In "The Six Fingers of Time", by R. A. Lafferty, available for free here on Project Gutenberg, there is a claim that having an extra finger or toe is associated with genius. The text gives some evidence for this claim:
Yet there were hints here of a Corsican who commonly kept a hand hidden, of an earlier and more bizarre commander who wore always a mailed glove, of another man with a glove between the two, hints that the multiplex-adept, Leonardo himself, who sometimes drew the hands of men and often those of monsters with six fingers, may himself have had the touch.
The Corsican is undoubtedly Napoleon Bonaparte. The last one is Leonardo da Vinci, in whose painting The Last Supper one of the apostles did have six fingers on one hand (introduced when the painting was restored and later fixed). Who are the other two?