The agouti gene is very important in the determination of horse color because it regulates the distribution of the black pigment. The effects of the Agouti (A) gene are therefore only visually obvious in the presence of the Black (E) allele. The dominant Agouti allele -- A -- restricts the distribution of black pigment in hair to a points pattern (so any black that shows up on the horse will be found mostly on the legs, mane, tail and ear rims). The Agouti color distribution is found on horses that are bay or a dilution of bay (buckskin, perlino, amber champagne, zebra dun, etc.).

The recessive allele -- a -- causes the distribution of black pigment over the entire horse and, when homozygous in the presence of the E, produces a uniformly black horse.

Regardless of what color it is bred to, any horse who is homozygous for the Agouti gene i.e., has a double Agouti AA allele -- will NEVER produce a solid black or smoky black foal. In fact, a horse who is homozygous for Agouti will always produce foals, either with the black restricted to the points (bay, buckskin, perlino or amber champagne), or with no black at all (i.e., a chestnut, palomino or cremello foal is possible if both parents carry an "e" gene) who also carries an Agouti gene. A horse who is homozygous for the Agouti gene will ALWAYS pass one Agouti gene to every single foal that the horse produces.

A Perlino with a double Agouti will never sire a Smoky Black or Smoky Cream foal because those colors cannot have an Agouti gene.