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Equine Color Part 2 - Agouti

FILED UNDER: Equine Color


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Feb 2010


Author: Katie


So yesterday I gave an overview of the base colors, Black and Red. After reading that, you might be wondering why there aren't more pure black horses running around, seeing as black is dominant to chestnut. The answer is Agouti, the modifying gene that causes a black horse to be appear bay.

Every bay horse has a black base coat. The Agouti gene simply restricts the black to the horse's points; the muzzle, the mane and tail, and the legs. There are actually two different types of the Agouti gene. One is regular Agouti (A) which restricts the black to the the points and the other is Seal Agouti (At) with doesn't restrict the black quite as much and results in a Seal Bay. A horse that doesn't have Agouti (a) will not have any restriction of black pigment.

Seal Bay Horse vs Regular Bay Horse

Agouti in either form is dominant to black, so regardless of whether a black horse has one copy (heterozygous) or two copies (homosygous) of Agouti, that horse is going to be bay. Regular bay is dominant to seal bay. The way to remember this is regular bay restricts more of the black then seal bay does so if both Seal Bay and Regular Bay are hanging out together, Regular Bay is going to supersede Seal Bay and restrict the black pigment further than Seal Bay does. But, if Seal Bay is chilling all alone, or with another Seal Bay gene, Seal Bay is going to express itself and since Regular Bay isn't be there to overshadow Seal Bay the black pigment isn't restricted as much.. A personal note, seal bay is my least favorite horse color.

Agouti and Chestnut Horses

Agouti does not interact with red genes at all because red horses do not have any black pigment to restrict. A chestnut horse may very well be carrying Agouti, but there will be no visible indication that it is there. The only way to know whether a chestnut is carrying Agouti is to perform a genetic test or to breed it to a true black and see if it ever produces a bay. If a bay is produced you can be certain that the Agouti gene came from the chestnut, because a pure black horse can not be carrying Agouti, otherwise it would be bay itself.

Possible Breeding Scenarios...

Seal Bay Horse vs Regular Bay Horse

In this first breeding scenario we have a visibly bay horse crossed with a chestnut horse. Because the chestnut horse does not carry any Agouti genes in this scenario half of the foals will be bay and half will be black. Because the black parent is homosygous for black, there is no possibility for a chestnut horse.

Seal Bay Horse vs Regular Bay Horse

In this second scenario there is a hidden Agouti gene (A) on the chestnut horse, for this reason even though neither parents are bay, there is a 50% chance of a bay foal.

Seal Bay Horse vs Regular Bay Horse

In the third scenario the chestnut parent is homozygous for Agouti and the bay parent is heterozygous for bay and homozygous for black, so the results of this cross is always going to be bay.

Seal Bay Horse vs Regular Bay Horse

There's a good chance if you breed bay to bay that you're always going to get a bay. In this hypothetical cross there is a very slim chance of getting a pure black foal and no chance whatsoever of a chestnut. Seal Bay Horse vs Regular Bay Horse

In this final scenario the two parents are both heterozygous for black and bay. This gives you the most possibilities in foal color outcomes. While bay is by far the most likely outcome you could also have a chestnut or black foal pop up.

From all of the sample crosses above, it should be becoming clear why there are so many bay horses running around.

Sooty Bay

Let's make this pot even more flavorful and add in some Soot. The "Sooty Gene" acts like the name would indicate, it makes a horse look they rolled around in some smoke ashes, or soot. The gene causes portions of the horse, normally the top and shoulders, to darken. This can be seen on Bays, Buckskins, Palominos and other colors too. It's simply layered on top of these other color traits. This is why in some cases Palominos look so pale, while other look much darker.

Sooty Bay

Next, Cream Dilution...

Photo Credits:,%20Quarter%20Horse.jpg

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2011 11 12

I really love these articles; they are very informative and clearly explain how this can apply to real life. I wish you would continue writing on this fascinating subject, especially as you are just getting into the interesting colors.

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