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.
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).
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“.
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.)