A locus – Agouti
Agouti encodes a paracrine signaling molecule which causes hair follicle melanocytes to synthesize reddish yellow pheomelanin instead of black or brown eumelanin, so-called pigment type-switching. Agouti signal peptide (ASIP) has several alleles that are involved in coat colour in dogs.
There are four alleles present in dogs in the dominance hierarchy ay > aw > at > a.
One of the most common alleles in domestic dogs is known as the ay allele, inherited as the dominant allele in this series. Dogs carrying an ay allele are uniformly yellow (commonly referred to as fawn, tan or sable), though hairs often have black tips that give a sandy appearance. The ay allele is associated with two ASIP coding variants in 22 breeds, suggesting a single common origin for the allele in dogs.
The at allele in dogs gives rise to a characteristic phenotype with a black dorsum and yellow (or tan) markings on the head, ventrum and/or legs (black-and-tan).
The aw allele, the presumed ancestral ASIP allele (wild type) causes some hairs to have a band of eumelanin, phaeomelanin, eumelanin pigment from base to tip. These banded hairs are typically along the dorsal region of the torso.
Recessive inheritance of the a allele is identified as a coding variant predicted to inactivate the protein, It is the only cause of black in the German Shepherd Dog and Shetland Sheepdog.
Berryere, T.G., Kerns, J.A., Barsh, G.S., and Schmutz, S.M. (2005). Association of an Agouti allele with fawn or sable coat color in domestic dogs. Mamm Genome 16, 262–272.
Kerns, J.A., Newton, J., Berryere, T.G., Rubin, E.M., Cheng, J.-F., Schmutz, S.M., and Barsh, G.S. (2004). Characterization of the dog Agouti gene and a nonagoutimutation in German Shepherd Dogs. Mamm Genome 15, 798–808.