What Causes 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 factor VIII or IX. These genes are located on the X chromosomes (KRO-muh-somz).

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.

Below are two examples of how the hemophilia gene is inherited.

Inheritance Pattern for Hemophilia—Example 1

The diagram shows one example of how the hemophilia gene is inherited. In this example, the father doesn't have hemophilia (that is, he has two normal chromosomes—X and Y). The mother is a carrier of hemophilia (that is, she has one abnormal X chromosome and one normal X chromosome).

Each daughter has a 50 percent chance of inheriting the abnormal gene from her mother and being a carrier. Each son has a 50 percent chance of inheriting the abnormal gene from his mother and having hemophilia.

Inheritance Pattern for Hemophilia—Example 2

The diagram shows another example of how the hemophilia gene is inherited. In this example, the father has hemophilia (that is, his X chromosome is abnormal). The mother isn't a hemophilia carrier (that is, she has two normal X chromosomes). Each daughter will inherit the abnormal gene from her father and be a carrier. None of the sons will inherit the abnormal gene from their father, and, therefore, none will have hemophilia.

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 who have 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.