Hemophilia is a genetic disorder that affects blood clotting. The two most common types are hemophilia A and hemophilia B. Hemophilia A is caused by a mutation in the gene for the factor VIII protein, while hemophilia B is caused by a mutation in the gene for the factor IX protein. These genes are located on the X chromosome, which means that hemophilia is inherited in an X-linked recessive pattern. Males have one X chromosome and one Y chromosome, while females have two X chromosomes. Males inherit the X chromosome from their mothers and the Y chromosome from their fathers. Females inherit one X chromosome from each parent. Males can have hemophilia if they inherit an affected X chromosome that has a mutation in either the factor VIII or factor IX gene. Females can also have hemophilia, but this is much rarer. Hemophilia may be hidden in a family for many generations if it passes only through females who do not have symptoms of the disorder.