Tag Archives | brown dilution

Translating horses to dogs

DogRed

Before I return to basic dog colors, here is a short guide for translating horse color and dog color. I have posted about this a few times before, but since there is going to be a lot of new material in the next few posts, I thought a refresher on this might eliminate at least some of the potential for confusion.

Yellow = Red

In horses, we talk about two types of melanin pigment: black and red. The latter type, known as pheomelanin, is sometimes referred to as red, yellow or even orange, depending on the species. In horses, we use red. In mice, which are the most common model for research on pigmentation, it is referred to as yellow. This is also true in dogs. Because pheomelanin in horses ranges into the deeper tones—and perhaps because “yellow” was once used in some regions to mean palomino or buckskin—red is the traditional term. These are just different names for the same thing.

Red ≠ Liver or Chocolate

Which brings us to the other area of potential confusion. When horsemen talk about red, they mean pheomelanic pigment. When some dog people speak of red, what they mean is a dog like the one at the top of this post, otherwise known as liver or chocolate. On this blog, this type of coloration in dogs, which is not thought to occur in horses, is referred to by its traditional name of brown while red is reserved for pheomelanin.

Seal brown ≠ Brown (b)

Brown is the other confusing term, because it means something very different in horses than in almost any other species. In other animals, brown is an alternate form of black (eumelanin). The dog at the top of this post is the brown version of tricolor (black, tan and white). In posts on this blog, brown usually means the seal brown coloring of the sort seen on the tobiano mare below.

Browntobi

To avoid confusion, when speaking of the non-equine brown I follow the term with the genetic notation “(b)”. When speaking about both, I either use the genetic notations (b and At) or the more specific equine term “seal brown”. (Dogs also have a different color called seal brown, but I will leave that for another day!)

Agouti vs. agouti

In horses, as in other mammals, Agouti is a locus (a specific location on a chromosome) that is involved in pigment-type switching. In other animals, agouti is also the name given to a specific coloration that involves black and red banded hairs. Horses do not appear to have banded (agouti) hairs, but dogs do. The general convention used by this blog is that when referring to a specific locus, the name is capitalized and italicized (or underlined when in a block quote). When used to mean a pattern of banded hairs, agouti appears in regular lowercase letters. This will be an important distinction later in this conversation, because in dogs some of the alleles at Extension and Agouti produce patterns that have both agouti and non-agouti areas. So Agouti is the locus where a number of the alleles for basic coloring reside, and agouti is a specific type of coloring involving banded hairs.

I am also going to post a quick comparison of the canine equivalents – from a genetic standpoint – to bay, black and chestnut. With luck using that as a common reference point, this will not be quite as confusing.

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Color and pattern variations

ChestnutBashkir

As I mentioned in the previous post, one of the things that attracted me to the original Sponenberg book was that it proposed a system for categorizing colors. In later books, the author revised his system from one that grouped things by point color to the more familiar base colors and modifiers that most of us use today. Not only did that system make it easier to explain coat color inheritance to others, but it promised a common language so that horsemen involved with different breeds might find it easier to communicate with one another about the subject. Needless to say, a lot of us who talked about the subject spent a lot of time explaining that no, there was no “Arabian chestnut” that was different from, say, “Morgan chestnut”. Chestnut was chestnut. Color from breed to breed was more alike than it was different.

In some sense, that is still true. We know that the vast majority of chestnut horses, like the American Curly Horse above, carry the same mutation to the MC1R gene. Somewhere in their distant past, chestnut horses all trace back to a common ancestor – the founder – that carried that original change. Although this “common ancestor” story has made news lately, especially when it comes to hair and eye color in people, as I mentioned during the discussion about founders, that is actually what would be expected. It would be expected that grey horses would trace back to the one horse that carried the initial mutation for grey, and that tobianos would trace back to the original tobiano. The question is, would this happen more than once? Could their be multiple grey mutations? Is there more than one tobiano?

In the case of chestnut, it turned out there were at least two. In 2000, another form of chestnut (ea) was identified in the Black Forest Horse. Technically the ‘new’ mutation does not cause the horse to produce red hair; horses with this version have the original mutation for red. They just also have another mutation that interferes with the test used to detect the original mutation. What is important, though, is that the two forms are functionally equivalent. That is, horses that carry the common mutation (e) and those that carry the more rare form (ea) do not look different from one another. They both can be light or dark in shade, or have pale or self-colored manes and tails. Whatever variation there may be from one chestnut to the next, it would seem the controls for those traits are to be found elsewhere in the genetic code.

A good example of functionally equivalent mutations can be found in dogs. There are three known variations that produce chocolate in dogs – bS, bd, and bc. Given the isolated nature of the different dog breeds, it is thought that there may be other less common mutations as well. Like the two chestnut varieties in horses, though, these mutations are not thought to produce a visually different outcome. From a testing standpoint, the versions of chocolate are slightly different, but the results are the same. For that reason, test results in dogs are usually reported simply as b, without indicating the specific version. (I should note that as far as I know, the different forms of chocolate in dogs are independently arising mutations that are similar, rather than the mutation-then-another-mutation situation with chestnut in horses.)

Boykin
This Boykin Spaniel falls on the darker end of the spectrum for chocolate, but not quite so dark as the German Shorthair pictured in the earlier post on diluted dogs.

PaleChocolate
This Australian Shepherd illustrates the paler end of the shades of chocolate. It does not appear that shade varies according to the type of brown mutation present.

The idea that there are equivalent mutations that produce the same visual result makes testing for colors more complicated because it can produce confusing test results. Prior to the discovery of the alternate form of chestnut in horses, it was possible to get test results back on a chestnut that indicated the horse was bay or black. So far the spread of that particular mutation has been fairly limited to some of the European draft breeds and some of the rustic Spanish ponies. (The latter are thought to have the mutation from more recent introductions of draft horse blood.) It is not hard to imagine a situation where an equivalent mutation might be more widespread, however, leading to confusing test results.

Even greater confusion might result if you had mutations that already produced a pretty broad range of outcomes. The visual range of a chestnut horse, or a chocolate dog, is pretty subtle compared to the range of what we have called sabino in horses. Both of the horses below have patterns that have, in the very recent past, been called “sabino”. Not many horsemen would think that two mutations producing patterns of white this different from each other could be thought of as “functionally equivalent.”

Markings2

VenomXsabino

To confuse things further, there are mutations to the same gene that have not, by tradition, been put in the same general category. It is likely that this horse, a true dark-headed roan, has a mutation to the same gene as the sabino horses above. Again, this is not something most people could think of as equivalent. Tobiano, the pattern many horsemen think of as the  “opposite” of sabino and the other patterns in the overo category, also alters that same gene (KIT). Meanwhile the splashed white patterns, which are thought of a single category, include mutations that occur in two different locations (MITF and PAX3). So things that look completely different can be mutations to the same gene, while things that look alike can be mutations to different genes. 

KSRoan

And that is the challenging part about where things stand in color genetics at the moment. Advances in molecular research are expanding our understanding at a phenomenal rate. In just a short time, more than thirty white patterns have been formally identified, and we know there are many many more. We know more about how these different patterns are related than ever before. The downside is that what has been discovered does not exactly match up with the structure that many of us have come to depend upon when explaining white patterns. Just naming the different patterns is a challenge, if you believe that the primary benefit of a uniform naming system – one that is used across breeds and countries – is clarity. This challenge of names and structure will be the topic of the next post.

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Cornaz albinos

Patty
Patty, an albino Pekingese from an early 20th century experimental breeding program

So the mutation responsible for white Dobermans has been identified, and is similar to the Cream and Pearl dilutions in horses. So what about Angel, the albino Shih Tzu that started this discussion? Is she also the canine equivalent of a cremello?

Probably not. An albino Lhasa Apso tested negative for the mutation found in Dobermans. What’s more, she did not have a mutation to that gene (SLC45a2). Whatever caused her pink skin and cream coat, it appears to be unrelated to the color in Dobermans. Because the Shih Tzu and Lhasa Apso shared a stud book up until 1934, it seems more likely that Angel would have the same mutation as the Lhasa. Both breeds also share a history with the Pekingese, which has one of the most throughly documented families of albino dogs. The influence of that family may explain why albinism in dogs is often seen in the smaller Asian breeds.

The albino Pekingese were the focus of an experimental breeding program conducted around the turn of the last century. Extensive information on the foundation animals appears in A Monograph on Albinism in Man, published in 1913. Although the program was disrupted by World War I, it did continue for a time and a follow-up article was published in 1929. Because the information was so detailed, it is possible to know the founder for the color in the Pekingese.

AhCum

That is Ah Cum, the “grandfather” of the Pekingese breed. He was an ordinary red sable, but because he was the common ancestor in all the known albinos in that breed, the authors of the study believed that the albino gene came from him. There can be little doubt that his son, Ch. Goodwood Lo, carried the recessive gene for the color.

Lo

What is interesting about this particular family, and this experiment, is that many of the dogs were photographed. The written notes on the dogs can be less than helpful, because those studying the dogs did not yet understand something that those of us who study animal coloration take for granted now, which is the concept of base colors and modifiers. So instead of seeing these dogs as a basic color, like sable or black-and-tan, that had been diluted down to a nearly white color by a modifying gene, the researchers assumed they were dealing with separate colors. They called the near-white dogs “Dondo Albinos” and the somewhat darker dogs “Cornaz Albinos”. That latter term is still used for this color in many breeds where albinos are known to occur.

It should be noted that the authors knew these dogs did not have pink eyes, or even necessarily blue ones. They considered an eye to be albinotic if the pigment was reduced. In fact, their discussion of equine eyes touched on a question that has often been on my mind. We often hear that horses do not have “true albinism” because there has not yet been a documented case of pink eyes. What I have often wondered was whether a pink eye is actually possible in all animals given the varying structure of the eye. Is an eye without pigment always pink or red?

In regard to the colour of the iris as seen during life in the imperfectly albinotic eyes, the present observations confirm in an interesting manner our previous knowledge that when the mesoblastic pigment is absent the iris is either white (the so-called “wall” eye) or blue or slaty blue according to its thickness and texture, a thick and fibrous iris being white and opaque throughout or translucent only at its thinnest part. In the horse even the thinnest or pupillary zone is probably too thick to be translucent.

Here the authors – two of whom are ophthalmologists – seem to suggest that in some animals an eye without pigment might not necessarily appear pink or red. Yet they also mention the difficulty in finding a horse with perfectly unpigmented eyes.

We have hitherto not succeeded in meeting with a perfect albino horse; the epiblastic pigment of the iris seems peculiarly persistent.

The Pekingese family was considered an example of ‘imperfect’ albinism, which meant that there was some trace of pigment either in the eyes, skin or hair. That is still what this kind of coloring is called in the dog world: albino. If something like this turned up in the horse world, there is little doubt that it would be considered a dilution, just as champagne and pearl were when they were identified. But as I mentioned, when this breeding program was undertaken the concept of a diluting modifier was not understood. (To give some perspective on the understanding of inheritance at the time, James Cossar Ewart’s famed Penycuik Experiments disproving telegony – the idea that previous matings left a taint that could influence later offspring – had been published only a dozen years earlier. Crick and Watson’s discovery of the double-helix structure of DNA was still forty years away.)

To a modern student familiar with how diluting modifiers work, the underlying colors on some of these Pekingese is obvious. Hints of the dark ‘spectacle’ markings common in sable Pekingese can be seen in the photo of the dog at the top of this post. The dog below looks to be a dilution of the black-and-tan pattern, judging from the coloring on the face and forelegs.

Fo

It also appears that some of the darker dogs may have been carrying some combination of the Cornaz albino dilution and the more common dog dilution, Brown. The color of the darker Cornaz albinos was described as “scraped chocolate”. One of the ancestors of the foundation stock was described as “liver and white”, and there was at least one puppy from the experiment that was noted as having a brown, not pink, nose. When later generations were crossed on black Pomeranians – which the researchers, anticipating the “designer dog” trend by a hundred years, called Pompeks – one of the first generation litters resulted in two chocolate puppies. In this way, it seems possible that the Cornaz dilution combines with Brown to produce an intermediate shade, much like Cream combines with Pearl or Champagne in horses.

Some of the puppies were surprisingly dark at birth, but still had pink – not chocolate – noses. The authors noted that the color at birth tended to be darker than the mature color, which is also true for Champagne foals. This Japanese Chin shows the kind of deeper coloring that some of the adult dogs in the study were said to have. Although it is not (yet) possible to test for the Cornaz coloring, it would be interesting to test some of the darker Cornaz albinos for Brown.

Fe

One thing that I have found surprising, since the initial post about Angel, is the number of albino-like dogs, and the range of breeds where they have occurred. It is possible that some do share the same mutation as the Dobermans, either due to outcrossing or because the mutation predates the formation of those breeds. Others likely share whatever mutation is responsible for the albino Lhasa Apso. It is also possible that there are still more mutations unrelated to the one in Dobermans and the one in the Asian breeds. With the exception of Pearl, dilutions in horses have so far proven to be dominant, or at least incompletely dominant. Because the diluted colors in dogs are more often recessive, it is far easier for them to hide for generations, especially when they are rare in the population. If these are older dilutions, then it is possible that albinos may appear unexpectedly in different breeds, just as chocolates and blues do.

So what does this all have to do with horses? That’s the topic for the next post.

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