Agouti ("A") is the genetic modifier that acts on the dominant form of the Extension locus (E) to create Bay.

"E" dominant horses are normally Black. But in the presence of Agouti, black pigment is restricted to the "points:" the lower legs, mane and tail, leaving the main body and head as red.

Jeremy Dunn, an Extreme Mustang Makeover finalist, with his beautiful bay mustang, Ojos.

AGOUTI - the BAY Modifier

These Bay Mustangs at a BLM adoption show the range of Bay colors - from bright orange to dark brown. All have the black mane, tail, and lower legs that signify Bay.

Bays are born with brown or red bodies but their manes and tails may be light-colored, and their lower legs are very light. They will all darken to black as they mature:

L - R: Newborn Foal with white legs and brown/red mane and tail, weanling dark mane and tail but still splotchy legs, yearling with black mane and tail and nearly black legs


While BAY is the result of a genetic modifier acting upon a BLACK base (Actually "Dominant "E"), BAY itself can be modified to create these color patterns:

Buckskin is Bay diluted by the Creme Gene

Dun is bay with the dun gene. Bay Duns are also called Zebra Duns

Zebra (Bay) Dun Kiger horse owned by Kim Bauer of Oregon

Bay Roan is Bay with Roan gene. (red body, black points)

Red (Bay) Roan adopted by Pat Hyatt of California 

Champagne on Bay

Callie, owned by Cathy Hill

Silver on Bay

Pinto Patterns on Bay

LIVER CHESTNUT is a red-based color pattern that can closely mimic Bay.  Liver Chestnut horses are dark red to almost black in body color, with darker, sometimes black, manes and tails. Their lower legs, however, are not black and thus they can be distinguished from bays by their lower legs.

If you see a Bay-like horse whose points are not quite black, or a Bay-like horse with black mane and tail but red legs, that is a Liver Chestnut.

This liver chestnut Morgan would be a bay except his lower legs are not black



Black or Red is determined on the section (Locus) of the DNA strand that is called the Extension Locus. The Extension locus can be either dominant or recessive. It is called "E" (if dominant) or "e" (if recessive) in genetic notation.

Horses with two recessive "e" "genes" are red.

Dominant "E" is normally black. However, the dominant "E" gives the horse the capability of producing both red and black hairs. This happens when "E" is modified by the "A" or Agouti gene.

The "A" gene (Agouti) restricts black (which would otherwise cover the entire horse) to the "points," leaving the body red.

Agouti has no effect on a Red base coat, since Red has no black to restrict. Agouti can be carried by a red horse, however, and transmitted to offspring. Thus, a red and a black horse can produce bay offspring if the red horse carries Agouti.

University of California, Davis, began in 2003 to offer a test for Agouti (Bay/Black).

You can download forms for this and other tests from their website-- follow
the links from

Wild-Type Bay ("A+")

Some geneticists, including Sponenberg, contend that a third allele is possible at the Agouti locus. It is denoted as "A+" and is called "Wild Type Bay."

The Wild-Type allele A+ expresses itself in that the points (particularly the lower legs) tend to have less black than regular bay color. The black portions of the cannon and lower leg are mixed with (sometimes very diluted appearing light golden) red pigmentation rather than being solid black.

Wild Type Bay also refers to an effect that mimics, and may also be partially caused by, the Pangare/Mealy gene, with the muzzle, underside, flanks, and behind the elbow lightened.

Angelo, a "Wild-Type Bay" brown horse.
Wild-Type Bay is, not surprisingly, fairly common in wild horses - but is in no way restricted to them.

Preliminary testing by a laboratory in France indicates that a form of the AGOUTI gene ("A+" or "Wild Type Bay") may be the agent that creates the Seal Brown pattern, which has up until now been attributed to the Pangare gene working on a Black base.

Apparently, all Seal Browns so far have tested by the French lab are positive for Agouti, indicating that they are perhaps a form of very dark, sooty Wild Type Bay.

-from "The Horse" Second Addition. Authors J. Warren Evans, Anthony Borton, Harold F. Hintz, L. Dale Van Vleck.
Copyrighted 1990. On page 479-481.

However, if you breed 2 Seal Browns you don't necessarily get a Bay, which indicates that more research is needed.

- Thanks to Joycelyn Kasmir for this information!


Color genetics are the same for all horses, regardless of breed or ancestry.

Since this is a Mustang website,
I use and prefer pictures of wild, or formerly-wild horses wherever possible.


Bay overo pinto

Typical Bay coloring

Genetic Technicals (in case you are feeling the need to become confused....):
  • Dr. Philip Sponenberg (pp. 104-109, HORSE COLOR by D. Philip Sponenberg and Bonnie V Beaver) describes the genetics of Bay as:
    " AB-CC-dE - -sty"
    which breaks means: * first gene pair AB: A (for Agouti) restricts color to the points, and B is the color Black;
    * second pair CC= doubly non-diluted;
    * dE= non-dilute with colored points;
    * -sty=clear, non-smutty body color.
    In other words, Bay is Black with a modifier ("A") that restricts the black to the points.

  • Dr. Ann Bowling of UC Davis describes the genetics of Bay as:
    E, A, CC, dd, gg, ww, toto
    which means it is Black (E, for eumelanin, the black pigment), with Agouti (A), doubly undiluted (CC) not dun ("dd"), not white ("ww") not Tobiano (toto);
    In other words, Bay is Black with Agouti, which restricts black to the points

They're describing the same thing, just using different letters.

Hit Counter