Background on Carriers
Generally, carriers are individuals who have or “carry” a gene for a condition but do not have the condition themselves. In hemophilia, women are carriers because the gene causing the condition is on the X chromosome. If a woman has a gene for hemophilia, she usually is not affected because women have two X chromosomes; therefore, they have a gene on their second X that compensates for the hemophilia gene. Men, however, only have one X chromosome, so if they have the gene causing hemophilia they will be affected. Carriers can pass the hemophilia gene they carry onto their children, resulting in sons with hemophilia and daughters who are also carriers.
Men who have hemophilia will pass their X chromosome to all of their daughters; therefore, they will all be carriers (also known as obligate carriers). Sons receive a Y chromosome from their fathers, rather than an X, and therefore will not be affected or carry the gene. A woman is an obligate carrier of hemophilia if any of the following apply:
- (1) she is the biological daughter of a man who has hemophilia,
- (2) she is the biological mother of more than one son with hemophilia, or
- (3) she is the biological mother of at least one son with hemophilia and has at least one other blood relative with the disorder.
For a woman who is at risk to be a carrier, the most accurate method of carrier testing is to compare her DNA to that of a male family member with hemophilia. If she has the same change in her gene for hemophilia as her affected family member, she is a carrier for hemophilia. It is possible for boys with hemophilia to be born to women who are not carriers. This occurs when there is a new genetic change during the development of one or more of the mother’s eggs, which is then passed onto the child. More commonly, however, in cases where there is not a family history of hemophilia, the mother is a new carrier.
The gene causing hemophilia normally contains the instructions for the body to make clotting factor. When women are carriers, they have one gene that functions properly and one that does not. Therefore, they are expected to have half the amount of clotting factor as other women. Occasionally, carriers can have less clotting factor than expected and experience bleeding symptoms. It is recommended that women who are carriers or are at risk to be carriers have their clotting factor levels evaluated. This evaluation is available at your local comprehensive hemophilia treatment center.
