Polydactyl cat

A polydactyl cat is a cat with a congenital physical anomaly called feline polydactyly (also known as polydactylism or hyperdactyly), which causes the cat to be born with more than the usual number of toes on one or more of its paws.

A point mutation in the noncoding region of the ZRS regulatory sequence causes abnormal SHH expression during limb development, causing variable polydactyly through following developmental processes rather than the mutation alone, explaining why the phenotypic expression of polydactyly varies. Cats with this genetically inherited trait are most commonly found along the East Coast of North America (in the United States and Canada) and in South West England and Wales.

The breed, American polydactyl cats, are bred with normal-toed cats by breeders, since the trait is inherited dominantly and can be passed on to offspring reliably. These cats spread through maritime trade, notably from Boston, since sailors valued polydactyl cats and saw them as lucky. They became known as "Hemingway cats" after Ernest Hemingway's love for his own collection of polydactyl cats.