Telling the story a different way

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As I stated on my biography page, my background was originally in art—not genetics. When I started writing about horse color, I was primarily speaking to other artists, so it made sense to use a visual framework to explain it. I wanted to organize the concepts about horse coloration in a way that would resonate with my audience, and that would make a rather technical subject seem less intimidating. That visual approach can be seen in the slides that I put together for a presentation I made fifteen years ago.

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A Visual Framework

I still use this basic framework (but not cartoons drawn with magic markers!), as the current slide at the top of this post shows. If asked to give a 30-second rundown, this is how I would lay out this “base color + modifiers” approach:

  • You start out with a basic color: bay/brown, black or chestnut
  • You can add various kinds of modifiers to that basic color: shade/intensity, dilutions, progressive depigmentation, white patterns, reversions
  • Every horse’s color, no matter how complex, can be reduced to a base color + modifiers.
  • Every modifier has a version for each base color (although not all combinations result in a visible alteration).
  • Modifiers have relationships with one another, both hierarchical (this trait trumps that trait) and interactive (one changes the other, sometimes in surprising ways)

The visual framework has proven to be a very effective way to present the story of horse color, not just to artists, but to non-scientists in general. It certainly is the most common approach taken by online horse color resources. It is how I laid out my own book. It is not, however, the only way to tell this story. In some cases, it isn’t necessarily the simplest way to tell the story.

The visual framework can invite certain misunderstandings, like the idea that “bay is a modified form of black“. It also leaves out a concept that is pretty central to the science of genetics, which is that of a wild-type. I talked a bit about that in a previous post. Recent discoveries about the color dun (D) have given that idea new relevance, which will be the subject of my next post. But for now, what I want readers to consider is that the visual framework is just one way to tell the story of horse coloration. And really, that is all the whole concept of “base + modifier” is: a way to tell this story. It is not the science of coat color genetics itself, but a way to talk about it. There are others.

An Evolutionary Framework

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We can also tell the story from an evolutionary perspective. That story might go something like this.

At one time, wild horses were uniformly bay dun, or perhaps more likely—given the appearance of other wild equids like the Przewalski’s Horse above—mealy wild bay dun. Their coloration was the result of a pigmentary process, controlled by a number of genes. While still in the wild, mutations to those genes changed part of that process, which in turn changed the color of the horses. Later, as horses were domesticated, additional mutations were selected for by human beings because they found them appealing or felt that the new color better served a particular purpose.

This is quite a different way of looking at horse color. Someone coming from a visual framework, looking at a yellow dun horse, would think of dun as a “modifier of bay”—as something added to the “normal color” of a horse. But from an evolutionary standpoint, bay dun is the wild-type, and our visual explanation is actually backwards. Dun was not added to bay, but rather dun horses became bay because something (a mutation) happened to remove the dun. The wide range of equine color came about in the same way; the wild-type coloration was altered by a variety of mutations, each of which changed some aspect of the original pigmentation of the species. Telling the story of when and how those changes came about is another way to explaining horse color.

Here is the 30-second rundown for this approach:

  • The wild-color for horses is bay dun. This is no longer speculation, but knowledge gained from multiple studies that involved the testing ancient remains.
  • Fairly early, the ancestors of domestic horses acquired a mutation to partially—but not fully—remove the dun coloration. They also acquired a mutation that resulted in progressive loss of color in the coat (Leopard Complex).
  • Mutations to the sites controlling black and red pigment resulted in all-black and all-red phenotypes.
  • Another mutation in domestic horses resulted in individuals with no sign of the dun coloration. Additional mutations occurred giving diluted phenotypes as well as those with white (depigmented) areas. These are believed to have been retained and selected for because human beings liked them.
  • Among modern domestic horses, individuals still appear carrying new mutations. This is particularly true for white markings or patterning.

Not a revolution, but an experiment

What I have described are two very different ways of explaining horse color. I want to be clear that I am not saying that teaching horse color from a visual standpoint is incorrect, or that anyone should stop using it. I will be guest lecturing this upcoming Tuesday, using the slide at the top of the post, so obviously I haven’t overhauled my own approach! But I think it is worthwhile to point out that there are many ways to talk about horse color. They all have their strong points and their pitfalls. Certainly the visual approach is easier for many non-scientists because horse color is such a visual subject. But for those that want to explore the subject further, it can actually be a bit disorienting to read published papers or textbooks on animal coloration, because the evolutionary point of view is the scientific framework for that setting. It is also a more logical framework when the subject veers into the history of domestication and differentiation in horses, because it takes into account chronology. That means that at times an evolutionary framework might make a lot of sense, even for people who were originally taught a visual approach.

It happens that discussing the recent discovering about dun is one of those times. So, we’ll be using a slightly different framework of understanding on the blog in the coming weeks. I am going to ask that readers more familiar with “base and modifiers” set that aside for just a bit while we tell the story of horses and their colors a slightly different way.

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Modifier relationships – it’s complicated

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In the most recent post in this series on pigment-type switching, I talked about the fact that both horses and dogs have a wild color that is a combination of red and black pigments: bay in horses, and wolf sable in dogs. Those wild colors involve the interaction of two pigment-type switching genes, Extension and Agouti. Horses and dogs also have a mutation for all-red (located at Extension) and another for all-black (located at Agouti).

One of the most straight-forward ways to explain how this works in horses is to say that the two alleles at Extension determine if black hair is possible, and the alleles at Agouti direct the placement of the black pigment, when it is present. This flow chart might help visualize this.

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So far the alleles we have here for dogs line up with those for horses. But dogs have a lot more options at both of these loci, and a look at how those work together is a great way to appreciate the complex nature of pigment-type switching, and why the chart above is only part of the picture.

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I mentioned that in dogs one of the alleles at Extension, melanistic mask (EM), directs the placement of black pigment. Those familiar with horse color tend to think that is the role of the Agouti locus, which is why it is so tempting to think of Extension as the primary control, and Agouti as the modifier. However, when it comes to the alleles that control the two basic pigments, the loci themselves do not really have simple primary vs. modifier relationships. It is the individual alleles that act as modifiers, and just which allele plays that role varies with the different colors. In fact, as we’ll see later, in some cases there are multiple layers to these relationships.

So the melanistic mask allele (EM) directs the placement of the black pigment, but like horses, dogs have alleles at Agouti that also do this. In addition to the melanistic mask, the dog in the picture above has the Agouti allele for fawn (Ay), which in some breeds is called sable. Another common allele at this locus is black-and-tan (at). That is the color associated with breeds like the Doberman or Rottweiler. Both the fawn and the black-and-tan alleles direct the placement of black pigment into specific patterns.

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These two alleles can work simultaneously with the allele for melanistic mask. Fawn dogs with melanistic masks are common in many breeds. This dog has both the black-and-tan pattern and the melanistic mask, which has partially obscured the tan patches that are so clearly visible on the nose of Doberman above. (This was probably even more striking before age caused the muzzle of this dog to turn gray.)

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When present, black-and-tan and melanistic mask are both visible in the final coat. There are also alleles at Extension that modify specific alleles at Agouti, changing the nature of the original color. In Salukis, the color known as grizzle is an allele at Extension (EG) that modifies black-and-tan (at) . A separate allele at Extension (eh) alters black-and-tan in a similar fashion to produce “sable” English Cocker Spaniels. (The linked articles are in German, but they contain numerous images of sable (zobel) Cocker Spaniels.)

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Saluki photo by Pleple2000, courtesy of Wikimedia Commons.

As the grizzle Salukis and “sable” Cocker Spaniels show, it is possible for Extension to enable the production of black pigment, direct its distribution and even modify the distribution of black pigment set down by alleles at Agouti. It is, in fact, even more complicated than this, because recent studies suggest that the black-and-tan pattern is itself a modification of a pattern known as saddle tan, which is a separate allele at a completely different locus. So grizzle is probably a modification of a modification, with the “original” pattern of red and black pigment residing at a locus that is neither Extension or Agouti.

This shows that even with dogs, where pigment-type switching has been more extensively studied, we only have a partial picture of the process. However, if we place the pieces of the puzzle that were outlined in this post into our previous flow chart, it gives a broader picture than the small window that the (known) alleles in horses allow. Here is an expanded chart showing the relationships between these alleles. If you hover over the image with your mouse, it will drop out the additional alleles to show the more limited picture provided by those alleles that correspond with the pigment-type switches in horses.


Saluki photo by Pleple2000 and Cocker photo by Louis Mayer, courtesy of Wikimedia Commons.

To make things more complicated, even this expanded flow chart covers only a portion of the pigment-type switches in dogs. While horses have two major sites that are known to control pigment-type (Extension and Agouti), dogs have a third (K Locus). That third locus, which controls dominant black and brindle, is an interesting topic for another day, but I want to take us back to horses, and what this bigger (if incomplete) picture might tell us about some of the mysteries still unsolved in that species.

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A brief commercial interruption

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I like writing books about horse color, but I am probably the worst person for book promotion. I would rather spend my time researching (there is never as much time for that as I would like) or working on the next book or blog post. But with the Christmas season upon us, it is probably a good idea to remind everyone that horse books make good presents. At least, that’s what I have been telling my family for years now…

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The page spreads that illustrate this post are from the most recent book, Equine Tapestry: An Introduction to Colors and Patterns, published this past summer. It is intended to work as both a supplement to the original Equine Tapestry series and as a stand-alone book outlining the basics of equine coat color inheritance. It is written in non-technical language—considerably less technical than the recent blog posts on pigment-type switching—and there are detailed illustrations and color photos throughout. The book covers colors that are known and well-defined, as well as some that are the subject of speculation, like belton patterning (below) and manchado.

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You can order a paperback copy here, or by clicking on the cover thumbnail on the right side of this page. Those links take you directly to my author page. Ordering through an author page means a larger portion of the purchase price goes to the writer and less to the distribution company. Those with an Amazon Prime membership might want to check there as well, since they offer discounts—sometimes significant ones—from time to time. The hardcover edition is also available through Amazon. The older book, which is scheduled to go out of print early next year (to be replaced with a full-color second edition), can be ordered here.

I also want to thank those readers who have left reviews on Amazon for either of the two books. Many readers use customer reviews to help determine if a book might be suitable for them, and I appreciate the fact that many of you took the time to give potential readers a better idea of what to expect from both books.

So having done my promotional duties, I will return to work on the next post on pigment-type switching. With luck that one will be posted some time this weekend.

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