Teenagers &
Young Adults
Genetics and inheriting haemophilia
Basic genetics
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.
Inheriting haemophilia
Haemophilia is known as a recessive X-linked condition. This means that if a boy (who will only have one X chromosome) inherits the gene for haemophilia they will not produce very much or sometimes no clotting factor because their body's only instructions for making this protein are faulty. Since girls have two X chromosomes, they usually receive a normal gene on the second X chromosome. In X-linked gene disorders, the normal gene is dominant and this means that girls who inherit the gene for haemophilia usually do not show symptoms as their normal gene can generally produce enough clotting factor for their blood to clot properly.
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 shown below and then reading the following information which should explain it clearly. 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 normal genes have a child (see part (a) of the inheritance diagram), all of their daughters will receive one affected gene (from their father) and one normal gene (from their mother), making them ‘carriers' of - but not sufferers from - the condition. None of their sons receive 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 normal 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 normal gene (therefore becoming a carrier) or of receiving two normal 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.
