If they have a son, he has a 50% chance of having hemophilia. This is determined by which X chromosome he inherits. If he inherits the mother's normal X chromosome, the boy will not have hemophilia. If he inherits the mother's mutated X chromosome, he will.
In the same way, the couple's daughter has a 50% chance of being a carrier. She could inherit the mother's normal X chromosome, and be normal herself. On the other hand, she could inherit the mother's X chromosome carrying the hemophilia defect and be a carrier.
Can carriers have hemophilia?
Because carriers have one normal X chromosome which produces a certain amount of Factor VIII or IX clotting factor, they are protected 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 in carriers is very wide. It ranges from levels similar to those of hemophiliacs in some carriers to normal levels in others. 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 excessive 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.
In any case, all carriers should pay close attention to signs of abnormal bleeding. These signs include:
heavy, prolonged menstrual bleeding (menorrhagia)
easy bruising
frequent nose bleeds.
Is there always a history of hemophilia in the family?
No. There are several explanations for a boy being born with hemophilia when there is no history of hemophilia in the family.
• Hemophilia does indeed run in the family but there is no evidence of it because no hemophiliac boys have been born in living memory.
• The genetic mutation responsible for hemophilia occurred in the mother at the time of her conception. The mother is then the first person in the family to carry hemophilia. Her children could be affected either as carriers or as hemophiliacs.
• The mutation occurred at the conception of the hemophiliac child. Either the egg from the mother or the sperm from the father underwent a mutation. In such a case, the mother is not a carrier, and her other children would not be affected by hemophilia.
Figure 6 shows the family pattern when a hemophiliac boy is born to a mother who is not known to be a carrier.