How did I get haemophilia?

Haemophilia is usually inherited. This means that it passes down from one generation of a family to the next in their genes. The genes for blood clotting factors are found on the X chromosome. Females have two X chromosomes, while males have only one (in addition to a Y chromosome).

For females to have severe haemophilia, they must have two copies of the faulty gene – one on the X chromosome inherited from their father and one on the X chromosome inherited from their mother. This is very rare.

More commonly, females have one faulty copy of the gene and one normal copy. They are known as ‘carriers’ and usually have enough clotting factors from their one normal X chromosome to prevent serious bleeding issues. However, up to 50 percent of carriers may have an increased risk of bleeding.

As shown below, it is possible for carrier mothers to pass the faulty gene to their children, who may then inherit haemophilia or become carriers themselves.

In contrast, because males have only one X chromosome, they require only one copy of the faulty gene to have haemophilia. As a result, haemophilia occurs more often in males than females.

Inheritance of haemophilia in the case of a 'carrier' mother (with one copy of the haemophilia gene) and a father without haemophilia

  • Each son has a 50 per cent chance of inheriting the faulty gene from his mother and having haemophilia.
  • Each daughter has a 50 per cent chance of inheriting the faulty gene from her mother and being a carrier.

Adapted from the US National Heart, Lung and Blood Institute, 2013.

Inheritance of haemophilia in the case of an affected father and a mother who is not a haemophilia carrier.

  • Each son has a zero per cent chance of inheriting the faulty gene from his father and having haemophilia. 
  • Each daughter has a 100 per cent chance of inheriting the faulty gene from her father and being a carrier.

Adapted from the US National Heart, Lung and Blood Institute, 2013.

In about one-third of cases, there is no known previous family history of haemophilia, with the faulty gene having arisen from a spontaneous alteration in the affected person alone.

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