In other species: cattle , dog , horse , red fox , pig , sheep , jaguar , jaguarundi , American black bear , woolly mammoth , domestic cat , rabbit , goat , Arctic fox , rock pocket mouse , oldfield mouse , gray squirrel , lesser earless lizard , little striped whiptail , water buffalo , domestic yak , alpaca , , coyote , reindeer , Geoffroy's cat , Colocolo , ass , Arabian camel , Mongolian gerbil , raccoon dog , fallow deer , zebu , lorises , antarctic fur seal

Possibly relevant human trait(s) and/or gene(s)s (MIM numbers): 266300 (trait) , 155555 (gene)

Mendelian trait/disorder: yes

Mode of inheritance: Autosomal Recessive

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; now 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. As explained by Schneider et al. (PLoS Genet 10(2): e1004892; 2015), "The most common causes of melanism (black coat) mutations are gain-of-function alterations in MC1R, or loss-of function alterations in ASIP, which encodes Agouti signaling protein, a paracrine signaling molecule that inhibits MC1R signaling". Mutations in MC1R have been associated with white colouring in several species.

Molecular basis: By sequencing the MC1R gene in brown, black and bi-colour guinea pigs, Vidal (2018) identified one brown allele (e; MC1R*2) as c.749T>C, p.Leu250Gln and another brown allele (e; MC1R*4) as "a deletion of 2760 bp, including the entire MC1R coding region".

Associated gene: