Hemophilia is a genetic bleeding disorder that prevents the blood from clotting properly, meaning that someone with hemophilia bleeds much more than the average person does. Usually, when a person gets cut, blood cells called platelets run to the area of bleeding and plug up the hole. Later, the platelets release chemicals that activate proteins called clotting factors to mix with the platelets and form fibers. These fibers are what stop the bleeding and make the clot strong and durable. Hemophilia is caused by a dysfunctional or absent clotting factor, either Factor 8 or Factor 9. Without either of these factors, stable clots aren't able to form as quickly, causing people with hemophilia to bleed severely even days after they are cut.
Hemophilia is a genetic, recessive disorder on the X chromosome. It is sex-linked because X and Y chromosomes determine whether you are male or female. Males have an X and a Y chromosome, but females have two X chromosomes. Because of this, it is far more likely that a male will have hemophilia than a female. In fact, 1 in every 4,000 males has hemophilia. If a male inherits the recessive X chromosome, he has no other dominant X chromosomes to mask it, so he will have the disorder. Females, however, can receive a recessive and dominant chromosome and only end up a carrier of the disease since the dominant chromosome is there to mask the recessive. There are two possible genotypes of parents who give the disease to boys: (recessive gene in red) father-XY, mother-XX OR father-XY, mother XX. Five genotypes are possible of parents who give hemophilia to girls: father-XY, mother XX OR father-XY mother-XX OR father-XY, mother XX OR father-XY, mother XX OR father-XY, mother-XX. Boys always inherit the Y chromosome from their fathers, so if a man and a woman are going to have a boy the mother will determine whether he has hemophilia or not. If you are a carrier of or have hemophilia, it is fairly likely that you and your spouse will have a child with hemophilia. Here is one common scenario: If the female parent is a carrier (XX) and the male parent is unaffected (XY), there is a 25% chance of having a boy with hemophilia (XY). Here is one other example: If the male parent has hemophilia (XY) and the female parent is unaffected (XX), there is a 100% chance that if they had a girl she would be a carrier (XX).