Haemophilia A and B are usually inherited conditions.1 In some cases, haemophilia can occur without a family history, due to a change in the person’s genes. Up to 30% of people with haemophilia A or B have this sporadic form.1,5
Haemophilia is usually caused by inheritance of a damaged or ‘mutated’ gene from the parents. People with haemophilia can’t produce enough clotting factor because of this damaged gene. The genes that carry the code for clotting factors are found on X chromosomes.1 Men only have one X chromosome while women have two. This means that men who inherit the damaged gene always develop haemophilia.
If a woman inherits the damaged gene she will become a ‘carrier’ – she will have one damaged and one normal copy of the gene. Around a third of women who carry the haemophilia gene have bleeding symptoms themselves and are classed as having 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).5,6,7
When a mother is a carrier and the father does not have haemophilia, there is a 50% chance that each son will have haemophilia and a 50% chance that each daughter will be a carrier.5 When a father has haemophilia and the mother is not a carrier, no sons will inherit haemophilia but all daughters will be carriers of the damaged gene.