Hemophilia is a genetic disease: mutations on the X chromosome result in low levels of blood clotting factors VIII or IX. The mutations that cause low levels of clotting factor are carried recessively on the X chromosome. As women have two X chromosomes (XX), the mutated gene would have to be present on both chromosomes to cause the disease, and this is exceedingly rare. Since men have only one X chromosome (XY), one copy of the mutated hemophilia gene is enough to cause the disease, so males who inherit the gene will
be affected.

The hemophilia gene runs in families; it is passed on from parents to their children. While, women possessing the hemophilia gene usually do not have blood clotting factor difficulties, they do "carry" the gene, which can then be passed on to the next generation. A son can inherit the genetic mutation only from his mother and will invariably be affected by the disease. A person's chances of developing hemophilia can be determined by consulting the following chart:

Although a family history of blood clotting factor difficulties is important, it should also be noted that no family history can be traced in one third of hemophilia A cases. In these cases, a spontaneous genetic mutation is assumed to be the cause.

People with a family history of hemophilia may wish to undergo genetic counseling before having children. Genetic counseling can help couples determine the risk of a child being born with hemophilia or of carrying the gene. Genetic counseling may include genetic testing and prenatal screening. If genetic testing reveals a high probability of passing hemophilia to children, prenatal counseling is available to weigh all the risks.

 

Mother	Father	Child's Chances of Hemophilia
Carrier (possesses hemophilia gene)	Normal clotting factor genes	Fifty percent chance son will have hemophilia. Fifty percent chance daughter will be a "carrier."
Normal clotting factor genes	Hemophilia	Son possesses "normal" clotting factor gene.   Daughter will be a "carrier."
Carrier	Hemophilia	Fifty percent chance son will have hemophilia.   Daughter may develop hemophilia (rare occurrence).

Hemophilia A and hemophilia B present with the same symptoms, even though the two disorders affect different blood clotting factors. Genetic testing is the only way to differentiate between the two disorders.

Although Hemophilia A and B are common, genetic testing may reveal other varieties of the disease. A blood clotting disorder known as von Willebrand's diseaseis more commonly diagnosed, and easier to treat, than hemophilia.

Genetic testing may also reveal other, more rare coagulation disorders, such as those affecting the following blood clotting factors: