Translating horses to dogs


Before I return to basic dog colors, here is a short guide for translating horse color and dog color. I have posted about this a few times before, but since there is going to be a lot of new material in the next few posts, I thought a refresher on this might eliminate at least some of the potential for confusion.

Yellow = Red

In horses, we talk about two types of melanin pigment: black and red. The latter type, known as pheomelanin, is sometimes referred to as red, yellow or even orange, depending on the species. In horses, we use red. In mice, which are the most common model for research on pigmentation, it is referred to as yellow. This is also true in dogs. Because pheomelanin in horses ranges into the deeper tones—and perhaps because “yellow” was once used in some regions to mean palomino or buckskin—red is the traditional term. These are just different names for the same thing.

Red ≠ Liver or Chocolate

Which brings us to the other area of potential confusion. When horsemen talk about red, they mean pheomelanic pigment. When some dog people speak of red, what they mean is a dog like the one at the top of this post, otherwise known as liver or chocolate. On this blog, this type of coloration in dogs, which is not thought to occur in horses, is referred to by its traditional name of brown while red is reserved for pheomelanin.

Seal brown ≠ Brown (b)

Brown is the other confusing term, because it means something very different in horses than in almost any other species. In other animals, brown is an alternate form of black (eumelanin). The dog at the top of this post is the brown version of tricolor (black, tan and white). In posts on this blog, brown usually means the seal brown coloring of the sort seen on the tobiano mare below.


To avoid confusion, when speaking of the non-equine brown I follow the term with the genetic notation “(b)”. When speaking about both, I either use the genetic notations (b and At) or the more specific equine term “seal brown”. (Dogs also have a different color called seal brown, but I will leave that for another day!)

Agouti vs. agouti

In horses, as in other mammals, Agouti is a locus (a specific location on a chromosome) that is involved in pigment-type switching. In other animals, agouti is also the name given to a specific coloration that involves black and red banded hairs. Horses do not appear to have banded (agouti) hairs, but dogs do. The general convention used by this blog is that when referring to a specific locus, the name is capitalized and italicized (or underlined when in a block quote). When used to mean a pattern of banded hairs, agouti appears in regular lowercase letters. This will be an important distinction later in this conversation, because in dogs some of the alleles at Extension and Agouti produce patterns that have both agouti and non-agouti areas. So Agouti is the locus where a number of the alleles for basic coloring reside, and agouti is a specific type of coloring involving banded hairs.

I am also going to post a quick comparison of the canine equivalents – from a genetic standpoint – to bay, black and chestnut. With luck using that as a common reference point, this will not be quite as confusing.

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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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Extension in other animals


When talking about horses, the simplest way to explain the interaction between the two common pigment-type switches, Extension and Agouti, is this: Extension determines whether or not there will be black hairs in the coat, and Agouti determines where on the body those black hairs will go. That is a reasonably accurate description of how the distribution of red and black pigment is understood to work. Other animals, however, can be far more complicated.

One difference can be seen in animals—like the agouti Guinea Pigs—that have banded hairs. In horses, the distribution of the black pigment is spatial; black hairs are directed to specific areas of the body. In other species, the distribution of black pigment is about timing; as the individual hairs develop, the two pigments alternate to form bands of red and black. It is also possible to have both kinds of distribution, spatial and timed, on the same animal. The Guinea Pig pictured above has both a timed pattern (banded hairs) and a spatial pattern (tortoiseshell) of black hairs.

Banded hairs are produced when the pigment switches from red to black during the development of the individual hairs

The other difference is that in some species, the distribution of the black hairs is not exclusively controlled by Agouti; other genes, including Extension, can be involved in the arrangement of black pigment. The allele that distributes black in tortoiseshell Guinea Pigs is not at Agouti, which is where most people familiar with horses might guess it to be, but at Extension. Because black pigment is only enabled in portions of the coat, the allele is called “Partial Extension” (ep). It is recessive to the wild-type allele (E), which enables black pigment across the entire coat, but dominant to the allele for the complete restriction of black pigment (e).

ExtenEm ExMask

Dogs are another species where alleles at Extension can determine not just the presence, but the placement of black hairs. An example of this is the black mask seen on some fawn dogs, like the Great Dane and the Belgian Malinois pictured above. Like Partial Extension in Guinea Pigs, the Melanistic Mask (EM) allele was proposed as part of the set of alleles at Extension fairly early (1919). At first it was thought of as a partial extension of black pigment, much like the tortoiseshell pattern in the Guinea Pigs, but later it was seen as a “super-extension” where the black pigment extended more dramatically in specific areas of the coat.

The underlying fawn color, however, is controlled by an allele at Agouti. In common horse color terms, it could be said that the Melanistic Mask allele (EM) modified the Fawn allele (Ay) by adding a black mask and darkening the ears and (usually) the topline. This, of course, is in conflict with the idea that the Agouti locus is a “modifier of Extension“.

The recessive form of Extension (e) produces a clear red dog, just as it produces a red (chestnut) horse

So is the relationship in dogs flipped? Does the canine form of Extension modify Agouti, while the equine form of Agouti modifies Extension? Not really. Dogs have recessive red (e) just as horses do, and their alleles at Agouti still need black pigment enabled to have any effect, just as is true for horses. A recessive red dog like the Golden Retriever above, will hide what he carries at Agouti for the same reason as a chestnut horse; there is no black pigment to distribute. It is just that dogs have many more alleles at both sites. Horses, having far fewer, give an incomplete picture of how the two pigment-type switches interact. The larger picture does not lend itself well to reducing one locus to the “primary” control and the other to its “modifier”. It is accurate to say that some of the alleles at one site modify the effect of an allele at the other, but Extension and Agouti as a whole have a more complex relationship.

The back-and-forth between these two genes is even more obvious when looking at a more recently discovered allele at the canine Extension locus, which is Grizzle (EG). Since that involves another layer of complexity, that will be the subject for the next post in this series. After that, we will look at the third major pigment-type switch in dogs, which—if readers haven’t thrown up their hands and completely given up on the subject by then—will make basic horse colors seem refreshingly simple. (That third switch will also allow us to circle back around to some of the remaining mysteries about horse color, too.)

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