Polydactyl Cat
A polydactyl cat is a cat that has more than the usual number of toes on one or all of its paws as a result of a cat body type genetic mutation. In animals including humans, polydactyly (or polydactylism, also known as hyperdactyly) is the anatomical abnormality of having more than the usual number of digits on the hands or feet.
Normal cats have five toes on each front paw and four toes on each hind paw. Polydactyl cats may have as many as seven digits on front and/or hind paws, and various combinations of anywhere from four to seven are common, although each of the front and rear paws are typically the same. Polydactyly is most commonly found on the front paws only, with polydactyly of all four paws being less common. It is rare for a cat to have polydactyl hind paws only.
Polydactyly is a congenital abnormality, usually genetically inherited as an autosomal dominant trait with incomplete penetrance. Polydactyly is not life-threatening and usually not even debilitating to a cat. In some cases it appears to improve the dexterity of the animal. For example, a common variation of polydactyly with six toes on the front paws, with two opposing digits on each, (comparable in use to human thumbs) enables the cat to learn and perform feats of manual dexterity generally not observed in non-polydactyl cats, such as opening latches on some cabinets, doors and windows.
Some polydactyl kittens initially have more difficulty in learning to walk and climb than normal animals. However, their extra abilities, once developed, endear them to many owners.
Feline radial hypoplasia is a mimic of polydactyly and is considered a severe condition. It is not a result of the Pd gene normally associated with polydactyls. As a result, cats used in polydactyl breeding programs can be screened (by x-ray) for indicators of radial hypoplasia. Radial hypoplasia may cause the formation of extra jointed toes, it does not cause the mitten cat form.

History
The condition seems to be most commonly found in cats along the eastern coast of the United States and in South West England. Some sources state that these cats are rare in Europe because they were killed as witches' familiars, but other sources indicate that they are quite common in England. Polydacty Cats are very common in the Cardigan area of Wales and are known as Cardi-Cats. In June 2007, a cat from Wales was reported to have 26 toes, which may be a record for a cat in the UK.
Although there is some controversy over whether the commonest variant of the trait originated as a mutation in New England or was brought there from England, there does seem to be agreement that it spread widely as a result of cats carried on ships originating in Boston, and the prevalence of polydactylism among the cat population of various ports correlates with the dates when they first established trade with Boston.[1][2] Contributing to the spread of polydactyl cats by this means, sailors were long-known to especially value polydactyl cats for their extraordinary climbing and hunting abilities as an aid in controlling shipboard rodents. Some sailors also considered them to be extremely good luck when at sea.