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Causes of Hemophilia


A person is born with hemophilia. Hemophilia cannot be caught from someone else and it cannot spread like a virus or an infection. The gene that contains the instructions to properly make factor VIII and IX exist in the X chromosome.

Hemophilia is caused when the gene inside the X chromosome is defective. Males have one Y and one X chromosome. Therefore, since a male’s DNA contains only one X chromosome, hemophilia affects mostly males. Women have two X chromosomes, so even if one has a defective gene, the other X chromosome has the information needed to create clotting factors. A female can have Hemophilia because of X-inactivation or Lyonization. It is possible for one X to “shut-down” or not to express itself. If the X chromosome with the Hemophilia gene is the active chromosome than she will have lower levels. 50% of carriers have low factor levels. Hemophilia may be inherited, or it may be caused by a spontaneous mutation of the factor gene.

Inherited hemophilia

In 70% of cases of hemophilia, there is a known family history. The gene that causes hemophilia is passed from mother to child. A mother that carries the gene is called a carrier. A carrier has a 50% chance of having a son with hemophilia and a 50% chance of having a daughter who is also a carrier. In other words, a female carrier has a 25% chance of having a child with hemophilia. Please view the illustrations demonstrating the hemophilia inheritance patterns to learn more.

Spontaneous Mutations

Approximately 30% of cases of hemophilia are caused by a spontaneous mutation of the gene. In these cases, the mother is not a carrier of hemophilia and the child affected is the first in the family to have hemophilia and to carry the defective factor gene.


A female who has a defect in one of her X chromosomes is said to be a carrier of hemophilia. She carries the gene that may be passed to her children. A mother may be a carrier and may not know that she is a carrier if there is no family history of the condition.

Carrier Testing

A mother who has a son with hemophilia and who has no family history of the condition may or may not be a carrier of hemophilia. A mother who has a son with hemophilia should consider having her factor levels tested. She may have low factor VIII or factor IX levels, and that information could be critical in her well-being in cases of surgery or severe trauma. It is advisable that a mother who has a son with hemophilia be tested for her son’s genetic mutation to see if she is a carrier of hemophilia. This information is important when planning subsequent pregnancies, and may be necessary to share with other female family members who are thinking of having children.

Symptomatic Carriers

A carrier of hemophilia who experiences some bleeding symptoms is called a symptomatic carrier. Some carrier females have low factor levels and they may be considered to have mild hemophilia. Symptomatic carriers often experience frequent nose bleeds, heavy menstrual periods, and easy bruising.

Assisting and Advocating for the Bleeding Disorders Community