Anatomy: Animals Inside

Circulatory System

What is hemophilia?

Hemophilia is a rare, hereditary bleeding disorder in which the blood does not clot normally at the site of a wound and the person bruises easily. It occurs more in males than females and is thought to be caused by the lack of a blood clotting factor: Type A, the most common type (around 90 percent of the cases), is caused by a deficiency of factor VIII (one of the proteins that helps blood to form clots) and Type B, which is caused by a deficiency of factor IX. Simply put, the condition is caused by a defect in one of the clotting factor genes that lies on the X chromosome and most often occurs in males since that gene can be passed from mother to son. A son—with XY chromosomes—lacks a second X chromosome to make up for the defective gene, whereas a daughter—with XX chromosomes—is likely to be a carrier but unlikely to actually have the disorder. In order to have hemophilia, females must have the abnormal gene on both X chromosomes, which is a very rare occurrence.


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