Every person has two sex chromosomes. Women have two X-chromosomes while a man has one X and one Y chromosome. Both factor VIII and IX proteins are located on the X chromosome. A fault in the Factor VIII or IX gene on the X chromosome will result in Haemophilia A or B in males because there is not a normal X chromosome to balance the abnormality. In females carrying the haemophilia gene, the normal X chromosome compensates for the abnormal X chromosome.
An affected male cannot pass the disorder on to his sons but all his daughters will carry the haemophilia gene. Although females only rarely have symptoms of the disorder, they may carry the haemophilia gene and may pass on the disorder to their sons. Sons of women with the haemophilia gene have a one in two chance of being affected and daughters will have a one in two chance of carrying the haemophilia gene.
Reproduced with thanks from Haemophilia Foundation Australia - "Finding out about carrying the haemophilia gene".
This page last updated: November 2004
Review date: June 2005
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© The State of Queensland (Queensland Health) 2008.