Definition of Hemophilia B:

Hemophilia B is a hereditary bleeding disorder caused by a lack of blood clotting factor IX. Without enough factor IX, the blood cannot clot properly to control bleeding.

See also: Coagulation disorder

Causes, incidence, and risk factors:

Hemophilia B is caused by an inherited X-linked recessive trait, with the defective gene located on the X chromosome.

Females have two copies of the X chromosome, so if the factor IX gene on one chromosome is defective, the gene on the other chromosome can do the job of making enough factor IX.

Males, however, have only one X chromosome, so if the factor IX gene on that chromosome is defective, they will have Hemophilia B. Therefore, most people with hemophilia B are male.

If a woman has a defective factor IX gene, she is considered a carrier. This means the defective gene can be passed down to her children.

Boys born to a woman who carries the defective gene have a 50% chance of having hemophilia B, while their daughters have a 50% chance of being a carrier.

All female children of men with hemophilia carry the defective gene.

Risk factors for hemophilia B include:

• Family history of bleeding
• Being male