The genes that control the production of the clotting factors VIII and IX are located in the X chromosome.
Males (XY) have hemophilia when the gene for clotting factor VIII (hemophilia A, classical hemophilia) or clotting factor IX (hemophilia B, Christmas disease) on the single X chromosome is affected.
Women (XX) who are carriers generally don't have symptoms of hemophilia because only one X chromosome has a copy of the hemophilia gene. The other gene of the other X chromosome allows for normal production levels of clotting factors VIII or IX. Those women who have only one affected gene are called hemophilia carriers. Not all males with hemophilia have mothers who are carriers. Sometimes a mutation (a genetic change) occurs resulting in hemophilia. Currently, it is not known why this mutation happens.
Sons of women who carry the hemophilia gene have a 50 percent chance of inheriting the gene and having hemophilia. Daughters of women who are carriers have a 50 percent chance of also being carriers of hemophilia. In families where only one male is known to have hemophilia, it is usually possible to determine whether the hemophilia gene was passed from a mother who carries the gene or whether a new mutation occurred in the person with hemophilia.