When you're cut or bleeding internally, your body normally pools blood cells together to form a clot to stop the bleeding. This process is called coagulation. Coagulation involves blood particles called platelets and plasma proteins that encourage clotting (clotting factors). The cause of hemophilia is a deficiency of one of these clotting factors. Which type of hemophilia you have depends on which clotting factor is deficient:

  • Hemophilia A. The most common type, hemophilia A is caused by lack of enough clotting factor 8 (VIII).
  • Hemophilia B. This second most common type is caused by lack of enough clotting factor 9 (IX).
  • Hemophilia C. This type is caused by a lack of clotting factor 11 (XI), and symptoms are often mild with this type of hemophilia.

Hemophilia inheritance

Everyone has two sex chromosomes, one from each parent. Females inherit an X chromosome from their mother and an X chromosome from their father. Males inherit an X chromosome from their mother and a Y chromosome from their father.

  • The gene that causes hemophilia A or B is located on the X chromosome, so it can't be passed from father to son. Hemophilia A or B almost always occurs in boys and is passed from mother to son through one of the mother's genes. Most women who have the defective gene are simply carriers and exhibit no signs or symptoms of hemophilia. It's also possible for hemophilia A or B to occur through spontaneous gene mutation.
  • The gene that causes hemophilia C can be passed on to children by either parent. Hemophilia C can occur in both boys and girls.
Aug. 31, 2011

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