Introduction to haemophilia

Haemophilia is an inherited bleeding disorder in which the blood does not clot properly. People with haemophilia lack the normal levels of “clotting factors” which are particular proteins that are necessary to the blood clotting mechanism.

Haemophilia A (classical haemophilia), the more common type of haemophilia, is due to a deficiency of factor VIII (factor 8); haemophilia B (Christmas Disease) is caused by a deficiency in factor IX (factor 9).

Inheritance of haemophilia

Haemophilia is usually an inherited condition, caused by a gene alteration, or “mutation”. While only males have haemophilia, females carrying the affected gene are known as carriers, and can then pass the gene on to their children.

Two chromosomes determine the sex of an individual. These are the X and Y chromosomes. Females have two X chromosomes (XX), and males have an X and a Y chromosome (XY). The genes for both factor VIII and IX are found on the female sex chromosome (the X chromosome). A child inherits one sex chromosome from each parent – always an X from the mother and either an X or Y from the father.

• If the mother carries the haemophilia gene and the father does not have the condition, there is a 50:50 chance of a male child having haemophilia, as he will inherit one of his mother’s X chromosomes and his father’s Y chromosome. If it is the affected one he will have haemophilia. If the child is female there is also a 50:50 chance that she will carry the haemophilia gene, as she will inherit either the affected or the unaffected X chromosome from her mother.
• If the mother is not a haemophilia carrier, but the father has haemophilia, a female child will automatically be a carrier, as she will have an X chromosome from her mother and the affected X chromosome from her father. All male children will inherit their father’s Y chromosome and their mother’s unaffected X chromosome. They will not be affected and cannot pass haemophilia on to future generations.
• However, in a third of all cases of haemophilia there is no known previous family history of the condition. This means that it has arisen due to a spontaneous gene alteration, read more
  
  
  
  
  

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