Learn more about hemophilia A

For more information about hemophilia, visit these Web pages, or read these publications: the National Hemophilia Foundation website (https://www.hemophilia.org/About-Us/Fast-Facts, https://www.hemophilia.org/Bleeding-Disorders/Types-of-Bleeding-Disorders/Hemophilia-A, https://www.hemophilia.org/Bleeding-Disorders/Types-of-Bleeding-Disorders/Hemophilia-B), and Triplett DA. Coagulation and bleeding disorders: review and update. Clin Chem. 2000;46(8 Part 2):1260-1269.

It is estimated that more than 400,000 people in the world have hemophilia

Hemophilia A occurs in 1 in 5000 male births and is about 4 times as common as hemophilia B. The number of people with hemophilia in the United States is estimated to be about 20,000. Approximately 75% of people with hemophilia around the world receive inadequate treatment or have no access to treatment.

What is hemophilia?

Hemophilia is a bleeding disorder that affects the way blood clots. People born with hemophilia are missing some or all of a type of protein called clotting factor. The 2 main types of hemophilia are hemophilia A and hemophilia B. People with hemophilia A are missing some or all of the clotting factor VIII. People with hemophilia B are missing some or all of a different clotting factor called factor IX.

Clotting factors are important because they help in the clotting process. When a blood vessel is cut or injured, clotting factors help platelets (which are cellular fragments made from your bone marrow) form a plug in the wound that stops the bleeding.

Why do mostly males have hemophilia A?

Hemophilia A is caused by a defective gene located on the X chromosome. Females have 2 X chromosomes, and males have 1 X chromosome and 1 Y chromosome. The defective gene is passed down from a mother (who carries, but is unaffected by, the gene) to her son. Females are unaffected because they have 2 copies of the X chromosome, so if the factor VIII gene on an X chromosome is defective, the gene on the other X chromosome can make enough factor VIII. Because males have only 1 X chromosome and 1 Y chromosome, if their only copy of factor VIII gene on the X chromosome is defective, they will have hemophilia A.

Because females get 1 X chromosome from their mother, and 1 X chromosome from their father, a father who has hemophilia will pass the defective X chromosome to his daughters, and his daughters will be carriers. Because males get their X chromosome from their mother, they will not inherit the defective X chromosome, and therefore males born to a father with hemophilia and a mother who is not a carrier will not be affected.