Hemophilia is a group of bleeding disorders. It is caused by low amounts of specific clotting factors. These factors help to stop bleeding.

The most common types of hemophilia are:

  • Hemophilia A (classic hemophilia)—accounts for 80% of all hemophilia (1 in 5,000 males), caused by too little factor VIII
  • Hemophilia B (Christmas disease)—occurs in 1 in 20,000 males, caused by too little factor IX

Hemophilia is caused by a faulty gene located on the X chromosome.

Females carry two copies of the X chromosome. If the faulty gene is only on one X, the normal gene on the second X will take over. As a result, they will not get the disease. Instead, they are carriers of the gene. The risk for their male offspring to inherit the gene is 1 in 2 or 50%. The chance that their female offspring will inherit the gene is also 50%. These female offspring will be carriers like their mothers.

It is possible for a female to have hemophilia. For this to happen she must inherit the faulty gene from both her mother and her father.

Males carry only one X chromosome. If they get the faulty gene, the disease will develop.

It is possible for a new genetic mutation to occur. This means a person can get hemophilia even if neither parent carries a genetic mutation.

Genetic Outcome Possibilities
Fetus Chromosomes
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