Tag Archives | dilutions

Is this still a horse color blog?

Talking

So what do albino dogs have to do with horse color? Certainly the fact that there are “cremello” Dobermans is interesting, but why is that relevant to this blog?

The story about Angel and her unusual coloring brought together a couple of thoughts that have been on my mind lately. I thought I might explain them briefly here, and then expand on each in a separate post.

The shift from Mendelian Genetics to Molecular Genetics
When Pearson and Usher studied albinos, Mendel’s work had been only recently rediscovered. The results from their experiments with the Pekingese left them convinced that the problem was too complex for Mendel’s theories to accurately predict the outcome of their crosses. There were too many variables. In the hundred years since, science has not only identified many of the variables that eluded the two men, but it has begun to unlock the underlying mechanisms that produce the colors. Beyond just making modern genetic tests possible, this focus on the molecular level means that it is possible to understand the nature of color mutations in ways not possible before. This has implications for anyone interested in sound and ethical breeding decisions.

Just how many colors are there? 
It is clear when reading A Monograph on Albinism in Man and Albinism in Dogs that the authors were not only grouping together different dilutions in dogs, but also blue-eyed white dogs that were in all likelihood spotted or merled. Because it is possible to test for specific colors, we now know that some colors that look alike are different things, at least on a genetic level. In some cases colors have proven to be unrelated even though the end result can be difficult to distinguish. Still other things are genetically related, in the sense that they are mutations to the same gene, but they differ visually – sometimes dramatically so. In other cases, they may be visually similar, but their pattern of inheritance is quite different. As a result, the list of known (and suspected) colors and patterns has been growing rapidly, while grouping and categorizing them has become more challenging.

What do we call all these new colors?
With each new discovery, the old naming system has been strained. Just as Dilution is a color in dogs, but a category of (visually) related colors in horses, many of the colors  - particularly the pinto patterns – could more accurately be termed categories now. Yet many still speak of patterns as singular things. The most obvious problem has been with the what was once called sabino, and now sometimes jokingly referred to as KMOSS (“KIT Mutation of Some Sort”). What do we call these new categories, and the colors within them? This last topic should make an appropriate segue into the new paper on white markings, since that, too, is part of the same problem.

So bear with me for a few days while I pull these related thoughts together in a (hopefully) coherent way, and in no time we will be back to talking about horses again!

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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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Albino Shih Tzu

AngelFace1

This is Angel, the Shih Tzu with the mystery color mentioned in my previous post. When my friend Gretchen contacted me, she wondered if Angel might be a pale lilac dilute. That is a dog that has both the blue dilution (dd) and brown (bb). The palest shades of lilac are usually the lightest color, aside from white, that most dog owners are likely to see. Angel’s skin is not a muted brown, though, but a clear pink – quite a bit lighter than would be expected in a lilac dog.

Although lilac dogs have the palest overall tone, when it comes to skin coloring the lightest colors can often be found in recessive red dogs that are also homozygous for brown (ee and bb). In Labradors, this combination is sometimes called Dudley. As this comparison image shows, Angel’s nose color (left) is considerably lighter than the one on the Dudley Labrador (right).

Angel2

The pink skin was consistent on the pads of her paws as well. This extreme absence of skin pigmentation is particularly striking to anyone accustomed to looking at dogs. As I mentioned in the previous post, dogs do not have pink skin under their white markings in the same what that a horse will. Unlike horses, white markings that cover the nose do not give a dog a pink nose. Dogs seem very inclined to retain at least some pigment in their skin, which is what makes a truly pink-skinned dog stand out.

AngelPaw
Pink Angel toes!

Historically, dogs like Angel have been called albinos. That is true even though most have blue, green or even hazel eyes. They also have traces of color in their fur, making it possible to discern hints of things like black-and-tan patterns, black masks or white markings. To someone immersed in the world of horse color, that would sound like a mistake. It is an article of faith in the horse color community that albinos must be white with pink or red eyes. Yet albino is the term used not only by owners and breeders, but in scientific literature. Researchers have told me that albino is a term that can be used strictly or more loosely. That is one reason why it is so very difficult to get a definitive answer to the question, “Can a true albino have blue eyes instead of pink?” It really does depend on who you ask!

Angel3
Angel has hazel-colored eyes, but would still be called an albino dog

I had first become aware of dogs like Angel when I stumbled across a paper on albino Pekingese when doing research on the Hanoverian Creams. I was also aware of a handful of of other more modern cases, mostly among the smaller Asian breeds. What I did not realize until I began looking was that while still quite rare, this type of coloring is more widely distributed among the different breeds (and even among what are probably pariah dogs) than I had originally assumed.

The timing of this could not have been better, too, because at least one type of albinism (or extreme dilution, if you prefer) in dogs was identified in just the last few months. The particulars of that mutation will be especially interesting to horse color researchers, but it merits a separate post. In the meantime, I have set up a new Pinterest board with a wide range of images of albino dogs. I tried to pick ones that showed the range of color in the eyes and the fur. As with the other Pinterest boards, I have tried to be sure to link the pins back to their original site to preserve the photographic credits. In the case of some of these pins, following those links will take you to further information and more photos.

And just a final note. Angel is in the care of the Crossroads Shih Tzu Rescue in Tallahassee, Florida, and is looking for her forever home. You can find more information on her Petfinder page.

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