What causes Haemophilia?

Haemophilia is an inherited condition. The genes responsible for producing factor VIII and IX are on the X chromosome.

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Chance of a carrier having a son with haemophiliaEach of her sons will have a 50% (1 in 2) chance of having haemophilia
Chance of a carrier having a daughter who is a carrierEach of her daughters will have a 50% (1 in 2) chance of being a carrier
Chance of a man with haemophilia having a son with haemophiliaNone – unless the mother of his son is a carrier
Chance of a man with haemophilia having a daughter who is a carrierAll his daughters will be carriers

Females have two copies of the X chromosome, and males have one X and one Y chromosome. The mother produces an egg containing one X chromosome. The father produces sperm containing either an X or a Y chromosome. The fusion of a sperm with an egg creates the embryo, which will grow into a child, which then has two sex chromosomes. If the father contributes his X chromosome, a girl is conceived. If he contributes his Y chromosome, a boy is conceived.

A man with an altered haemophilia gene on his X chromosome will be affected with haemophilia. If a female has an altered haemophilia gene on only one of her X chromosomes, then she is known as a carrier. The term carrier can be misleading because some females who are carriers of haemophilia may have significantly reduced factor levels, which means they have a mild form of haemophilia themselves. You may hear doctors use the term heterozygote instead of carrier. The pattern of inheritance is known as sex- or X-linked recessive.

In some cases of haemophilia, there is no known family history. This may be because the alteration to the haemophilia gene is new, known as a spontaneous mutation, or that no affected males have been known in the family.

More information is available in our Understanding Haemophilia booklet.