Haemophilia inheritance
The human body has 46 chromosomes. There are conditions whereby people have a different number of chromosomes. However, these have no bearing on haemophilia.
The father's sperm contains 23 of these chromosomes. The mother's egg contains another 23. Together, they total 46. 44 of these are autosomal. 2 are called 'sex chromosomes'.
Haemophilia A & B (Factor VIII and IX deficiencies) are genetic disorders, carried on the X chromosome.
A mother can pass on one of 2 X chromosomes. If one is faulty (i.e. contains the haemophilia gene), she has a one in 2 chance of passing the faulty gene on to each of her children.
A father, on the other hand, has only one X chromosome. Let's assume it contains the haemophilia gene. His son will NOT have haemophilia because he did not inherit the faulty X gene. His daughter WILL have the haemophilia gene because she inherited the faulty chromosome. This daughter will be a carrier. She will have the X chromosome containing the faulty gene. However, she will not show symptoms of haemophilia because she has a second X chromosome (from her mother) to produce sufficient Factor VIII or IX.
What happens when the father has haemophilia and the mother is unaffected?
When the father has haemophilia and the mother is unaffected, none of their sons will have haemophilia. All of their daughters will carry the haemophilia gene.
What happens when the mother carries the haemophilia gene and the father is unaffected?
When the mother carries the haemophilia gene and the father is unaffected, there is a 50 per cent chance at each birth that a son will have haemophilia. There is a 50 per cent chance at each birth that a daughter will carry the haemophilia gene.
Last Reviewed: 24 May 2007
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