Hemophilia A (factor VIII deficiency) affects 1 in 5,000 males. Hemophilia B (factor IX deficiency) affects 1 in 25,000 males. Severe hemophilia in women is extremely rare. However, women are carriers of hemophilia. Many women who are carriers have bleeding problems. However, compared to von Willebrand disease, the total number is small.
The genes responsible for producing factors VIII and IX, the mutant genes in hemophilia, are situated on the X chromosome. This makes hemophilia a sex-linked genetic disorder.
Carriers have one normal X chromosome and one abnormal X chromosome. The normal X chromosome produces a certain amount of factor VIII or IX clotting factor. This protects carriers from the most severe form of hemophilia in which the level of clotting factor is less than 1%.
However, the variation in clotting factor levels among carriers is very wide. In some carriers the level is much below normal. In other carriers, the level is closer to normal. This is because the two X chromosomes, one of which carries the hemophilia gene, are not equally functional. If the hemophilia X chromosome happens to be functional in most cells, then the carrier will have a very low level of clotting factor activity.
Many carriers have a clotting level between 30% and 70% of normal and do not usually suffer from severe bleeding, although they may suffer from the most common symptom – heavy menstrual bleeding. However, some carriers have less than 30% of the normal level of factor VIII or IX. These women are considered to be mild hemophiliacs.
It is not known how many carriers of hemophilia A and B have bleeding problems. Estimates vary up to 60%.
For more information about carriers of hemophilia A and B see the following:
A comprehensive guide for carriers of hemophilia A or B.
Developed and published by the Canadian Hemophilia Society.
An interactive animated video that provides potential pre-teen and teenage carriers with relevant and age appropriate information about what it means to be a carrier of hemophilia A or B. It includes facts about hemophilia, inheritance of the hemophilia gene, carrier testing and the management of bleeding symptoms.
The resource also includes interactive buttons for viewers to test their knowledge and access tips, myths and fun facts of interest to young carriers.