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updated July 2012

      Genetic discussion of Agouti (bay) gene horses: The Agouti gene restricts black hair to the points, that is, the agouti gene makes a black horse into a bay horse. Now we know that the Agouti gene also affects the shades of red horses too. First, let's review the basics of the black and red based horse.

Extension series (E) - causes black or red, whereas the most recessive form of the extension series (e/e) = a red based horse. Any horse carrying an (E) will be black based.

Non-red (black based) parents may have a red foal, because red is recessive to black.

Red bred to red always gives red.


(A) Classic bay. A black horse with the agouti modification that restricts black hair on a black based horse to its points. This is the most dominant of the Agouti series, which explains why classic red bays are more common than the other variants.

(A+) Wild bay. I believe this is the most rare of the Agouti gene variants. This is a black based horse with agouti modification to the greatest extent. Visually similar to classic bay, except the black points are further restricted, in that the black is only up to the fetlocks, or may be absent altogether, along with an overall lighter body color, and some degree of washing out, bleaching or streaking of the mane. Although counterintuitive, A+ is recessive to A. A+ is dominant over At.

(At) Brown. Black horse with agouti modification to the least extent. Recessive to both A and A+. When in homozygous form, (At/At) the horse will have a further bleached (cocoa color) to its hair coat. When I learned this, I first thought that sounded counterintuitive... however upon reflection, to the contrary: This is a doubling of the lightening or restricting effect.

(a) Self. No agouti gene at that loci.


(A/A) (A/A+) (A/At) (A/a) Visually: a classic bay.

(A+/A+) (A+/At) (A+/a) Visually: a wild bay.

(At/At) Visually: usually will be a horse that is not as "true black" color in is body coat as the easier-to-recognize At/a horse; the coat will look more evenly "brown" or solid cocoa colored, (think very dark chestnut, or sun-faded) with black points.

(At/a) Visually: a "brown" or a "seal brown". A black bodied horse, with varying amounts of red hair on soft spots (muzzle, around eyes, in flanks or under elbows).

(a/a) Visually: no agouti to see here, so let's move along...

illustration on left is linked to its source website:

Morgan Colors- The Base Colors: Chestnut, Black, Bay and Brown Morgan Horses; author, Laura Behning, Covington, GA,


Semantics: Depending upon where you live, or what discipline you work in, the words red, chestnut and sorrel may or may not be interchangeable. To keep things interesting, now let's add in some more equine genetic terms to explain how the Agouti gene affects the red horse, so now there are even more ways to use these terms. While we have fun with this, keep in mind I am a cowgirl, and that both sorrels and chestnuts are red based horses, as opposed to black based horses, and we'll see what we come up with, here.

Note: All shades of red may or may not have a flaxen (blonde) mane and/or tail and/or fetlocks.

(A) A sorrel horse with a plain A (classic bay) should have, simply put, no effect on red hair. A is dominant over the other agouti gene variants, so again, being the most dominant of the Agouti series, likely explains why classic bright red sorrels are more common than other shades of red.

Buffalo Gal, one of my "primitive sorrel" Curly Horses

(A+) I do not know, (yet) if or how the A+ Agouti gene affects red hair. I have my suspicions however... for many years I have raised a rare strain of hypoallergenic Curly Horse that include what I refer to as "primitive" chestnuts and sorrels... this bloodline traces back to horses owned by Native Americans in the 1700 & 1800's. Perhaps I was not too far off. What I call "primitive sorrels" have a bit duller or lighter red coats that are more prone to sun-fade, with faded feathering on their fetlocks and silvery-grey-flaxen manes & tails; while the chestnuts have mottled brown on the lower legs and faded feathering, with grey manes & tails. (¿Quién Sabe?) A+ is recessive to A and dominant over At.

(At) Visually, a dark sorrel or medium chestnut (depending upon your particular venacular). At is recessive to A+ and dominant over a.

(a) Self. No agouti gene at that loci.

So now let's look at how the shades of red are affected by the various combinations of Agouti gene pairs (and for the sake of clearer representation, let's drop the /slashes/ between the gene pairs when illustrating these gene combinations):

(ee AA) (ee AAt) (ee Aa) - Visually: a sorrel (which would explain why the "plain" red sorrel is the most common red based horse color).

(ee AtAt) (ee Ata) - Visually, dark sorrels to light & medium chestnuts.

(ee aa) - Liver chestnut - and if you think about it, this makes sense, because underneath everything, this combination has no agouti to restrict any pigments - and - the more recessive the gene combinations, the more rare they are.


UC Davis Test for Agouti only presence or absence of an agouti
The standard/conventional Agouti test, specific for the recessive 'a'. Everything that is not 'a' is interpreted as 'A' - but can be either bay-A, or seal brown-At)

Pet DNA Test for Agouti A, At, a
The 'a' and At alleles are caused by separate changes in Agouti, which do not overlap. Thus to obtain a comprehensive Agouti allelic status (when it cannot be deduced from the breeding records), one has to perform two DNA tests; The At-specific test will differentiate between bay A and At (this test done alone cannot distinguish between bay ‘A’ and ‘a’), both offered by PDSAz