Six fingers! It happens sometimes — especially to a few choice species, including humans. It's called polydactyly. And there's an amazing story behind how we tracked down the reason people, cats, and flies seem to grow extra digits.
Inigo Montoya, the most quoted fantasy character in history, strode through both the book and the movie version of The Princess Bride looking for the six-fingered man who killed his father. He figured the man shouldn't be hard to find. Actually, polydactyly in general is pretty common, occurring in about one in five hundred to one thousand babies. Count Rugen was exceptional in that his fingers all worked, whereas most polydactyl people only have fleshy fingers that branch from their pinkies or thumbs, but it was common enough that people went looking for the answer.
To find one, they had to go all the way back to fruit flies. Fruit flies have been showed to have a remarkable amount of DNA that is analogous to humans, and they are both more numerous and easier to experiment on (lucky thing for humans, that). Researchers could tinker with different genes, and see what happened. One gene, named hedgehog gene, caused a large amount of spiky hairs to grow on the flies. The hedgehog gene, or versions of it, was found in many other animals. There was the Indian hedgehog gene, the Desert hedgehog gene, and finally the Sonic hedgehog gene. This last was studied in mice, and was shown to affect things like gut and digestive system development. It was also shown to correlate with polydactyly in both mice and chickens.
The gene itself doesn't mutate to grow the extra fingers. Instead a different gene is mutated, and affects the way the Sonic hedgehog gene is expressed. When the gene spits out a certain concentration of protein at a certain area, the growth of fingers is kickstarted. The mutation causes the Sonic gene to put out protein at high concentrations in additional areas of the developing hand, and extra fingers grow.
This same pattern happens in mice, humans, and in the famous polydactyl cats. Polydactyl cats are also known as Hemingway cats, because a number of such cats live in the house Hemingway wrote in in Key West. Local legend has it that Hemingway got the cat from a ship's captain and introduced the trait into the local cat populace. After testing, it looks like all the North American polydactyl cats are descended from the main line - the first scientific record of which dates back to Nova Scotia in 1868. It's likely, then, that Hemingway cats made their way down to Key West by land, and Ernest just took them in.
Top Image: Lucy Provencher