Silkie the "SHH" gene, called the ZPA Regulatory Sequence (ZRS). This causes ectopic "SHH" expression in the anterior of the developing limb bud, leading to increased tissue growth and digits. While the feet of the Silkie display polydactyly, the wings have the standard tridactyly (three digit) arrangement. The Japanese Silkie initially develops additional digits in the wing as an embryo, but these are lost prior to hatching. The genetic cause behind Silkie polydactyly differs from those that cause polydactyly in the Dorking chicken breed, which is due to ectopic "FGF4" expression in the AER, with ectopic "SHH" a secondary effect. The
The Great Silkie of Sule Skerry is the title of the Orcadian texts, about twice in length. There is also a greatly embellished and expanded version of the ballad called "The Lady Odivere". "The Great Silkie of Sule Skerry" was collected from a lady from Snarra Voe, Shetland, and 7 verses from its transcription was published by Capt. F. W. L. Thomas in the 1850s. It was later included in Francis James Child's anthology, and catalogued as Child ballad number 113. Alan Bruford has noted that "silkie" is an anomalous spelling for "selkie", and in other ballad specimens, the mythical being instead of being a "great"