Hemophilia is a bleeding disorder that is inherited from one or both parents. The condition is an X-linked genetic disorder, meaning it mainly affects males rather than females.
In hemophilia, there is a deficiency in the level of clotting factors in the blood, which means the blood does not clot properly after an injury and bleeding can last for an abnormally long period. In hemophilia A, factor VIII is deficient and this condition accounts for most cases of hemophilia. Hemophilia B, on the other hand is caused by a deficiency in factor IX. There is also a much less common form of the disease, hemophilia C, which is an autosomal genetic disorder that leads to a deficiency in factor XI. This condition can affect males or females and is more common in Ashkenazi Jews than other population groups.
The severity of this condition depends on how deficient a patient is in the relevant clotting factor. The level of severity is grouped according to the following levels of deficiency:
- Those with less than 1% active factor are classified as having severe hemophilia
- Those with 1% to 5% active factor have moderate hemophilia
- Those with 5% to 40% active factor have mild hemophilia
All human cells contain 22 pairs of chromosomes called autosomes and one pair of sex chromosomes, to give a total of 46 chromosomes. In males, the sex chromosomes are made up of an X chromosome and a Y chromosome, while in females there are two X chromosomes. Male offspring inherit their X chromosome from their mother and their Y chromosome from their father, while female offspring inherit an X chromosome from each parent.
Hemophilia is inherited through a mutation in the X chromosome. Since males only have one X chromosome, they will develop symptoms of hemophilia if that chromosome carries the mutation. However, because the condition is recessive, the inheritance of one abnormal X chromosome does not usually cause symptoms in females because they usually also inherit another normal X chromosome. These females are then classified as carriers of the disease and can pass on the mutation to male offspring even though they do not develop symptoms themselves. Technically, a female can develop hemophilia if she is born to a female carrier and a man affected by hemophilia, but this is very rare.
Reviewed by Sally Robertson, BSc