Polydactyly

Polydactyly is a gene in chickens that causes the bird to have a fifth toe on the back of the foot, near the fourth toe (1)(2). There are actually two separate genes that are responsible for this in chickens. One is found in Asiatic breeds, while the second is from European breeds (4). These two, while distinct genes, both act in the same way (4) and were only discovered with advanced gene mapping techniques in chickens. They also are located on very close loci on Chromosome 2(4).

In both cases, the polydactyly is caused by the changes to the Sonic Hedgehog protein, the same protein that controls the finger and toe growth in cattle, pigs, sheep, mice, cats, dogs, and even humans. (9)(11). Strangely, this is a different gene than found in some ducks, including Muscovy. (10) The fifth toe is not recognized in any duck breeds. The increase in protein during embryo growth is what causes this mutation in both lines (5)(9).

Polydactyly is completely dominant, meaning that the chicken will have five toes whether it has one copy of the mutation or two (8). The small toe protruding from the stub of the fourth toe that sometimes crops up in heterodactyly breeds is actually caused by a separate mutation (11) that is unrelated to polydactyly though is often hiding in Silkie and other breeds.

The APA recognizes the Houdan (European), Dorking (European), Silkie (Asiatic), and Favorelles (European) to have five toes (12). The Chinese You and some forms of commercial broilers also have the polydactyly gene. (11)

Breeding with Polydactyly:

Note: Because it is completely dominant, you cannot tell between birds with one or two copies of the polydactyly gene. For the sake of clarity here, Polydactyly refers to birds with two copies of the gene, and Five Toes refers to birds with one copy of the gene.

Polydactyly x Polydactyly = 100% Polydactyly

Polydactyly x Wildtype = Five Toes

Five Toes x Five Toes = 25% Polydactyly, 50% Five Toes, 25% Wildtype

Five Toes x Wildtype = 50% Five Toes, 50% Wildtype