What is Hemophilia?
Hemophilia is an inherited bleeding disorder, predominantly affecting males. There are two major forms of the disease: hemophilia A (factor VIII deficiency) and hemophilia B (factor IX deficiency). People with no family history can also develop hemophilia A. The National Hemophilia Foundation says this accounts for a third of all cases. It usually happens after a new or spontaneous gene mutation.
Hemophilia may result from gene mutations: point mutations involving a single nucleotide, deletions of all or parts of the gene, and mutations affecting gene regulation. About 50% of cases of severe hemophilia A result from a major inversion of a section of the tip of the long arm of the X chromosome. Because factor VIII and factor IX genes are located on the X chromosome, hemophilia affects males almost exclusively. Daughters of hemophiliacs will be obligatory carriers, but sons will be normal. Each son of a carrier has a 50% chance of being a hemophiliac, and each daughter has a 50% chance of being a carrier. Rarely, random inactivation of one of the two X chromosomes in early embryonic life will result in a carrier's having a low enough factor VIII or IX level to experience abnormal bleeding.
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