Again, the pattern of inheritance is a simple one just as for PKU and eye colour. The inheritance of this genetic disease is a bit more complicated because it is sex linked. Women normally have two genes for blood clotting, one on each X chromosome. Men only have one blood clotting gene because they only have one X chromosome. This gene is not present on a Y chromosome.
Some women carry a defective gene for haemophilia.
These women are called carriers. Their daughters may be normal or may be carriers. Their sons are either normal or actually have the disease. A haemophiliac male has problems when blood vessels are damaged because his blood will not clot properly. He can pass the gene on to his daughters who will be carriers.