Polydactyly

Polydactyly is defined as the presence of more than the usual number of digits and characterised by broad phenotypic diversity. Two morphological types of polydactyly were reported so far: “mitten-paw” and “patty-foot”. Mitten-paws have an extra digit which is shorter than the other digits and separated from them, the distance between the extra digits and the first normal digits is greater than the distance between two normal digits. Patty-foots have extra digits of the same length and are not separated from the normal digits, which means that the distance between extra digits and normal digits and between two extra digits is the same.
Polydactyly may involve a single limb, two, three or all four limbs, but mostly, the forelimbs or all limbs seem to be affected. The number of additional digits is also varying.
Three variants were found to be responsible for feline non-syndromic preaxial polydactyly, located in a regulatory sequence of the Sonic Hedgehog (SHH) gene, called zone of polarising activity regulatory sequence (ZRS), known to be involved in limb development. These variants were first described in outbred domestic cats in the UK (UK1 and UK2 variants) and in Hemingway cats in the USA (Hw variant). All three variants are inherited as autosomal-dominant traits, cats with one or two copies of these variants are likely to be polydactylic.