Dogs in general have four digits in the hind limb, presumably due to an evolutionary adaptation. However, in a few breeds, including Great Pyrenees, five or six digits have been intentionally maintained through breeding.
In addition, most breeds often display this extra digit as a genetic variation, although the number of digits (five) in the forelimb is essentially unchanged. This type of dominant genetic alteration has been commonly called 'dewclaw,' but is more accurately described as hind-limb-specific preaxial polydactyly.
Polygenic disorders causing polydactyly have been reported in the Australian Shepherd with lumbar scoliosis, short malformed tibias and fibulas, polydactyly, brachygnathism, cleft palate and cleft lip. It has also been described in a litter of St. Bernards with cleft palate, anotia, incomplete bifid tongue, preaxial hind paw polydactyly, and an extra thoracic vertebra and rib.
Ectrodactyly has also been reported in dogs affected by congenital polyarthrodysplasia, a condition characterized by congenital elbow luxation, ectrodactyly, patellar luxation, hydrocephalus, retained testicle, deformed tail and Legg-Calve-Perthes disease (femoral head necrosis).
Rarely, polydactyl mutations involving both the fore and hind limbs occur, as reported in breeds like the Norwegian Lundehund, although their genetic bases are poorly understood.
Other conditions such as ectrodactyly (lobster-claw) can also occur.
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