How haemophilia is inherited

The genes responsible for making clotting factors VIII and IX are on the X chromosome. This is one of the two chromosomes that determine what sex we are. Females have two X chromosomes (XX) and males have one X chromosome and one Y chromosome (XY). If a female inherits an X chromosome with ‘haemophilia’ clotting factor genes, the other X chromosome will usually be normal, so they are still able to produce safe levels of clotting factors. Although some female ‘carriers’ of the ‘haemophilia’ X chromosome do have occasional clotting problems, they are rarely as serious as the problems in males.

If a male inherits a ‘haemophilia’ X chromosome, there is no back-up X chromosome available, and they will not be able to produce normal levels of the clotting factors they need. Most males who inherit the ‘haemophilia’ X chromosome will have clotting problems – some will be extremely severe.