- Men have only one X chromosome. If that chromosome carries the abnormal hemophilia gene, the man will have the disease hemophilia. Both hemophilia A and B are linked to the X chromosome, which means they primarily affect men.
- Women have two X chromosomes. Even if one of the X chromosomes carries the defective gene, the normal gene on the other X chromosome will protect the woman from hemophilia. The gene does not affect them, but they carry it and pass it on to their children.
- If a woman is a carrier, she has a 50% chance that her sons will have hemophilia. Likewise, she has a 50% chance that her daughters will be carriers of hemophilia. This assumes that the woman's partner is not a hemophiliac.
- A man who has hemophilia has a 100% chance that his
daughters will be carriers, since they must inherit the defective x
chromosome from the father. His sons will not be affected if the mother is not a carrier.
- The transmission of this gene to offspring accounts for 70% of the cases of hemophilia. The remaining 30% occur from spontaneous changes in genes responsible for causing hemophilia.