Haemophilia

Haemophilia (also spelled hemophilia in North America, is a group of hereditary genetic disorders that impair the body's ability to control blood clotting or coagulation, which is used to stop bleeding when a blood vessel is broken.

Haemophilia A (clotting factor VIII deficiency) is the most common form of the disorder, occurring at about 1 in 5,000–10,000 male births.

Haemophilia B (factor IX deficiency) occurs at about 1 in about 20,000–34,000 male births.

Causes

• Haemophilia A is a recessive X-linked genetic disorder involving a lack of functional clotting Factor VIII and represents 90% of haemophilia cases.
• Haemophilia B is a recessive X-linked genetic disorder involving a lack of functional clotting Factor IX. It is similar to but less common than haemophilia A.
• Haemophilia C is an autosomal genetic disorder (ie not X-linked) involving a lack of functional clotting Factor XI. Haemophilia C is not completely recessive: heterozygous individuals also show increased bleeding

Similarly to most recessive sex-linked, X chromosome disorders, only males typically exhibit symptoms. This is due to the fact that females have two X chromosomes while males have only one, lacking a 'back up' copy for the defective gene the defective gene becomes manifest more easily in males. Because females have two X chromosomes and because haemophilia is rare, the chance of a female having two defective copies of the gene is very low, thus females are almost exclusivelyasymptomatic carriers of the disorder. Female carriers may inherit the defective gene from either their mother, father, or it may be a new mutation. Only under rare circumstances do females actually have haemophilia. Affected males typically inherit the defective gene from their mother, or it can be a new mutation.

Haemophilia has featured prominently in European royalty and thus is sometimes known as "the royal disease".