Blood cells called platelets are very important for blood clotting. These cells have a sticky surface that allows them to clump together to stop the flow of blood.
Platelets also need clotting factors. These are proteins that form a "web" around the platelets, helping them to stay in place.
The mutated haemophilia gene means a child with the condition doesn't have enough clotting factors in their blood.
Several different clotting factors are present in the blood. They are numbered using roman numerals. For example, in haemophilia A there's not enough clotting factor VIII (8) in the blood. In haemophilia B, there's not enough clotting factor IX (9) in the blood.