Although an individual's haemophilia can usually be traced in the ancestry, in about 30% of cases there is no family history of the disorder, and the condition is speculated to be the result of spontaneous mutation in an ancestor. Victoria's appears to have been a spontaneous or "de novo" mutation, most likely inherited from one of her parents, and she is usually considered the source of the disease in modern cases of haemophilia among her descendants. Queen Victoria's father, Prince Edward, Duke of Kent, was not a haemophiliac, but the mutation may have arisen as a germline mutation within him. The rate of spontaneous mutation is known to increase with paternal age (and is higher in fathers than in mothers at all ages); Victoria's father was 51 at her birth. The probability of her mother having had a lover who suffered from haemophilia is minuscule given the low life expectancy of 19th-century haemophiliacs. Her mother, Victoria, Duchess of Kent, was not known to have a family history of the disease, although it is possible that she was a carrier, but among her children only Victoria received the mutated copy.