This is a short explanation of the common factors that dilute coat color in dogs, to provide background for the subsequent posts about albinism.
A friend of mine who works in rescue recently sent a picture of a Shih Tzu, asking my opinion on her unusual color. Because I am very fond of dogs, posts about them appear here from time to time. They are a very interesting species for those who study color because, like horses, the genetic mechanisms involved have been the subject of a great deal of research in the last few years. In this particular case, the subject happens to touch on a number of relevant topics, including the possibility (thanks to genetic testing) of differentiating between visually similar colors, and the challenge presented when it comes to categorizing and naming colors.
But before I post about Angel, the unusual Shih Tzu, I thought it might be a good idea to explain – for those more familiar with horses – the more common diluted colors in dogs. I suspect that Angel is an albino dog, which is to say that she probably does not have some combination of the factors discussed here. I’ll talk more about just what albino does mean in the context of dogs in subsequent posts. For now, this is a post about what Angel is not.
Some quick dog-to-horse translations
I should probably start by saying that dogs, like all other mammals, have two basic types of pigment: eumelanin (black) and pheomelanin (red). There are some differences both in how color works in dogs, and also in the classifications and semantics. For instance, what horsemen call red pigment is often called yellow in dogs. That is understandable, since pheomelanic dogs are more often a tan or golden color, while the deep chestnut brown common in red horses is relatively rare. However, because red is familiar to readers of this blog, that is the general term I will be using for pheomelanin.
The other problematic term is Brown, which here refers to the color found in chocolate Labradors. Brown in horses is something quite different. To make matters even more confusing, in some dog breeds this color is called red. It is also sometimes called liver, but like brown that means something very different in horses. Brown is also not usually classified as a dilution, but is most often referred to as an alternate form of black pigment. For simplicity, I have used the term “dilution” in a more general sense, meaning anything that makes the fur, skin and eyes lighter. Just be aware that Brown is often spoken of as being separate from the other colors described here, and that using “dilution” as an overall category is somewhat unique to the horse color community.
The Doberman Pinscher on the left in the top image is a Blue dilute. While the color is most commonly called blue, the formal term for the mutation is Dilution. It alters black pigment in both the skin and the hair to a slate-gray. Dilution also alters red pigment to a more muted tone, though the effect is more subtle than with the black.
This blue Italian Greyhound is another example. Notice that his eyes are not particularly pale. While many Dilute dogs have gray or amber eyes, there does seem to be a range of possible eye colors. The coat can vary quite a bit, too, so that some Dilute Blues are quite dark. On genetically red dogs in particular, it can be easy to miss some of the darker shades of Dilution because the only sign may be a dark blue (rather than black) nose.
Dilution is a recessive gene, which is why blue dogs can occur unexpectedly in litters. It is believed that there is more than one version of the mutation for blue, though at the moment only one form can be detected by testing. There is no known equivalent mutation in horses. There is a mutation to the same gene (MLPH) in cats which dilutes black to blue and red to cream. That mutation is responsible for the distinctive blue coloring in the Korat, Chartreux and Russian Blue breeds.
The brown Miniature Pinscher in the picture at the top of the post has had her black fur and skin is replaced with chocolate brown. Like most brown dogs, her eyes are amber. Brown does not alter red pigment, which is why the tan points are still a clear red-gold. In colors that involve a mix of red and black fur, this mutation will turn the black areas brown while the red areas remain unchanged.
Like the blue dilution, brown dogs are found in a range of shades from dark to light. In my own experience, it seems that brown dogs with some degree of red pigment (sables, brindles, tan-pointed) are often paler in shade than brown dogs that are genetically black, like the German Shorthaired Pointer pictured below.
Dogs have black-pigmented skin, which is why genetically red dogs like Golden Retrievers and Irish Setters have black noses.
Like Dilution, in dogs Brown is recessive. There are three known versions of the mutation, all of which can be identified by testing. There does not appear to be a visual difference between each one, or combinations of the three.
Although mutations to the same gene (TYRP1) are found in many mammals, including mice, cats, and cows, there is no similar mutation in horses. You can find references to Brown in older papers on horses, though, because for some time it was theorized that chestnut was the result of Brown. The paper mentioned in the previous post about Silver in Shetlands assumes the presence of Brown in horses, which makes for confusing reading if you are not aware of this discarded theory! (An alteration to the same gene in people is responsible for the blonde hair seen in some Solomon Islanders.)
Isabella or Lilac
This is not a separate mutation, but rather a combination of the two previous colors. Dogs that are both Brown (bb) and Dilute (dd) are a pale shade of taupe gray with pale gray skin and green-gray eyes. The best-known breed with this kind of color is the Weimaraner, but it can occur in any breed where both brown and blue are found.
Cream and White
Both Blue and Brown have been identified at the molecular level. There are also dogs that appear to be pale cream or even white, though they have ordinary black pigmented skin and dark eyes. These American Eskimos are a good example of the whiter form of the this kind of coloring. In some breeds, like the Afghan Hound, the color of the coat could more accurately be called cream. Even some Golden Retrievers and yellow Labradors could more accurately be called cream than yellow. (We used to say of our own very pale dog that he was a vanilla, rather than a yellow, Labrador.)
To date all the white and cream dogs have proven to be genetically red (ee). Because they have black noses, eye and lip rims, and dark eyes, the assumption is that they have some factor that dilutes red (but not black) pigment. Sponenberg proposed calling the factor Intensity (I), so that is sometimes how it is notated. The mechanism behind the color is not yet known, however. An added complication with dogs is that white patterning that covers the face – even extensive areas of the face – does not alter the color of the nose. (Notice the extensive white on the face of the blue Pit Bull, yet his nose retained color.) Theoretically, a dog with an extreme piebald pattern might not look much different from a dog with extremely diluted red pigment, so it is possible that some all-white dogs are not actually dilutes at all.
That is a quick overview of the more common things that make for pale dogs. The next post will have pictures of Angel, and a little historical background on albino dogs.