HOW IS HAEMOPHILIA PASSED ON?
The gene for Haemophilia is carried on the X sex chromosome (females are XX; males are XY). A man with Haemophilia passes the faulty gene to his daughters who then carry this gene and may pass the condition onto their sons. A man with Haemophilia does not pass the faulty gene onto his sons because they receive a copy of his Y chromosome (their X chromosome comes from the mother). Men are affected because the faulty gene on their X chromosome lacks the necessary instructions to produce the clotting factor which causes blood to clot. Women are usually unaffected because they have two X chromosomes and the working copy of the gene tends to override the faulty copy. However, about a third of female carriers have low clotting factor levels and may be mildly affected. If a carrier mother decides to have children, all her daughters have a 50% (1 in 2) chance of being a carrier. This depends on whether they receive the X chromosome containing the faulty or the working gene. Sons also have a 50
