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.
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.
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.
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.)
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.)
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.
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.