What is the genetic basis of haemophilia?

Haemophilia is a genetic disorder caused by a mutation in the genes that code for blood clotting factors.

Haemophilia is a sex-linked recessive disorder, which means it is carried on the X chromosome. The genes responsible for producing blood clotting factors VIII and IX are located on the X chromosome. In individuals with haemophilia, these genes have mutations that result in the production of insufficient or defective clotting factors, leading to the symptoms of the disorder.

In males, who have only one X chromosome, inheriting the mutated gene from their mother will result in haemophilia. This is because they do not have a second X chromosome to provide a normal copy of the gene. Females, on the other hand, have two X chromosomes. If they inherit the mutated gene from one parent, they will typically be carriers of the disorder but will not show symptoms, as the normal gene on their other X chromosome can compensate. However, if a female inherits the mutated gene from both parents, she will also have haemophilia.

The specific type of haemophilia a person has depends on which clotting factor is affected. Haemophilia A, the most common type, is caused by a deficiency in factor VIII. Haemophilia B, also known as Christmas disease, is caused by a deficiency in factor IX. Both types are clinically identical and are characterised by prolonged bleeding.

The mutation in the gene can occur spontaneously, or it can be inherited from a parent. Genetic testing can identify carriers of the gene and can also diagnose haemophilia in unborn babies. There is currently no cure for haemophilia, but it can be managed with regular injections of the missing clotting factor.