Causes of Hemophilia

If you have inherited hemophilia, you’re born with the condition. It’s caused by a defect in one of the genes that determine how the body makes blood clotting factors VIII or IX. These genes are located on the X chromosomes.

Chromosomes come in pairs. Females have two X chromosomes, while males have one X and one Y chromosome. Only the X chromosome carries the genes related to clotting factors.

A male who has the abnormal gene on his X chromosome will have hemophilia. A female must have the abnormal gene on both of her X chromosomes to have hemophilia; this is very rare.

A female is a “carrier” of hemophilia if she has the abnormal gene on one of her X chromosomes. Even though she doesn’t have the condition, she can pass the gene on to her children. Females who are carriers usually have enough clotting factors from their one normal X chromosome to prevent serious bleeding problems.

Very rarely, a girl is born with hemophilia. This can happen if her father has hemophilia and her mother is a carrier.

Some males with the disorder are born to mothers who aren’t carriers. In these cases, a mutation (random change) occurs in the gene as it is passed to the child.