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 family will experience mild, moderate or severe symptoms.

Chromosomes

Chromosomes are strands of DNA. They contain a detailed set of instructions that control a wide range of factors, including how the body's cells develop and the baby's sex.

There are two types of sex chromosomes: the X chromosome and the Y chromosome. All humans have a pair of sex chromosomes. Men have an XY pair and women have an XX pair.

Boys inherit their X chromosome from their mother and their Y chromosome from their father. Girls inherit an X chromosome from each parent.

Haemophilia is inherited through a mutation in the X chromosome.

How the mutation is inherited

The chances of a child inheriting the haemophilia mutation depend on which of their parents has the mutated gene.

Only the mother is affected

If a woman with the mutated X chromosome and an unaffected man have a baby, there's a:

• one in four chance of having an unaffected baby boy
• one in four chance of having a baby boy with haemophilia
• one in four chance of having an unaffected baby girl
• one in four chance of having a baby girl with one affected chromosome

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.

Only the father is affected

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.

However, any daughters the man has will become carriers of the mutated haemophilia gene and may pass it on to their children. 

Both parents are affected

If a woman with the mutated chromosome and a man with haemophilia have a baby, there's a:

• one in four chance of having an unaffected baby boy
• one in four chance of having a baby boy with haemophilia
• one in four chance of having a baby girl who is a carrier of haemophilia
• one in four chance of having a baby girl with haemophilia

This means it's possible for a female to have haemophilia, although it's very rare.

No family history

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.

Although family history is a strong indicator, some studies have shown there's no known family history of haemophilia in up to one-third of new cases.

How haemophilia affects the blood

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.