In practice, this would have required the Duchess's lover to be haemophiliac – an extremely unlikely survival, given the poor state of medicine at the time, or the Duchess herself to be a carrier of haemophilia, since haemophilia is X-linked, meaning that her mother would have been a carrier, if haemophilia was not otherwise previously expressed in the Duchess's parents. Actual evidence to support this theory has not arisen, and haemophilia occurs spontaneously through mutation in at least 30% of cases.