Inheritance

Haemophilia is referred to as an ‘X-linked recessive condition’, meaning that the defective FVIII or FIX gene is located on the ‘female’ or ‘X’ chromosome.  The daughter of a man with haemophilia will always be a carrier of haemophilia, i.e. she carries the gene, but does not suffer from the symptoms of haemophilia.  The sons of a woman that carries the defective gene will have a 50% risk of suffering from haemophilia.  The same woman’s daughters will have a 50% risk of being a carrier of haemophilia.

Approximately one in every 5000 men are born with haemophilia A (defective FVIII gene). Haemophilia B (defective FIX gene) is inherited in the same way as haemophilia A, but it is five times less common. Clinical impact is similar to haemophilia A and it can only be differentiated from haemophilia A through a blood test. It is sometimes called Christmas disease after the family name of the first patient in whom it was diagnosed.