Hemophilia is a hereditary bleeding disorder of specific blood clotting factors. There are several types of hemophilia, including hemophilia A and B. Hemophilia A is 7 times more common than hemophilia B. Hemophilia B is the result of a deficiency of clotting factor IX. The disorder is caused by an inherited X-linked recessive trait, with the defective gene located on the X chromosome. Thus, the disorder occurs primarily in males. Females carry two copies of the X chromosome, so if the factor IX gene on one chromosome is defective, the other can compensate. Males, however, carry only one X chromosome, so if the factor IX gene on that chromosome is defective, they have the disease. Females with one defective factor IX gene are carriers of this trait. Half of the male babies born from women who carry the defective gene have a 50% chance of developing the disease. Half of the female babies born from women who have the defective gene have a 50% chance of being carriers. The severity of symptoms can vary with this disease, and the severe forms become apparent early on. Bleeding is the hallmark of the disease and sometimes, though not always, occurs if an infant is circumcised. Additional bleeding manifestations make their appearance when the infant becomes mobile. Mild cases may go unnoticed until later in life, when they occur in response to surgery or trauma. Internal bleeding may occur anywhere and bleeding into joints is common. Risk factors are a family history of bleeding and being male. Hemophilia B occurs in about 1 out of 32,000 men. | 
Hematology
