The Silkie is a unique chicken breed recognized for its remarkably soft, fluffy plumage, which lacks the stiff barbicels of normal feathers, giving it a fur-like appearance. Beyond its distinctive coat and dark, pigmented skin, the Silkie possesses an unusual anatomical feature: five toes on each foot. This extra toe is rooted in a specific, well-documented genetic alteration that has been selectively maintained by breeders for centuries.
The Standard Chicken Foot Versus the Silkie’s Fifth Digit
Most domestic chickens, descended from the Red Jungle Fowl, exhibit a foot structure with four digits. This typical configuration includes three toes pointing forward (digits two, three, and four) and one backward-pointing toe, known as the hallux (digit one), which aids in perching.
The Silkie, however, displays a condition called polydactyly, meaning “many digits.” The fifth digit typically presents as an accessory toe located just above and behind the hallux. This fifth toe is usually fully formed and separated from the fourth toe, a specific physical requirement outlined in breed standards. Its presence is a defining characteristic of the Silkie breed, differentiating it from the four-toed majority.
The Genetic Basis of Polydactyly
The development of the Silkie’s fifth toe is caused by a single, dominant genetic mutation. This mutation involves a regulatory sequence called the Zone of Polarizing Activity Regulatory Sequence (ZRS). The ZRS is an enhancer region located within an intron of the LMBR1 gene, which is far upstream of the signaling gene, Sonic Hedgehog (SHH).
During the embryonic development of the limb bud, the SHH gene sets up the anterior-posterior axis, determining the number and position of digits. In normal chickens, SHH expression is tightly controlled and confined to the posterior side of the developing limb. The Silkie’s ZRS mutation disrupts this control, causing the SHH signaling molecule to be expressed ectopically, or in the wrong place, specifically on the anterior side of the limb bud.
This abnormal, duplicated expression of the SHH signaling pathway creates a second organizing center in the limb bud. This leads to the formation of an extra toe structure. Because the mutation is dominant, a Silkie only needs to inherit one copy of the altered gene from either parent to express the five-toed trait.
Why Breeders Maintain This Unique Trait
For Silkie chickens, the five-toe structure is not considered a deformity but a mandatory characteristic of the breed standard. Poultry associations, such as the American Poultry Association, require the fifth toe for a Silkie to be eligible for exhibition and judging. The standard specifies a well-separated fifth digit as a sign of purity and quality within the bloodline.
Breeders actively select for this polydactyly trait to maintain the distinctiveness of the Silkie. Birds that only possess the standard four toes are often disqualified from shows, providing a strong incentive to ensure the continuation of the dominant mutation. This selection pressure has cemented the extra toe as a hallmark of the breed’s identity. The polydactyly gene is also found in other breeds, including the Dorking and Faverolles, demonstrating how human selection can turn a genetic anomaly into a defining breed feature.