So-called "Hemingway cats" are actually polydactyl cats, or cats with extra toes; they do not belong to a specific breed, nor are they typically bred selectively. However, polydactalism is a common abnormality, and polydactyl cats are available for adoption on The Animal Foundation website as of November 2015.
Cats normally have 18 digits: four toes on each hind paw and four toes plus a dew claw on the front paws. Polydactyl cats have additional toes on one or more feet, usually in the front. The defect occurs as the result of an autosomal dominant gene, the Pd gene. Although the trait is generally harmless, some forms of polydactylism cause disabling defects in the bones of the front legs. For this reason, intentional breeding of polydactyl cats is controversial, at best.
Polydactylism is common in many breeds, particularly the Maine Coon, which had polydactylism rates of about 40 percent before the breed was officially recognized in the United States. According to folklore, polydactylism evolved in the breed to help them walk in the snow and catch fish from rivers and lakes, but this is almost certainly not true. Today, the breed standard calls for normal feet, so polydactylism has become less common in Maine Coon cats.
The term "Hemingway cat" became associated with polydactylism due to the late author Ernest Hemingway's fondness for cats with the trait. His former estate in Key West, Florida, now the Ernest Hemingway Home & Museum, currently houses about 40 to 50 polydactyl cats.