The shank color on poultry can be black, blue, green, yellow or white. Generally speaking, the cause of shank color variations in chickens are a combination of genes that influence melanin pigmentations, polygenic modifiers, and environmental factors such as diets.
Some factors that affect shank color are inhibitor of dermal melanin (Id), extended black (E), white skin (W), yellow skin (w), and other genes such as sex-linked barring (B) and dominant white (I). The skin of a bird consists of two layers, the epidermis and dermis. Pigment can be present in one or both layers. The above genes affect the presence or absence of melanin and xanthophylls in the dermis and/or epidermis.
The E locus affects the relative distribution of melanin pigmentation in the epidermis. E extends melanic pigment to the shanks. Black shanked birds have varying amounts of pigment in the stratum corneum. The stratum corneum is the outer layer of the epidermis. You can assess the skin color on your black shanked birds by observing the foot pads. In blue shanked birds, the stratum corneum is devoid of pigment. A bird that is heterozygous for E may result in blue shanks.
The skin color of poultry can be classified as white or yellow. White skin and yellow skin are inherited differently than genes that affect plumage color. White skin (W) is dominant to yellow skin (w). There is a correlation between yellow and white color of the shank, beak and skin color on the body. Willow shanks are caused by deposition of melanic pigment into shanks that have yellow skin.
Id is a sex-linked inhibitor of dermal melanin. Id (which is the absence of black pigment) and id+ (which is the presence of black pigment) affect the dermal layer of skin on poultry. The id+ gene, either in the homozygous or hemizygous condition, allows the deposition of melanin in the dermis. The id+ gene expresses progressively as a homozygote and shank color changes gradually from light to dark with increased age up to 8 weeks. The Id and id+ genes behave in a sex-linked manner and their locus is on the sex chromosome. The birds which are homozygous, hemizygous or heterozygous for Id eventually have either white shanks (xanthophyll absent) or yellow shanks (xanthophyll present) provided no epidermal melanin is present in the shank skin. In the presence of id+ and absence of epidermal melanin, the shanks of the birds eventually become blue (xanthophyll absent) or green (xanthophyll present). A sex-linked allele, id^a causes green spotting on shanks due to small aggregates of melanin in the dermis. The number of aggregates per scale varies.
The Dominant white gene (I) can remove epidermal shank pigment and dilute dermal pigment. It does not remove dermal pigment. A study found that slate shanks can be achieved on dominant white birds that are homozygous for E and id+.
The barring gene has a restricting effect on melanic pigment in the shanks.
The eWh allele is known to inhibit the expression of dermal melanin. It can reduce and even hide the expression of it.
Besides genetic factors, nutrition has greatly influenced shank coloration in chickens. A number of research studies have found that the main compound with coloring function in meat and eggs are carotenoids. In order to obtain deeper yellow yolk and skin color, many natural pigments, such as corn gluten meal, curcumin and dehydrated alfalfa meal are used in poultry feeds. Dietary supplementation with marigold flower extract at the rate of 200 mg/kg of feed was found to enhance carcass and shank color. Another study in 2010 measured higher skin yellowness in all body parts of chickens fed 12 to 15 mg xanthophylls per kg feed. An additional study in 2004 reported that oxycarotenoids produced by microorganisms enhanced yellowness in the skin of broilers. Canthaxanthin increased pigment in shanks and skin when added at a level of 8.8 mg per kg.
More information will be added to this post as additional genes that influence melanin pigmentations, polygenic modifiers, and environmental factors are explored!
