Hemophilia B Causes

Most people with hemophilia B are born with the disorder, although it can develop later in life.1,2

Hemophilia B is caused by a lack of a 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. Because of 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.3

The role of family history1

Because hemophilia B is usually an inherited disorder, the role of genetics and family history is important to understand.

How genes cause hemophilia B1,4

Example of Hemophilia B Inheritance Patterns
Hemophilia B Inheritance Patterns

Everyone inherits two sex chromosomes, X and/or 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 only on the X chromosome. A woman who gives birth to a child with hemophilia is often referred to as a “carrier.” Often, she will have other male relatives with hemophilia as well.

Sometimes a baby will be born with hemophilia B 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 (often called a “spontaneous” or “sudden” mutation).

There are 4 possible outcomes for the baby of a woman who is a carrier for hemophilia B:

  • A girl who is not a carrier
  • A girl who is a carrier
  • A boy without hemophilia
  • A boy with hemophilia B

Although it is very rare, a female can be born with hemophilia if her father has hemophilia and her mother is a carrier.1,4

With each pregnancy, a woman who is a carrier has a 25% chance of having a son with hemophilia. Because the father’s X chromosome determines whether a 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, so the sons of a man with hemophilia will not have hemophilia unless their mother is a carrier.


  1. Hemophilia B. National Hemophilia Foundation website. https://www.hemophilia.org/Bleeding-Disorders/Types-of-Bleeding-Disorders/Hemophilia-B. Accessed October 21, 2014.
  2. Giangrande P. Acquired Hemophilia. Revised ed. Montreal, QC: World Federation of Hemophilia; 2012. http://www1.wfh.org/publication/files/pdf-1186.pdf. Accessed October 28, 2014.
  3. Fast facts. National Hemophilia Foundation website. http://www.hemophilia.org/About-Us/Fast-Facts. Accessed October 14, 2014.
  4. What causes hemophilia? National Heart, Lung, and Blood Institute website. http://www.nhlbi.nih.gov/health/health-topics/topics/hemophilia/causes.html. Published July 31, 2013. Accessed November 11, 2014.