littlecellbiologyblog:

Normally cats have five toes on their front feet and four on the back feet, yet some are known to have extra toes, a condition known as polydactyly. The author Ernest Hemingway received a six-toed cat, called Snow White, from a Ship’s Captain, and descendants of this feline are still resident at the Hemingway Museum today. These cats have become known as ‘Hemingway’s Cats’ or ‘Polydactyls’.

Normally the correct number of fingers/toes is regulated by a gene called Sonic Hedgehog (Shh), which is active in a specific region near the future little finger/toe. However, these cats have a genetic variation in which Shh also becomes active in the region near what should be the future thumb, leading to growth of one or more additional toes.