Haemophilia is caused by an inherited genetic mutation, which mainly affects males.
A genetic mutation is a permanent alteration in the DNA sequence that makes up a gene. This means that some of the body's processes won't work in the normal way.
The type of mutation determines whether a person will experience mild, moderate or severe symptoms.
The gene mutation is found on the X chromosome. It can be carried by either the mother or father, or both.
You can read more about chromosomes and how we inherit mutations by reading about genetics.
The chances of a child inheriting the haemophilia mutation depend on which of their parents has the mutated gene.
If a woman with the mutated chromosome and an unaffected man have a baby, there's a:
In the last situation, the girl becomes a carrier of the mutated gene. This means she can pass it on to her children, but won't usually have any severe symptoms of haemophilia herself.
However, some female carriers sometimes have bleeding problems, such as heavy periods.
If a man with haemophilia has a son with an unaffected woman, there's no chance the boy will get haemophilia.
This is because he always inherits his X chromosome from his mother, who in this case doesn't have the mutated gene.
However, any daughters the man has will become carriers of the mutated haemophilia gene and may pass it on to their children.
If a woman with the mutated chromosome and a man with haemophilia have a baby, there's a:
This means it's possible for a female to have haemophilia, although it's very rare.
In some cases, a boy is born with haemophilia even though there's no family history of the condition.
In such cases, it's thought the mutation developed spontaneously in the boy's mother, grandmother or great-grandmother, but until then a male member of the family had never inherited the mutated gene.
Some studies have shown there's no known family history of haemophilia in up to a third of new cases.
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.