Hemophilia B

  • Definition

    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

    Alternative Names

    Christmas disease; Factor IX hemophilia

    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