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

Christmas disease; Factor IX hemophilia

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

Images

Blood clots

X-linked recessive genetic defects - how girls are affected

Blood cells

X-linked recessive genetic defects - how boys are affected

X-linked recessive genetic defects

Next Section

Hemophilia B Symptoms

Review Date: 03/28/2010
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; James R. Mason, MD, Oncologist, Director, Blood and Marrow Transplantation Program and Stem Cell Processing Lab, Scripps Clinic, Torrey Pines, California. 3/28/10Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org)