As I mentioned in an earlier post, the relatively small number of alleles in horses gives us a limited window into how pigment-type switches can work. This does make the basic coat colors much easier to explain, but without the bigger picture it is possible to misunderstand some fundamental aspects of animal coloration. The most common misunderstanding that I encounter is the idea that horses are “basically black” or “basically red”. Instead of Extension being one of a class of genes that control the two types of pigment (a pigment-type switch) it becomes the location where the “basic color” (red or black) is determined. This seems logical, I suspect, because there is a natural tendency to assume that anything “basic” should be reducible to one gene. So with this thinking, a horse with the dominant allele at Extension (E) becomes a “black horse”, while one with two copies of the recessive allele (ee) is a “red horse”. Any other color is the result of a “modifier”.
But Extension is not a special site where some pure, basic color is ultimately determined. It is not the “red or black gene”—at least not in the sense that the black horse/red horse idea posits. It is part of a system where basic pigmentation (red and black) is controlled. Looking at a species that has a wider array of alleles at both Extension and Agouti can make this more obvious, which brings us to dogs. In this post I am going to outline some equivalent colors in dogs, and then follow it with a post that fills in the rest of the picture.
It is important to remember that the starting point for the color of an animal (as opposed to the color of pigment) is the wild color. In horses, that color is believed to be bay. In dogs the wild color is what is commonly called “wolf sable”. That term can be confusing, because in some breeds a genetically different color is also called sable. I have used a photo of my neighbor’s dog Caroline (above) to help highlight the difference between the two. Caroline is a German Shepherd-Rough Collie cross, and she looks quite a lot like a Collie except for her coloring. Obviously she lacks the white pattern associated with that breed, but the pattern of red (yellow) and black hair is also different. Here is my friend Melanie Miller’s Collie, Shelby, showing the other kind of sable. (Her Bully friend Barley probably also has that same gene as Shelby, but we’ll get to that later.)
If you can imagine the difference between the typical “sable” German Shepherd and a sable Collie, you can get the idea of what the wild color in dogs is. Other breeds that are commonly wolf sable are the Keeshond and the Norwegian Elkhound, but I used a picture of a German Shepherd cross because that is a familiar breed that retains the pheomelanic (red/yellow) pigment while many of the other wolf sable breeds have other factors that bleach it out, giving the impression is of a grey and black dog. This coloring is quite similar to that of the domestic dogs’ direct ancestor, the Gray Wolf. This Gray Wolf has retained a good degree of pheomelanic pigment, and looks quite similar to Caroline in color. (Photo by Carlos Delgado, and provided courtesy of Wikipedia Commons.)
Like bay in horses, wolf sable is a two-part genetic recipe. The dog needs to have the production of black pigment enabled at Extension, as well as the allele(s) for wolf sable (aw) at Agouti. As you can see, the original coloring of a dog is not red or black, nor is it something that is controlled by one primary gene. It is the product of a system of pigmentation controlled by a number of genes. Changes to that system alter the placement of the two pigments. They can also alter the proportion of the two pigments, up to and including eliminating one or the other.
Turning the animal all-red
Dogs also have a mutation to Extension that disables the production of black pigment. Like chestnut in horses, this mutation is recessive. Dogs with this form of Extension (e) do not have black pigment in their coat, so they are all-red (yellow). This is the familiar coloring of Golden Retrievers and yellow Labradors. A recessive all-pheomelanic coloring is common in many domestic animals.
Turning the animal all-black
Dogs also have a recessive black allele at Agouti, just like horses. The difference in dogs is that the gene for recessive black (a) is rare. It is, however, the only form of all-black possible in some breeds, like the Shetland Sheepdog. Most black dogs have a different gene, which I will discuss much later in this series. In dogs that have recessive black, though, it works like the color in horses. The dog must have black pigment enabled at Extension, and then must be homozygous for the recessive black gene (aa) at Agouti. Like recessive red, there are equivalents to this in a number of different species.
These all-red and all-black animals do not represent some essential essence of animal coloration. They are not the “basic” or “original” colors of their species. All-red (ee) and all-black (aa) are mutations that effectively turn off the production of one of the two original pigments. It can be helpful to think of them as the outermost limits for alterations to the regulation of the two pigments, with the other alleles sliding the scale to rearrange (and perhaps favor more or less) the two pigments. That arrangement can happen at either Extension or Agouti—or somewhere else entirely. I already mentioned the Melanistic Mask (EM) allele in a previous post, and how it points to the fact that Extension can direct the placement of black pigment as well as simply enable its production. I will expand on that with the next post about some of the other alleles, and hopefully give a more complete picture of what can happen with these two pigment-type switches.