How is haemophilia inherited?
Genes found on the X-chromosome cause haemophilia, so the disorder usually affects only boys. In some cases, the disorder appears 'out of the blue', following a genetic change in the unfertilised egg. In other cases, the affected boy inherits haemophilia from his carrier mother. A carrier woman has a one-in-four chance of having an affected son, and a one-in-four chance of having a carrier daughter. About a third of carrier women have mild symptoms themselves.