Haemophilia

Haemophilia is a sex-linked condition in which the blood does not clot properly. A person with haemophilia bleeds for longer after an injury and may have internal bleeding into joints and muscles, which can cause pain and joint damage.Blood clotting depends on a cascade of proteins called clotting factors. The genes for two of these are on the X chromosome. Haemophilia A, the most common form, is caused by a faulty allele for factor VIII. Haemophilia B is caused by a faulty allele for factor IX.Haemophilia is X-linked recessive. Using X^H for normal clotting and X^h for haemophilia, affected males are X^hY, carrier females are X^HX^h, and affected females would be X^hX^h, which is very rare.Worked example: a carrier woman (X^HX^h) and an unaffected man (X^HY) have children. Offspring: X^HX^H, X^HX^h, X^HY, X^hY.• probability that a son has haemophilia = ½
• probability that a daughter is a carrier = ½
• probability that any child has haemophilia = ¼Haemophilia is famous in European royal families. Queen Victoria was a carrier, and the allele passed through her daughters to royal families in Spain, Germany and Russia, where several sons were affected.Haemophilia is treated by injecting the missing clotting factor. Today this is often made by genetically modified cells (recombinant factor VIII), which avoids the infection risk of factors taken from donated blood.