Molecular genetics of coat color in livestock: insights into comparative biology and applications for animal products traceability

From a biochemical and physiological point of view, pigmentation in mammals has its base in the presence or absence of pigments, melanins, in hair and in the skin. Melanins are pigments with a different molecular weight and are formed by enzymatic oxidation of the tyrosine amino acid from which two kind of pigments derive: eumelanins (black/brown pigments) and pheomelanins (yellow/red pigments). Pigmentation is essentially determined by the distribution of these two pigments, respectively producing a black/brown and a yellow/red colour. Metabolic pathways leading to the synthesis of these two types of melanins are mostly known. Melanins are synthesized and accumulated within melanosomes, which are particular organelles of the cytoplasm of specialized cells, the melanocytes, lying between derm and epiderm. Afterwards, melanosomes are transferred into hair during their growing up, by means of an exocytosis process. During embryo development, melanocytes, starting from the neural crest, migrate in the different parts of the body, conferring pigmentation on the areas where they are present. In the areas where melanocytes are missing, white spots appear giving the characteristic spotting of some cattle, pig, horse and rabbit breeds. Moreover, in some parts of the body pigmentation can be modified depending on a more or less reduced melanocytes activity. The first studies on farm animals coat colour were made at the beginning of 1900. They come immediately after Mendel laws recovery. These researches were followed by some other studies, stating colour homologies among various mammals and since then these schemes are used to describe coat colour in livestock (Searle, 1968). The main loci, identified by means of several crossings and studies of segregation of different colours are: Agouti (A), Extension (E), Albino (C), Brown (B), Dilution (D), Roan (R), Silver and Spotted (S), each of them has an own allele series (Searle, 1968). The relative proportion between the two kinds of melanin is controlled by the loci Extension (E) and Agouti (A), which show epistatic effects. In several mammals, dominant alleles at the E locus produce a black coat colour while recessive alleles produce a red/yellowish colour. Alleles at the A locus determine a recessive black colour when the wild allele, but not the dominant or recessive allele, is present at the E locus. Other loci determine pigmentation extension and intensity. Among them the W locus (White Spotting), which some studies refer to the S locus (Spotted), acts on spotting extension, the Roan locus (R) determines the homonymous colour, the Dilute locus acts diluting pigmentation, the Silver locus determines follicular melanocytes loss, resulting in a grey coat color, the Brown locus determines brown colour, etc. A more complete list of loci affecting coat colour in mammals is reported by Searle (1968).