Skin and Shank Color
Genetics Mini-Series Article #10
Shank color genetics in chickens is anything but straight-forward. Traits believed to be dominant a century ago are now considered recessive. Traits which appear to be governed by one gene now appear to be one gene with five different loci. Some believe shank color to be controlled by two traits, others by three, and still others by four, and all agree that the expression of these traits is affected by a plethora of plumage genes and yet-unidentified modifiers. So, with that in mind, I’ll present my best stab at a working theory and encourage further research to those interested in shank color.Skin Color
Some describe skin color (yellow or white) as being restricted to the epidermis, or top layer of skin. Others say that skin color is present in both the epidermis and dermis (deeper layers of skin). You can often see a reddish color in addition to the skin color. This is often from increased blood flow due to hormones, most noticeable in a rooster’s shanks during mating season. A reddish tone, sometimes called horn, can also be a secondary effect of reddish or golden plumage.
Yellow skin is recessive (w). The yellow pigment is from carotenoids which are consumed by the chicken. A diet rich in carotenoids (often in the form of greens and corn) will enhance the yellow pigmentation. The yellow pigmentation in a hen’s shanks will fade throughout her laying cycle as nutrients are pulled into her eggs from the reserves in her body. Yellow pigmentation will also fade with age.
White skin is dominant and is the wild type (W+). White skin is characterized by a lack of yellow pigmentation.Dermal Melanin
Dermal melanin is a dark pigment in the deeper layers of skin. The presence of dermal melanin is recessive and is the wild type (id+). The absence of dermal melanin is dominant (Id). Dermal melanin is a sex-linked gene. Males carry two copies and females carry one. Dermal melanin itself is controlled by five different alleles which affect its expression and do not seem to be entirely understood (Id, ida, idc, idM, id+; see here). Dermal melanin comes in slowly and is usually not present in chicks (excepting those with birchen or extended black, explained more below). The best time to identify the presence of dermal melanin is from 8 to 16 weeks of age as dermal melanin can fade over time.
Factors inhibiting dermal melanin
Certain plumage patterns inhibit the expression of dermal melanin and may reduce or hide the presence of the id gene entirely. Barring (B) is one of these genes. Because barring is also sex-linked, males with two copies of the barring gene (B/B) will probably not show dermal melanin at all while females of the same line (B/-) will show some dermal melanin.
Other factors inhibiting the expression of dermal melanin to varying extents include: mottling (Mo), dominant white (I), recessive white (c), blue and splash (Bl), gold dilute (Di), silver (S), and wheaten (EWh). A light undercolor in the plumage, visible in the ‘fluff,’ can be evident where plumage genes dilute dermal melanin.
Factors enhancing dermal melanin
Base colors which extend areas of eumelanin tend to extend the black pigment into the shanks as well. Some people call this dermal melanin, but others seem to think that this melanin is actually spread into the epidermis rather than the dermis. Base colors which enhance dermal melanin include extended black (E) and birchen (EB) to a lesser extent.Other melanizers
Other genes can cause a darker pigment in the shanks, such as the dominant Fibromelanosis (Fm) gene used to darken the skin of Silkies.
Resulting Genotypes and Phenotypes
This chart describes leg color very generally, using only the W-gene, the Id-gene, and a bit of information from the E-series alleles.
These are common shank colors as they are sometimes called. Dusky white, dusky yellow, and straw are not commonly used terms. Dusky white might be characterized as simply white or slate, dusky yellow as willow, and straw as yellow or light yellow.
Also see GMS Supplement #2: Skin Color Punnett Squares and Test Crossing for visuals of the inheritance of white and yellow skin and a discussion of how to breed out unwanted recessive traits.
Shank Color References and Resources
This is certainly not an exhaustive explanation of shank color genetics and may not even be entirely accurate. Here are the resources I used which may be a great springboard for further investigation: