Genes' Impact On Coat Color

The colour of an animal's coat is determined by its genes. In dogs, for example, there are about 19,000 genes in their genome, but only a handful affect the physical variations in their coats. Similarly, in horses, only a few genes are responsible for the many different colour variations.

The two basic pigments that determine the colour of canines are eumelanin (black) and phaeomelanin (red). All different colour variations are created by these two pigments, which are both forms of melanin. Melanocytes are the cells within the hair follicles that add melanin to the hair as it grows and determine the basic coat colour. The more melanin, the darker the colour.

The role of Melanocortin 1 Receptor (MC1R) and Agouti Signalling Protein (ASIP) in determining coat colour

The melanocortin 1 receptor (MC1R) and agouti signalling protein (ASIP) are two of the key proteins involved in regulating mammalian skin and hair colour. The MC1R protein lies within the cell membrane and is signalled by melanocyte-stimulating hormone (MSH) released by the pituitary gland. When activated by one of the variants of MSH, typically α-MSH, MC1R initiates a complex signalling cascade that leads to the production of eumelanin. In contrast, the receptor can also be antagonised by agouti signalling peptide (ASIP), which reverts the cell back to producing the yellow or red phaeomelanin.

The MC1R gene is located on chromosome 18 in cattle and is responsible for melanic polymorphisms in at least three unrelated species: the bananaquit, the snow goose, and the arctic skua.

The ASIP gene is located on chromosome 20 in humans and is responsible for pigmentation genetics.

The yellow and black agouti banding pattern observed on most mammalian hair is caused by the pulsative nature of ASIP signalling through MC1R. Exceptions include particoloured bay horses, which have reddish bodies, and black legs, mane, and tail, where ASIP signalling is limited to regions instead of pulsating. Human hair, which is neither banded nor particoloured, is thought to be regulated by α-MSH signalling through MC1R exclusively.

The MC1R and ASIP genes have been found to be associated with coat colours in the Massese sheep breed.

The impact of melanin and its two types: eumelanin and phaeomelanin

Melanin is a substance in the body that produces hair, eye, and skin pigmentation. The amount of melanin in the body depends on factors such as genetics and sun exposure. It is produced in melanocytes, which are located in the innermost layer of the skin, pupils, irises, and parts of the brain, inner ear, and adrenal gland.

There are three types of melanin: eumelanin, pheomelanin, and neuromelanin. This answer will focus on the first two types, which are responsible for pigmentation.

Eumelanin

Eumelanin is the most common type of melanin and is responsible for dark colours in skin, eyes, and hair. It comes in two forms: black and brown. The amount of eumelanin in the body determines the darkness of these features. For example, people with brown or black hair have varying amounts of brown and black eumelanin. When there is no black eumelanin and only a small amount of brown eumelanin, it results in blonde hair.

Pheomelanin

Pheomelanin is the second type of melanin and is responsible for pigmentation in the lips, nipples, and other pinkish parts of the body. People with equal parts eumelanin and pheomelanin tend to have red hair. Pheomelanin also contributes to skin pigmentation, giving those with red hair a more pinkish hue.

The Impact of Melanin

The unique combination of eumelanin and pheomelanin is responsible for an individual's skin, hair, and eye colour. While the amount of melanin produced varies, all humans typically have the same number of melanocytes. People with more melanin generally have darker skin, eyes, and hair compared to those with less melanin.

In addition to pigmentation, melanin also provides protection from harmful UV rays. When spending time in the sun, the body produces more melanin, which absorbs UV rays and redistributes them toward the upper layers of the skin, protecting the cells from sun damage. However, it is important to note that melanin alone is not enough to protect the skin, and wearing sunscreen and appropriate clothing is crucial.

Melanin also offers protection against reactive oxygen species (ROS), which are byproducts of the body's cell processes. By scavenging for ROS, melanin boosts antioxidants and eliminates free radicals, helping to prevent stress, premature ageing, and health issues such as diabetes and cancer.

How genes affect the distribution of melanocytes and create patterns of white spotting or speckling

Melanocytes are melanin-producing neural crest-derived cells located in the stratum basale of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and heart found in many mammals and birds. Melanin is a dark pigment primarily responsible for skin colour. The amount of melanin in the body depends on factors including genetics and how much sun exposure one's ancestral population had. Melanocytes produce two types of melanin: eumelanin (black or brown) and pheomelanin (reddish-yellow). The ratio of eumelanin to pheomelanin determines skin colour.

The life cycle of melanocytes consists of several steps, including the differentiation of melanocyte lineage from neural crest cells, migration and proliferation of melanoblasts, differentiation of melanoblasts into melanocytes, and proliferation and maturation of melanocytes at target places. Melanocytes of the epidermis and hair are cells that share some common features but generally form biologically different populations living in unique niches of the skin.

The embryonic development of melanocytes gives an opportunity to better understand skin diseases such as melanoma and vitiligo. Melanocytes in the epidermis and hair follicles are controlled by different genetic and epigenetic factors derived from keratinocytes, fibroblasts, melanocytes, the pituitary gland, other organs, and environmental factors such as UV radiation.

The precise mechanisms that control the organisation and number of melanocytes in the epidermis are unknown. However, it is known that melanocytes, keratinocytes, and dermal fibroblasts communicate with each other through secreted factors and cell-cell contacts. Keratinocytes control melanocyte growth and activity through a system of paracrine growth factors and cell adhesion molecules. Dermal fibroblasts secreted factors such as stem cell factor (SCF) and neuregulin 1 (NRG1) influence the growth, pigmentation, shape, dendricity, mobility, and adhesive properties of melanocytes.

The distribution of melanocytes in the skin depends on the environment (mainly UV radiation) and factors secreted by keratinocytes and fibroblasts. The density of melanocytes in the skin is also influenced by age, with a decrease in melanocyte density of 10-20% every decade after 30 years of age.

In summary, the distribution of melanocytes and the creation of patterns of white spotting or speckling are influenced by a combination of genetic, epigenetic, and environmental factors that regulate the proliferation, differentiation, and maturation of melanocytes in the skin.

The function of the agouti locus in turning bay to black, and the extension locus in turning bay or black to chestnut

The agouti locus and the extension locus are genes that, in combination, determine a horse's base colour. The agouti gene controls the distribution of black pigment and determines whether a horse will have a bay or black base coat colour. The agouti locus has three alleles: 'A', which is dominant and confines the black hair to the points (ear rims, lower legs, mane and tail) to produce a coat colour called Bay; 'At', which is a possible third allele that has been recognised to explain the colour Seal Brown; and 'a', which is recessive and results in a uniformly black horse.

The extension locus controls the production of the black pigment 'eumelanin' in the melanocytes. If the gene is altered and non-functional (ee), the black pigment cannot be produced and the horse has a chestnut coat colour. Horses with one or two intact copies of the extension gene (EE, Ee) are either bay or black depending on the agouti locus.

The combination of the agouti and extension loci determines the three base colours that occur in horses: chestnut (reddish), black, and bay (brown). A change at the agouti locus is capable of turning bay to black, while a mutation at the extension locus can turn bay or black to chestnut.

The influence of the K (dominant black) locus on coat colour

The K-Locus, also known as the dominant black gene, is due to a mutation in the Beta-defensin gene (CBD103). This gene binds with proteins and other pigment cells to produce the different variations of the K-Locus. The K-Locus is named for the solid black coat it can cause in dominant black dogs. The K locus is dependent on the E-Locus. If the E-Locus genotype is e/e (recessive), the K-Locus is not expressed. However, if the E-Locus is coded as E/E or E/e, the K-Locus is still expressed.

The dominant black gene consists of three different alleles, or variants. The first allele, which is dominant, is notated as "KB," or dominant black. The dominant black allele is actually a mutation that reduces or eliminates the expression of the agouti gene (A-Locus). Because this mutation is dominant, a dog only needs to have one copy of the mutation to affect the agouti locus. If a dog is KB/KB or KB/n that means that they will be solid black in colour. The second allele is known as the "brindling" allele, and is represented as "Kbr." The Kbr allele is a separate mutation that allows the A locus to be expressed. However, the expression causes a brindling of the agouti patterns. The A locus represents several different colours, such as fawn/sable, tricolor, tan points, or recessive black. The Kbr allele is recessive to the KB allele. This means that if a dog’s genotype is KB/K­br, they will still be black in colour. Kbr is, however, dominant over the third allele, Ky.

The third allele is represented as "Ky." This allele allows the agouti gene to be expressed without brindling. If a dog is Ky/Ky at the K locus, the A locus then determines the dog's coat colour. The Ky allele is recessive to both KB and Kbr. This means that if the dog has a genotype of KB/Ky or Kbr/Ky, the dog will not express the A locus like a Ky/Ky dog would. A KB dog would be black and the Kbr dog would express a brindled A-locus allele. For example, a dog that is Ay/Ay at the A locus could be fawn/sable if the dog is Ky/Ky. However, if that same dog is KB/KB at the K locus, the A locus expression will be hidden. Its coloration will be determined at the B and E loci and there is a good chance the dog will be black. However, if that same dog is Ky/Ky at the K locus, it will then be able to express the coloration of the A locus, and will be fawn/sable.

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