The basic (horse) coat colors

The three basic colors: bay, black and chestnut. Some include seal brown, making it four basic colors.

Since some time has passed since I last posted to the blog, I thought it might be worthwhile to do a quick recap of the basic colors before I ventured back into the more complicated realm of basic colors in dogs. This will also allow me to reframe the discussion of red and black hair in a way that might make the next set of posts a little less confusing.

This series of posts began in response to a common simplification used when explaining horse color, and the misunderstandings that arise from presenting color in this way. That problematic explanation usually goes something like this:

Horses are basically red or black. The genes at Extension determine if the horse is black (“E”) or red (“e”). If a horse is black (“EE” or “Ee”) and ‘has agouti’, then black is modified to bay. If ‘agouti’ is absent, then the horse is just black.

I do understand that the simplicity of this is appealing, but it is not correct. It paints an inaccurate picture of what goes on with color and encourages a number of unnecessary misconceptions. Countering it with an accurate explanation that is still simple is a challenge. The latest series of posts was an attempt to do that, and here is a brief summary. (Note that in these two paragraphs I have boldfaced the distinction between talking about horses and talking about pigments.)

In mammals, there are two types of pigment: red and black. The original coat color of a species, known as the wild color, is typically some combination of both red and black pigment. In horses, the wild color is bay. Cells have the ability to make either of the two pigment types (red or black), and the genes that control them are called “pigment-type switches”.

In horses, like many other mammals, the primary pigment-type switches are Extension and Agouti. The un-mutated (“wild-type”) form of those two genes (“E” and “A”) combine to produce the color we know as bay. The other two basic horse colors, chestnut and black, are the result of mutations to one of those two genes. At Extension, there is a recessive mutation (“e”) that restricts the black pigment to the skin, which means the hair is all-over red. This results in the color we call chestnut. At Agouti, there is a recessive mutation (“a”) that removes all restriction from black pigment, so if the horse can have black hair—that is, if he is not chestnut—he will be all-over black. This is the color we call black. Additionally, it is believed that there may be other variants (alleles) at Agouti that restrict black pigment more or less strongly than either bay (“A”) or black (“a”). These are proposed to be responsible for colors like seal brown and wild bay.

In previous posts in this series I have explained that Extension is the locus that “permits the expression of black pigment” and Agouti is the locus that “determines the location of black pigment, if present.” That is true. All I have done in the paragraph above is turn it around so that the three basic colors are seen, not through what is happening to black pigment, but to what is happening to the normal production of pigment in horses. In this case, that is a mutation (e) that has taken away the ability to make black pigment, and another mutation (a) that has allowed black to express throughout the coat. That is actually what is being identified when you submit a horse for the standard Extension and Agouti tests. The test for Extension is looking for the mutation for chestnut (e) and the test for Agouti is looking for the mutation for black (a). Explaining it in this way makes the tests easier to understand, but it is even more important when we begin to talk about basic dog coloring. That’s because more common black-centric way of explaining the relationship between Extension and Agouti, and the implication that Extension sits at the top of a hierarchy of basic color controls (and that Agouti “modifies” it), does not work as well once you start adding more alleles, or more pigment-type switches.

As the most recent post explained, in dogs (and other animals), the distribution pattern of black and red hair is not solely controlled by the Agouti locus. That is, Agouti is not necessarily “modifying” the outcome at Extension. The Extension locus itself can direct the placement of black pigment, as it does in the Melanistic Mask (EM) allele in dogs. In fact, the relationship between the two loci is even more complex, so that alleles at either (or both) locations can influence the distribution of red and black hair. What’s more, looking at dog color it becomes obvious that a specific “turn on the black” allele is not required in order to have a coat with some portion of black hair. It would be more helpful to think of black hair as being part of the normal coloration that is there unless some mutation occurs that prevents it. For that matter, the same can be said for the presence of red hair. That is because ultimately these variations in basic color are really just mutations to the original wild color—to the original production and distribution of color in the species—which typically has some portion of both colors.

If the construct of “basic color” is understood to be “different distributions of black and red pigment on the animal”, with all-red and all-black simply occupying the extreme ends of that spectrum, and the pigment-type switches as being nothing more than the loci that control the options in that category, that should make some of these dog colors less confusing. That’s important, because after I talk about those, I hope to make a post about the third major pigment-type switch in dogs and what that might tell us about horse color.

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4 Responses to The basic (horse) coat colors

  1. Nancy Cornwell November 20, 2014 at 12:13 pm #

    I thought bay was black with agouti? So wouldn’t it be black. So two basic colors .

    • equinetapestry November 24, 2014 at 11:08 am #

      Bay horses have the production of black pigment enabled because they have the dominant allele at Extension (E), but they are not black horses. What this series of posts has been about is that there are to basic colors of pigment, but those are different from the basic colors of horses.

  2. Laurie December 1, 2014 at 2:29 am #

    Are you in effect saying that horses start out bay then mutations kick in to produce the final color?
    Just trying to reword and understand.

    • equinetapestry December 1, 2014 at 12:26 pm #

      The thinking is that the species was originally bay, and without the later mutations (one for recessive red, e, and one for recessive black, a) that is what they would be.