Blood Home > Causes of Hemophilia

Hemophilia is an inherited disease. Specifically, hemophilia is caused by mutations in the F8 or F9 genes. In most cases, hemophilia is passed from a female carrier to her son; however, it is possible for boys with hemophilia to be born to mothers who are not carriers when there is a random mutation in the gene as it is passed on to the child. Although it is very rare, it is possible for a girl to be born with hemophilia.

What Causes Hemophilia?

Hemophilia is an inherited disease. Hemophilia is caused by a defect in one of the genes that determines how the body makes blood clotting factors 8 and 9. These genes are located on the X chromosomes, which determine whether a baby is a boy or girl.
 

F8 and F9 Genes

Mutations in the F8 or F9 genes cause hemophilia. Mutations in the F8 gene cause hemophilia A, while hemophilia B is caused by mutations in the F9 gene.
 
The proteins made by these genes play a critical role in the process of blood clotting. Mutations in either gene prevent clots from forming in response to injury, which lead to excessive bleeding that can be difficult to control.
 

Understanding Chromosomes and Inheritance

Hemophilia is inherited in an X-linked recessive pattern. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome, which is one of the two sex chromosomes.
 
Chromosomes come in pairs. Women have two X chromosomes, while men have one X and one Y chromosome. A woman is a "carrier" if she has a defective gene for factor 8 or factor 9 on one of her X chromosomes. She can pass the defective gene on to her children.
 
  • If she has a son, there is a 1 in 2 chance (50 percent) that he will have hemophilia.
  • If she has a daughter, there is a 1 in 2 chance (50 percent) that the daughter will be a carrier.
 
A man who has hemophilia cannot pass the disease on to his sons; however, all of his daughters will be carriers.
 
In very rare cases, a girl may be born with hemophilia. This can occur if her father has hemophilia and her mother is a carrier.
 
Written by/reviewed by:
Last reviewed by: Arthur Schoenstadt, MD
Last updated/reviewed:
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