Polydactyl cat

Normal cats have a total of 18 toes, with five toes on each fore paw, and four toes on each hind paw; polydactyl cats may have as many as nine digits on their front or hind paws. Various combinations of anywhere from four to seven toes per paw are common.^[9] Polydactyly is most commonly found on the front paws only; it is rare for a cat to have polydactyl hind paws only, and polydactyly of all four paws is even less common.^[4]

Preaxial polydactyly: Hw mutant in two right forefeet

Maine Coon kitten

Cat's paw pad, edited to show seven yellow circles, indicating digits. The circle with a question mark indicates what might be a separate digit. The rightmost circle is for a small, underdeveloped, clawless digit.

Embryologists distinguish two principal forms of feline polydactyly: preaxial and postaxial.^[1] Preaxial polydactyly involves supernumerary digits on the medial (inner) side of the paw, corresponding to the thumb side in humans, whereas postaxial polydactyly affects the lateral (outer) side, corresponding to the little finger. Postaxial polydactyly is comparatively rare in cats, and the additional digits in such cases are often incompletely formed. By contrast, preaxial extra digits are typically well developed.^[1]

Polydactyly is a congenital malformation that is most commonly inherited in an autosomal dominant pattern. In several cases, the condition is associated with mutations affecting regulatory elements that control expression of the sonic hedgehog (SHH) gene during limb development.^[11] SHH encodes a signalling protein that plays a central role in embryonic pattern formation, including the specification and growth of limbs and digits.

Genetic work studying the DNA basis of the condition indicates that many different mutations in the same ZRS area can all lead to polydactyly.^[11] In the case of preaxial polydactyly of the Maine Coon cat (Hemingway mutant; Hw) a mutation of the cis-regulatory element ZRS (ZPA regulator sequence) is associated. ZRS is a noncoding element, 800 kilobase pairs (kb) remote to the target gene SHH. An ectopic expression of SHH is seen on the anterior side of the limb. Normally SHH is expressed in an organiser region, called the zone of polarising activity (ZPA) on the posterior limb side. From there it diffuses anteriorly, laterally to the growth direction of the limb. In the mutant mirroring smaller ectopic expression in a new organiser region is seen on the posterior side of the limb. This ectopic expression causes cell proliferation delivering the raw material for one or more new digits.^[12][13] An identical sequence at this position serves the same function in human and mice and cause similar symptoms when mutated. Different mutations have different specific effects: for example, while the Hw mutant tends to mostly induce extra fingers in the fore limbs, many other mutations affect the posterior limbs too.^[11]

Polydactyly is a spontaneous complex phenotypic variation, developed in one generation. In the concrete preaxial form of the Hw mutant the variation is induced by a single point mutation in a noncoding cis-regulatory element for SHH. In an extensive phenotypic variation like this, one or more complete digits at each single limb are developed including nerves, blood vessels, muscles and ligaments. The physiology of the digits can be perfect. This complex phenotypic result cannot be explained by the mutation alone. The mutation can only induce the variation. In the consequence of the mutation, thousands of events, each different from the wildtype, occur on different organisation layers, such as expression changes of other genes, cell-cell signal exchange, cell differentiation, cell and tissue growth. The summarised small random changes on all layers build the raw material and the process steps for the generation of the plastic variation.^[9]

The mentioned form of polydactyly of the Hw mutant shows a biased variation. In a recent empirical study first the number of extra toes of 375 mutant Maine Coon cats were variable (polyphenism) and second, the number of extra toes followed a discontinuous statistical distribution. They were not equally distributed as one might expect of an identical single point mutation. The example demonstrates that the variation is not explained completely by the mutation alone.^[9]

Polydactylism is found in random-bred cat populations, and not an indication of purebred ancestry. Polydactylism is considered a disqualifying fault in most breeds, and makes kittens of any breeding line inadmissible for registration with some of the major cat registries. The registries argue that polydactylism is a congenital abnormality, which confers no welfare benefit upon cats, and in some cases the additional toes may be prone to injury; furthermore, claws on the extra toes may fail to wear down naturally, potentially causing harm to the animal.^[14] Due to these welfare concerns, polydactyl cats are barred from registry and showing in for example the FIFe,^[15] GCCF,^[14] and WCF.^[16] This also holds true for oligodactylism (less toes) in some registries.^[15] The feature is permitted by some of the other major cat fancier clubs, for example with CFA and TICA; however only in specific cat breeds, including the Highlander, Maine Coon, and Pixie-bob.^[1] In TICA the polydactyl Maine Coons are categorised as a sister breed called the Maine Coon Polydactyl, and allowed polydactylism in their cats is limited to no more than 7 toes on any foot.^[2]

The American Polydactyl is recognised as a cat breed with the Rare and Exotic Feline Registry (REFR),^[17] however not by any of the major registries. The cats are selectively bred in compliance with a breed standard in order to maintain a standardised breed with specific physical and behavioural characteristics, in addition to extra digits.

• Polydactyly in early tetrapods
• Ship's cat
• List of cat body-type mutations