia B is a deficiency in clotting factor IX.
The bleeding patterns and consequences of these two types of hemophilia are similar.
Hemophilia is caused by several different gene abnormalities. They are sex-linked, which means that the gene abnormalities are inherited through the mother and that nearly everyone with hemophilia is male (see also X-Linked Inheritance).