Expert: John Locke - 12/8/2008

Question
Hemophilia, a disease the prevents blood clotting normally in humans, is controlled by a recessive x-linked gene

a) why is hemophilia more common in males than in females?
b) a woman has hemophilia. Could either of her parents have normal blood?
c) Would a hemophilic father always have hemophiliac sons?

Answer
Thanks for using AllExperts.

Hemophilia is an X-linked disease, meaning that the defective gene involved in the disease process is carried on the X chromosome. A single X chromosome is active in any single cell, and if that X chromosome is defective, it will cause defective plasma clotting factors to be produced. Males have one X chromosome and one Y chromosome; inheriting a defective X chromosome guarantees that all of their cells will express the defective clotting factor, and thus they will have hemophilia. Females have two X chromosomes; inheriting a single defective X chromosome will not cause disease, because the healthy chromosome can produce enough functional clotting factor that hemophilia will not occur. A female must inherit two defective X chromosomes to express hemophilia; given that the mutation is fairly rare to begin with, the odds of having a girl with two defective X chromosomes are considerably lower than having a boy with one such defective chromosome.

As noted above, because the disease can be carried without expressing it in females, it is entirely possible for the woman in b) to have had a mother who did not have hemophilia. Her mother would have been a carrier for the disease and her father would have had to be hemophiliac.

Since fathers pass on Y chromosomes to their sons, a father will never contribute to a son having hemophilia. It is an X-linked disorder, as noted above. A hemophiliac father could potentially have hemophiliac sons--but the defective X chromosome would come from their mother.