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Haemophilia

Haemophilia is a genetic condition where the clotting factor in the blood is either partly or completely missing, leading to a life-long tendency to excessive bleeding with very slow clotting of the blood. It affects females as carriers and males who inherit the condition.

Blood clotting involves a complicated process occurring in stages. The sequence is called a cascade and it involves a series of chemical factors described as factor I to factor XIII. There is no factor VI. These factors are proteins, and there is a gene that sets out the chemical structure of each one. The genes for two of the factors, VIII and IX, are on the X (sex) chromosome.

The blood-clotting cascade ends in the formation of a substance that converts a blood protein called prothrombin into the protein thrombin, which actually forms the substance of the blood clot. Thrombin tangles up a lot of small cell fragments in the blood, called platelets, to form a mass that can effectively seal off small bleeding blood vessels.

A gene mutation is a change in the gene code. This change is likely to change the structure of the protein that the gene causes to be made. There are different kinds of mutations, some having a more serious effect on the structure of the protein than do others. If a male person has a significant mutation in the gene for factor VIII or factor IX, he will have haemophilia.

Haemophilia A occurs in about 1 baby in 5,000 live births. Haemophilia B is much less common, occurring in about 1 in 30,000. Haemophilia can be mild, moderate or severe, depending on the amount of functioning factor VIII or IX the mutated gene is able to produce. About 6,000 people are affected with haemophilia in the UK.