By Nanna M. Andersen | Published: 27.11.21 | Edited: 19.02.24 The basic coat colourations are defined by the ratio of the two pigments eumelanin (black/brown) and pheomelanin (red/orange/yellow). Their ratio is primarily controlled by the agouti signalling protein (ASIP) and melanocortin 1 receptor (MC1R) ligand-receptor system, which determines the ratio between the two types of melanin. Therefore, basic coat colouration can range from black/brown (dominance of eumelanin) to red/orange/yellow (dominance of pheomelanin) including all kinds of intermediate phenotypes depending on the ratio of eumelanin to pheomelanin, because the basic colouration is further influenced by additional coat-colour-associated genes resulting in a huge variety of different phenotypes. According to a paper by Tomohisa Hirobe, both pigments were found in the hairs of Agouti, Black, Pink-eyed agouti, Pink-eyed black, Recessive Yellow, and Pink-eyed Recessive Yellow mice. We will focus on Experiment 2. F2 (pink-eyed black x agouti). While he found both yellow and black pigments in all coat colors, some exhibit such minute amounts of yellow pigment that it was negligible. One of these were Black. The increase in yellow pigment in the Agouti is attributed to the banding of the ticked hairs. His study measured Total melanin (TM), black pigment (Eu), and yellow pigment (Pheo), and the results are depicted in the picture below. In another study by Autumn J. Bernal and Randy L. Jirtle they study the dominant A^vy allele in mice, located on the Agouti locus, which causes the production of yellow pigment instead of black pigment, resulting in a dominant yellow coat. This type of yellow coat is associated with serious health issues, which is unfortunate. Unlike the dominant A^vy allele in mice, we typically encounter recessive Yellow genes in other species, although this gene appears to be extinct within the fancy rat population, it did once exist. In 1914, W. E. Castle documented a Black-eyed Yellow (yy). I truly wish we had this gene. In other species, the most common recessive Yellow gene typically exhibits the genotype "ee" and is found on a locus called "Extension." The "ee" genotype results in a Yellow phenotype, as seen in the picture below. The picture illustrates the various shades in which Yellow can manifest in rabbits and guinea pigs. The Blue phenotype The Dilution locus (D) is controlled by the melanophilin (MLPH) gene, it's not known to me what Grey locus is controlled by and how similar their pigment distribution actual is. The Blue phenotype decreases pigment saturation of the black pigment to a metallic grey-blue. Eumelanin (black pigment) is more greatly affected than pheomelanin (yellow pigment) which shows on Blue Agouti rats were the black/brown pigment is greatly diluted while the yellow/orange banding is only slightly diluted. References
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