Haemophilia is a genetic condition which means it is passed on when someone's parents have a haemophilia gene. There is usually a history of haemophilia in the family, but one in three children with haemophilia is born into a family where no family member has ever been known to have the condition. From then on, it is passed on in the usual way to the children of the affected person.
If a mother is a carrier of haemophilia, then there is a fifty-fifty chance of any son having haemophilia, and if she has a daughter, there is a fifty-fifty chance of the daughter being a carrier, like her mother. A fifty-fifty chance is the same as guessing 'heads or tails' when a coin is tossed.
If a father has haemophilia, then all his daughters will be carriers, but all his sons will be completely unaffected, and won't pass on the gene if they have children of their own.
There are some families where all the brothers are affected by haemophilia, or only one. All different combinations are possible.
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von Willebrand's can be passed on by either parent.
Rarely, it is possible for someone to develop haemophilia with no history of haemophilia in the family, this is known as acquired haemophilia. It happens when the person's own body’s attacks factor VIII in a person who has no history of a bleeding disorder. This can be the result of an illness or pregnancy.
Haemophilia can also arise in your family with no previous history - the gene could have been there for a while without affecting anyone.
