Haemophilia A results from a congenital deficiency of factor VIII (VIII:C) coagulant activity.
It is an X-linked recessive disorder affecting 1 in 10 000 males. Heterozygous females act as carriers but may be symptomatic when lyonisation - random inactivation of one of the X chromosomes in each cell - results in preferential expression of the abnormal X chromosome. Carriers with low normal levels of factor VIII - 40-50% - may show symptoms under severe haemostatic stress e.g. after major trauma.
The condition 'breeds' true so that in families all affected members have severe disease or all have mild disease.
Last reviewed 01/2018