Can women have haemophilia?

Around a third of women who carry the haemophilia gene have bleeding symptoms due to mild haemophilia. The extent of their bleeding symptoms depends on the relative expression of the faulty gene to the normal gene (this effect is called Lyonisation). In the UK, there are just under a thousand women diagnosed with haemophilia.

In extremely rare cases, it is possible for a daughter to inherit two faulty haemophilia genes. This happens when the father has haemophilia, and the mother is a carrier, so the daughter inherits an affected X chromosome from both parents.