Agouti coloration genetics

The agouti protein causes red to yellow pheomelanin to be produced, while the competing molecule α-MSH signals production of brown to black eumelanin.

[7] The mahogany locus interacts with Agouti and a mutation there can override the pigmentation and body weight effects of lethal yellow.

The hair cycle promoter involved in these colors is thought to have arisen about 2 million years ago in an extinct species of canid, which later hybridized with wolves.

In normal horses, ASIP restricts the production of eumelanin to the "points": the legs, mane, tail, ear edges, etc.

In 1951, zoologist Miguel Odriozola published "A los colores del caballo" in which he suggested four possible alleles for the "A" gene, A+, A, At, and a, in order of most dominant to least.

[25] Pangaré is an ancestral trait also called "mealy", which outlines the soft or communicative parts of the horse in buff tan.

It is still hypothesized that to some extent, the darkening of coat color in some bays may be regulated by unrelated genes for traits like "sooty".

[31] Most donkeys have creamy to gray-white areas on the belly and around the muzzle and eyes, called light points or pangare.

[32] In rabbits, the wildtype is agouti with a light belly, and a recessive non-agouti allele causes a black coat.

A third allele, possibly a mutation to a regulator or promoter region, is thought to cause black and tan color.