Bleeding disorders are generally inherited, meaning they are passed from parent to child in their genes, so you would have the condition from birth. There are some bleeding disorders that you can develop later in life because of another illness or condition, but this is quite rare. These are known as acquired bleeding disorders.
When a baby is conceived, two sets of genes are brought together, one from each parent. Genes are small sections of DNA within the genome that code for proteins. Everything about us, from our eye colour to our height, is coded in our genes. Different sets of genes carry information for different characteristics.
Sometimes genes carry faults that can be passed on to our children. For most bleeding disorders, it is necessary to inherit a specific gene fault from both parents before you develop the condition. People who inherit a gene fault from one parent are called carriers. In most bleeding disorders, they will not have the condition but could pass the gene fault on to their children. Carriers may have lower levels of the clotting factor than normal and may have mild symptoms (but also may have no symptoms at all).
The best-known bleeding disorder, haemophilia, is caused by a gene fault on the X chromosome, one of the two sex chromosomes. This means that it is far more common in boys, because they only have one X chromosome so only need to inherit a gene fault from one parent to have the disease. This is not the case with most bleeding disorders, which are not linked to the sex chromosomes and affect men and women equally.
There are five possible situations that can arise with parents passing on a clotting factor gene fault:
- one parent is a carrier and the other has ‘normal’ genes
- both parents are carriers
- one parent has the condition and the other has ‘normal’ genes
- one parent has the condition and the other is a carrier
- both parents have the condition.
The following show how children will be affected in each situation.
One parent is a carrier and the other has ‘normal’ genes
There is a one in two chance that each child will be a carrier and a one in two chance that each child will be a carrier and not have the condition.
Both parents are carriers
There is a one in four chance of each child being completely unaffected and a one in four chance of each child having the condition.
There is a two out of four chance of each child being a carrier.
One parent has the condition and the other has normal genes
All the children will be carriers, but none will have the condition.
One parent has the condition and the other is a carrier
There is a one in two chance of each child having the condition and a one in two chance of each child being a carrier.
No children will be unaffected.
Both parents have the condition
All children will have the condition.