Coat Color in Horses

Horses come in a wide variety of color morphs, and these are produced by the interaction of several different genes.

Extension ("red factor") and Agouti
Two genes interact to produce the most common horse coat colors. The extension locus, also known as the "red factor" controls the production of melanin pigments. The dominant allele (E) encodes the production of brown/black eumelanins, and the recessive allele (e) encodes the production of yellow to red phaeomelanins.

• genotype E- produces black/brown coat
• genotype ee produces chestnut.

The agouti locus, which you've already met in other mammals (gerbils) controls the deposition of eumelanin. In horses, the dominant allele of the gene (A) results in eumelanin deposition only at the "points"--mane, tail, nose, ear rims and legs--and the recessive allele (a) results in eumelanin deposition all over the body.

• genotype A- produces bay pigmentation
• genotype aa produces uniform pigmentation.

In the absence of any modifier loci affecting their expression, the E and A loci can combine to produce the following basic horse colors:

genotype EE A- will produce a brown bay:

genotype Ee A- will produce a red bay:

genotype ee -- will produce a chestnut/sorrel horse. (The genotype of the A locus is irrelevant, as the animal cannot produce eumelanin.)

genotype E- aa will produce a black horse.

---

Modifiers
Modifiers are genes that affect the degree to which a gene at another locus is expressed. In horses, there are several modifiers, including

• champagne - a dominant allele that dilutes hair pigmentation. Black (eumelanin) is diluted to brown, and red (phaeomelanin) is diluted to gold.

• cream - wild type (c) is recessive, and this allele does not dilute the hair pigmentation. A mutant allele (C^cr) exhibits incomplete dominance: In heterozygous condition, (CC^cr) it dilutes red to yellow. In combination with other genes, this will produce palomino (dilute chestnut), buckskin (dilute bay), or "smoky" black (dilute black).
  
  In homozygous condition (C^crC^cr), it dilutes pigment further, producing cremello, perlino, or smoky cream).
  
  These are the closest that horses come to albino: A true albino (complete lack of melanin) has never been confirmed in Equus caballus. Strange, but true!

  There are far more horse colors and interacting genes that cause them than we will cover here. But if you're interested here's a fun toy for horse breeders, and a listing of horse colors if you're not sure what they are.