Hemophilia A and B are caused by an inherited defect in a pair of chromosomes. Hemophilia is a sex-linked genetic disease. It is also called an X-linked disease because the defect is on the X chromosome. Fathers pass the defective gene on to their daughters, but not to their sons, and mothers may be carriers. See a picture of the hemophilia inheritance pattern .

Hemophilia almost always occurs in boys. Males get the disease by inheriting the defective gene from their mother. It is very rare for girls to have hemophilia because they must inherit a defective gene from each parent.

The genetic defect affects how much clotting factor a person will produce and how the factor will function. The less normal clotting factor you have, the more severe the hemophilia.

Although hemophilia is a genetic disorder, about one-third of all people with hemophilia have no family history of the condition. In these cases, hemophilia occurs spontaneously when a normal chromosome develops an abnormality (mutation) that affects the gene that determines the production of clotting factor. A child who inherits this mutation may be born with hemophilia or may be a carrier. Only females can be carriers.