In other species: , coyote , dog , red fox , American black bear , domestic cat , jaguar , ass , horse , pig , reindeer , cattle , goat , sheep , rabbit , domestic guinea pig , domestic yak , alpaca , gray squirrel , woolly mammoth , rock pocket mouse , oldfield mouse , lesser earless lizard , Geoffroy's cat , Kodkod , jaguarundi , Colocolo , little striped whiptail , water buffalo , Arctic fox

Possibly relevant human trait(s) and/or gene(s) (MIM number): 266300

Mendelian trait/disorder: yes

Considered a defect: no

Key variant known: yes

Year key variant first reported: 2018

Cross-species summary: The extension locus encodes the melanocyte-stimulating hormone receptor (MSHR; also known as MC1R). This receptor controls the level of tyrosinase within melanocytes. Tyrosinase is the limiting enzyme involved in synthesis of melanins: high levels of tyrosinase result in the production of eumelanin (dark colour, e.g. brown or black), while low levels result in the production of phaeomelanin (light colour, e.g. red or yellow). When melanocyte-stimulating hormone (MSH) binds to its receptor, the level of tyrosinase is increased, leading to production of eumelanin. The wild-type allele at the extension locus corresponds to a functional MSHR, and hence to dark pigmentation in the presence of MSH.

Molecular basis: Adopting the comparative candidate gene approach, Almathen et al. (2018) sequenced the MC1R gene in "91 animals from ten Arabian camel populations, that included the four predominant coat color phenotypes observed in the dromedary (light brown, dark brown, black and white)". They identified a missense variant (c.901C>T; p.R301C) associated with white versus non-white: "All nonwhite individuals (black, dark, and light brown, n = 70) were homozygous for the 901C allele, whereas all white dromedaries (Wodh population, n = 21) were either homozygous for the 901T (n = 8) or heterozygous (n = 13) for the 901C/T alleles . . . (P < 0.0001, df = 1)."

Associated gene: