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This website is intended for carers and patients living with haemophilia.
Genetics and inheriting haemophilia
We are made from billions of cells, each containing coils of DNA, which in turn hold our genetic information (the body's instruction manual). A gene is a section of DNA with a specific function. Genes can control everything from very complex bodily functions to the colour of someone's hair. They are made up of a particular sequence of bases (building blocks) and these DNA sections determine a person's genetic code (the body's instruction manual).
DNA is contained in the cell within the nucleus (the cell's control centre) and these coils of DNA are packaged up inside structures known as chromosomes. We all have two copies of each chromosome and receive half our chromosomes from our mother's egg and half from our father's sperm cell. Because of this we end up with a copy of every one of our genes from each parent. There is only one exception to this and this is in the ‘sex chromosomes', in this case the chromosomes can be different, a boy inherits two different sex chromosomes, one X and one Y chromosome, whereas a girl inherits two of the same sex chromosome, two X chromosomes.
To understand how haemophilia is inherited, it is useful to know about chromosomes. These provide the blueprint for how the human body develops, such as the creation and growth of different cells and the sex of a baby.
There are two types of sex chromosomes, known as X and Y. All humans have a pair of these chromosomes which determines their gender. Males have an XY pair and females 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.
The genes – or instructions - for blood clotting are found on the X chromosome, which means that heamophilia is described as an X-linked disorder. Since girls have two X chromosomes, they usually receive a healthy gene on the second X chromosome. The healthy gene will be dominant, meaning that girls who inherit the gene for haemophilia remain carriers, but do not usually show symptoms as their healthy gene can generally produce enough clotting factor for their blood to clot properly. However, this is not the case for boys who inherit the gene for haemophilia, because they only inherit one X chromosome. This means that they have no “back up” set of instructions for the clotting factor so are unable to produce very much, or in some cases none at all.
It can be quite complicated to visualise how haemophilia is inherited and what the chances are of someone having haemophilia or being a carrier, so it is worth looking at the diagram and reading the explanation. And remember, with the same parents, the likelihood of having a child with haemophilia is the same for each pregnancy.
Diagram of haemophilia inheritance (where is the gene for haemophilia A)
When a man with an affected gene on his X chromosome and a woman with two healthy genes have a child (see part (a) of the inheritance diagram), a daughter will receive one affected gene (from their father) and one healthy gene (from their mother), making them a ‘carrier' of the condition, although they will not suffer from it. No son receives the affected gene as they must inherit the Y chromosome (not the X chromosome) from their father.
If the mother is a carrier and the father has healthy genes (see part (b) of the inheritance diagram), each son has a 50 per cent chance of receiving the affected gene from the mother. Any daughter has 50 per cent chance of receiving one affected gene and one healthy gene (therefore becoming a carrier) or of receiving two healthy genes.
It is worth noting that in 30 per cent of newly diagnosed children their haemophilia is caused by a new mutation in the X chromosome, either in the mother or the child, where there is no previous family history of haemophilia. If the altered gene occurs in the mother it can also be passed to other children that she has, as detailed above. In addition, it may have arisen in her mother so her sisters, aunts and cousins may also be carriers