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The genes that control the production of clotting factors VIII and IX are located on the X chromosome. Females have two X chromosomes, while males have one X and one Y chromosome. A female carrier of hemophilia has one abnormal (or mutated) gene on one of her X chromosomes and one normal gene on the other X chromosome. When a hemophilia carrier is pregnant, there is a 50/50 chance that the gene will be passed on. If the gene is passed on to a son, he will have the disease because he only has one X chromosome. If the gene is passed on to a daughter, she will be a carrier. If the father has hemophilia, but the mother does not carry the gene, then none of the sons will have the disease, because they get their one normal X from their mother, but all of the daughters will be carriers because they receive one normal X chromosome from their mother and one abnormal X chromosome from the father. About a third of children with hemophilia are in families with no history of the disorder. These cases are due to a "new" gene mutation. Tests are available to possible carriers to determine if they carry the gene.
Carriers of the hemophilia gene usually have normal levels of clotting factors and few significant symptoms. Sometimes carriers do have increased bleeding and may be prone to bruise easily, bleed more with surgeries and dental work, and have frequent nosebleeds or excessive menstrual bleeding.
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