Hemophilia is a rare bleeding disorder in which the blood does not clot properly, leading to spontaneous bleeding as well as bleeding following injuries or surgery. It is usually an inherited disorder caused by a mutation or change in one of the genes that provides instructions for making the clotting factor proteins needed to form a blood clot. These genes are located on the X chromosome, and males inherit the X chromosome from their mothers and the Y chromosome from their fathers. Therefore, males are more likely to develop hemophilia than females. Hemophilia can also be acquired in rare cases when a persons immune system attacks clotting factor 8 or 9 in the blood.

People with hemophilia have low levels of either factor VIII (8) or factor IX (9), which are proteins in the blood that help with clotting. The severity of hemophilia that a person has is determined by the amount of factor in the blood. The lower the amount of the factor, the more likely it is that bleeding will occur, which can lead to serious health problems.

In summary, hemophilia is caused by a mutation or change in one of the genes that provides instructions for making the clotting factor proteins needed to form a blood clot. This change or mutation can prevent the clotting protein from working properly or to be missing altogether.