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.