Of the various types of hemophilia, the most common of these lifelong bleeding disorders are due to an inherited deficiency of factor VIII or factor IX (Table 1). The genes for these blood coagulation factors lie on the X chromosome, and when mutated, they cause the X-linked recessive traits hemophilia A and B. Since these disorders are X-linked, they usually occur in males. Usually, the affected boy has inherited the mutant gene (X_H ) from his carrier mother (X _H/X ), but about 30 percent of cases arise from a spontaneous mutation, and there is . . .