How do you get hemophilia?

People are born with hemophilia. They cannot catch it from someone like a cold.

Hemophilia is usually inherited, meaning that it is passed on through a parent’s genes. Genes carry messages about the way the body’s cells will develop as a baby grows into an adult. They determine a person’s hair and eye colour, for example.

Sometimes hemophilia can occur when there is no family history of it. This is called sporadic hemophilia. About 30% of people with hemophilia did not get it through their parent’s genes. It was caused by a change in the person’s own genes.

How is hemophilia inherited?
The hemophilia gene is passed down from parent to a child. The genes for hemophilia A and B are on the X chromosome. For this reason, hemophilia is called an X-linked (or sex-linked) disorder.

The figure below explains how the hemophilia gene is inherited. When the father has hemophilia but the mother does not, none of the sons will have hemophilia. All the daughters will carry the hemophilia gene.

If a woman inherits a copy of the altered gene from either of her parents, she is said to "carry" the hemophilia gene and is therefore called a "carrier." In other words, she has one normal and one altered copy of the gene. She can pass either gene onto her children. For each child, there is a 50% chance that a son will have hemophilia and a 50% chance that a daughter will carry the gene. On average, carriers of hemophilia will have about 50 per cent of the normal amount of clotting factor, but some carriers have far lower levels of clotting factor. Click here for more information on carriers and women with hemophilia.

Father with hemophilia
Mother carrying hemophilia gene

Acquired hemophilia
In rare cases, a person can develop hemophilia later in life. The majority of cases involve middle-aged or elderly people, or young women who have recently given birth or are in the later stages of pregnancy. This condition often resolves with appropriate treatment.


Updated September 2016


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