According to the Hemingway Home website:
“Cats normally have five front toes and four back toes. About half of the cats at the museum have the physical polydactyl trait but they all carry the polydactyl gene in their DNA, which means that the ones that have 4 and 5 toes can still mother or father six-toed kittens. Most cats have extra toes on their front feet and sometimes on their back feet as well. Sometimes it looks as if they are wearing mittens because they appear to have a thumb on their paw.
Ernest Hemingway was given a white six-toed cat by a ship’s captain and some of the cats who live on the museum grounds are descendants of that original cat, named Snow White. Key West is a small island and it is possible that many of the cats on the island are related. The polydactyl cats are not a particular breed. The trait can appear in any breed, Calicos, Tabbies, Tortoise Shell. White, Black, etc. They vary in shapes, sizes, colors and personalities.”
Learn more at Curiosity.com (https://curiosity.com/topics/ernest-hemingways-house-is-full-of-six-toed-cats-curiosity/)