Haemophilia almost always affects males due to the way it is passed down from a parent to their child.
How haemophilia is inherited
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
- the colour of a baby’s eyes
- 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.
Genetic mutation
Haemophilia is caused by a genetic mutation in the X chromosome. A genetic mutation occurs when the normal instructions that are carried in certain genes become "scrambled". This means that some of the body’s processes will not work in the normal way.
If a woman with the mutated X chromosome and an unaffected man have a baby girl, there is a:
- 50% chance that the baby girl will inherit a pair of normal X chromosomes and will be unaffected by haemophilia
- 50% chance that the baby girl will inherit a normal X chromosome from her father and a mutated X chromosome from her mother
In the second circumstance, the normal X chromosome will "balance out" the mutated X chromosome. The girl will become a carrier of the mutated gene.
If a woman with the mutated X chromosome and an unaffected man have a baby boy, there is a:
- 50% chance that the boy will inherit a normal X chromosome from his mother and a normal Y chromosome from his father and will be unaffected
- 50% chance that the boy will inherit a mutated X chromosome from his mother and a normal Y chromosome from his father and will be born with haemophilia
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. This is because girls always receive the mutated X chromosome from their father.
The type of mutation in the gene is directly connected to how severe the symptoms will be. For example, if a man with severe haemophilia has a carrier daughter who gives birth to a son with haemophilia, her son’s haemophilia will also be severe.
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 that the mutation that causes haemophilia develops spontaneously in the boy’s mother, grandmother or great-grandmother but, until now, a male member of the family has never inherited the mutated gene.
Although family history is a strong indicator, some studies have shown that there was no known family history of haemophilia in up to 50% 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 agents. These are proteins that form a "web" around the platelets, helping them to stay in place.
The mutated haemophilia gene means that boys with the condition do not have enough clotting agents in their blood.
Several different clotting agents are present in the blood. They are numbered using roman numerals. For example, in haemophilia A, there is not enough clotting agent VIII (8) in the blood. In haemophilia B, there is not enough clotting agent IX (9) in the blood.
