Causes of Hemophilia B

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

Hemophilia B is the result of a lack of a certain protein in your blood called factor IX, 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. Due to the shortage of factor IX, a person with hemophilia B is unable to form blood clots normally, leading to the prolonged bleeding and easy bruising seen in this condition.

The role of family history

An individual’s family medical history can also explain why hemophilia B happens. Since hemophilia B is largely an inherited disorder, the role of genetics and family history is important to understand.

Here’s how it works:

Everyone inherits two sex chromosomes, X and Y, one from each of his or her parents. 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 and every pregnancy:

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 his Y chromosome, which means the sons of a man with hemophilia will not have hemophilia unless the mother is a carrier.