A defect in one of the genes that determines how the body makes blood clotting factor VIII or IX causes hemophilia. These genes are located on the X chromosomes (KRO-muh-somz).
Chromosomes come in pairs. Females have two X chromosomes, while males have one X and one Y chromosome. Only the X chromosome carries the genes related to clotting factors.
A male who has a hemophilia gene on his X chromosome will have hemophilia. When a female has a hemophilia gene on only one of her X chromosomes, she is a "hemophilia carrier” and can pass the gene to her children. Sometimes carriers have low levels of clotting factor and have symptoms of hemophilia, including bleeding. Clotting factors are proteins in the blood that work together with platelets to stop or control bleeding.
Very rarely, a girl may be born with a very low clotting factor level and have a greater risk for bleeding, similar to boys who have hemophilia and very low levels of clotting factor. There are several hereditary and genetic causes of this much rarer form of hemophilia in females.
Some males who have the disorder are born to mothers who aren't carriers. In these cases, a mutation (random change) occurs in the gene as it is passed to the child.
Below are two examples of how the hemophilia gene is inherited.
Inheritance Pattern for Hemophilia—Example 1
Each daughter has a 50 percent chance of inheriting the hemophilia gene from her mother and being a carrier. Each son has a 50 percent chance of inheriting the hemophilia gene from his mother and having hemophilia.
Inheritance Pattern for Hemophilia—Example 2
Each daughter will inherit the hemophilia gene from her father and be a carrier. None of the sons will inherit the hemophilia gene from their father; thus, none will have hemophilia.