Causes of Hemophilia A

Most people with hemophilia A are born with the disorder; in rare cases it can be acquired later in life as the result of a spontaneous gene mutation.

Hemophilia A happens when there is a shortage of a protein in your blood called factor VIII, which is essential for normal blood clotting. When a blood vessel is injured, a blood clot is formed to seal cuts and breaks at the site of injury to stop the bleeding. A person with hemophilia A is unable to form blood clots normally, leading to the prolonged bleeding and easy bruising seen with this condition.

The role of family history

The cause of hemophilia A can also be explained by family history. Since hemophilia A is largely an inherited, or congenital disorder, the role of genetics and family history is important to understand.

Here’s how it works:

Everyone is born with two sex chromosomes, which are inherited from his or her parents. There are two types of sex chromosomes: X and Y. A female inherits one X chromosome from her mother and one X chromosome from her father (XX). A male inherits one X chromosome from his mother and one Y chromosome from his father (XY). The gene that causes hemophilia is located on the X chromosome.

A woman who gives birth to a child with hemophilia often has other male relatives who also have hemophilia. Sometimes, a baby will be born with hemophilia when there is no known family history. This means either that the gene has been "hidden" (that is, passed down through several generations of female carriers without affecting any male members of the family) or the change in the X chromosome is new (a "spontaneous mutation").

There are four possible outcomes for the baby of a woman who is a carrier. These four possibilities are repeated for each pregnancy she has:

With each pregnancy, a woman who is a carrier has a 25% chance of having a son with hemophilia. Since the father's X chromosome determines that the baby will be a girl, all daughters of a man with hemophilia will be carriers. Sons are determined by the father through the Y chromosome, which means none of his sons will have hemophilia unless the mother is a carrier.