Causes of haemophilia 

Haemophilia is caused by an inherited genetic mutation.

It mainly affects males due to the way it's passed from a parent to their child.

A genetic mutation occurs when the normal instructions carried in certain genes become "scrambled". This means that some of the body's processes will not work in the normal way.

The type of mutation determines how severe the symptoms will be, so the condition will always be the same severity within a family.


To understand how haemophilia is inherited, it is useful to know about chromosomes.

Chromosomes are blocks 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 is 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 will become a carrier of the mutated gene. This means she can pass it on to her children, but she will not usually have any severe symptoms of haemophilia herself. However, female carriers sometimes have some bleeding problems, such as heavy periods.

Only the father is affected

If a man with haemophilia has a son with an unaffected woman, there is no chance that the boy will get haemophilia. This is because he will always inherit his X chromosome from his mother.

However, any daughters that 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 is 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 is 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 is no family history of the condition. In such cases it is 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 that there is 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 that a child with the condition does not 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 is not enough clotting factor VIII (8) in the blood. In haemophilia B there is not enough clotting factor IX (9) in the blood.

Page last reviewed: 03/04/2013

Next review due: 03/04/2015