Base Colors
What is Extension?
The Extension locus, also known as red factor, is the Melanocortin 1 Receptor (MC1R) gene which is responsible for how red and black pigment is produced. A red horse must be homozygous for recessive extension allele (e/e), while a black or bay horse may be either homozygous or heterozygous for the dominant extension allele (E/E or E/e).
What is Agouti?
The Agouti locus, or the agouti-signaling protein (ASIP), is responsible for how black pigment is distributed. Dominant Agouti (A/A or A/a) will restrict black pigment to the points of the horse, including the mane, tail, legs, and ear tips. Recessive Agouti (a/a) will distribute black pigment throughout the entire body of the horse.
Black
In order to be black, a horse must be E/E or E/e and a/a.
Bay
In order to be bay, a horse must be E/E or E/e and A/A or A/a.
Red
In order to be red, a horse must be e/e and any combination of Agouti.