Victoria's youngest son, Leopold, was affected by the blood-clotting disease haemophilia B and two of her five daughters, Alice and Beatrice, were carriers. Royal haemophiliacs descended from Victoria included her great-grandsons, Tsarevich Alexei of Russia, Alfonso, Prince of Asturias, and Infante Gonzalo of Spain. The presence of the disease in Victoria's descendants, but not in her ancestors, led to modern speculation that her true father was not the Duke of Kent but a haemophiliac. There is no documentary evidence of a haemophiliac in connection with Victoria's mother, and as male carriers always suffer the disease, even if such a man had existed he would have been seriously ill. It is more likely that the mutation arose spontaneously because Victoria's father was over 50 at the time of her conception and haemophilia arises more frequently in the children of older fathers. Spontaneous mutations account for about a third of cases.