From Dog
Hindlimbs of a St. Bernard pup showing polydactyly[1]
Polydactyly in a dog[2]

Polydactyly is a rare autosomal-recessive genetic disease characterized by additional phalanges in the fore or hindlegs of dogs[3].

Dogs in general have four digits in the hind limb, presumably due to an evolutionary adaptation[4]. However, in a few breeds, including Great Pyrenees, five or six digits have been intentionally maintained through breeding[5][6].

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[7]. 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[1].

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)[8].

Rarely, polydactyl mutations involving both the fore and hind limbs occur, as reported in breeds like the Norwegian Lundehund[9], although their genetic bases are poorly understood.

Other conditions such as ectrodactyly (lobster-claw) can also occur[10].


  1. 1.0 1.1 Villagómez DA & Alonso RA (1998) A distinct Mendelian autosomal recessive syndrome involving the association of anotia, palate agenesis, bifid tongue, and polydactyly in the dog. Can Vet J 39(10):642-643
  2. Horrorzine
  3. Ostrander EA et al (1993) Identification and characterization of dinucleotide repeat (CA)n markers for genetic mapping in dogs. Genomics 16:207-213
  4. Galis, F et al (2001) Why five fingers? Evolutionary constraints on digit numbers. Trends Ecol Evol 16:637–646
  5. Park K et al (2008) Canine polydactyl mutations with heterogeneous origin in the conserved intronic sequence of LMBR1. Genetics 179(4):2163-2172
  6. Alberch P (1985) Developmental constraints: Why St. Bernards often have an extra digit and poodles never do. Am Naturalist 126:430-433
  7. Senders CW et al (1986) Observations about the normal and abnormal embryogenesis of the canine lip and palate. J Craniofac Genet Dev Biol Suppl 2:241-248
  8. Montgomery M & Tomlinson J (1985) Two cases of ectrodactyly and congenital elbow luxation in the dog. J Am Anim Hosp Assoc 21:781–785
  9. Fogle, B (2000) Domestic Dog Breeds: The New Encyclopedia of the Dog. Dorling Kindersley, London
  10. Barrand KR et al (2004) Ectrodactyly in a West Highland white terrier. J Small Anim Pract 45(6):315-318