Tag Archives | cream

Where the wild things are

NotTob

In response to the previous post about the Agouti locus, a reader questioned the importance of the fact that bay was the original color of wild horses—that it “came first”, before black or chestnut. Why should that matter?

This question touches on the reason why I have come believe that the way color gets explained matters so much. I have mentioned in previous posts that equine coat color has become far more complex since I first began writing about it. (At the risk of revealing my age, that was around 1990.) There was a time when everything could be explained in terms of the colors themselves, while the technical aspects of genetics could be skipped or at least minimized. In hindsight, while some of these explanations made certain concepts easier to grasp, they could also be misleading. Let me give an example of a common misunderstanding from fifteen years ago, the explanation that was used to clarify the situation, and how that same concept—so useful then!—is somewhat problematic now.

At the time in question, the concept of dominance was difficult for a lot of people. It was not uncommon to get questions like, “Which is more dominant, grey or bay?” Or, “I know bay is dominant to chestnut, so shouldn’t palomino be recessive to bay?” These questions arose because it was not clear that a color could not be dominant or recessive to an unrelated color. My approach was to point out that colors could only have this kind of relationship with their opposites. Tobiano, for example, could not be dominant or recessive to buckskin; tobiano could only be dominant or recessive to “not-tobiano”. The opposite of a color was not a different color, but the absence of the color. This was a very clear way to get the idea across that the genes for different colors were separate things, and that each presented an independent chance for inheritance.

BuckTobi
Fifteen years ago, many did not understand that each aspect of this horse’s color—bay, cream and tobiano—involved a separate, unrelated gene.

It was a simple explanation, but behind it lurked some puzzling questions for anyone who cared to look a little closer. If tobiano was dominant to “not-tobiano”, what exactly was this “not-tobiano”? If tobiano was believed to have arisen after domestication, how on earth were those wild ancestors carrying around a gene for the absence of something that did not yet exist? The idea of “not-tobiano” worked when it came to predicting breeding outcomes, but looked at in this light it made no sense.

That is why something like the situation with bay as an ancestral color matters, because the key to understanding what is really going on with “not-tobiano” can be found there. As I mentioned in the previous post, bay (or bay dun) is the most likely ancestral, or wild, color for horses. The other two basic colors, chestnut and black, were later mutations to the two genes responsible for bay. Another word for those alleles that were already there is wild-type. The wild-type is the allele that is typical for a given population. Wild-type is the “normal” setting—the default—for a gene. “Not-tobiano” and all those other “not-colors” were really just that: the wild-type for their particular gene. In the case of things like dilutions and white patterns, the wild-type is usually just the instructions for normal pigmentation.

Shifting from a color-based approach to a gene-based approach

Looking at colors in terms of the wild-type eliminates the misunderstandings that come from thinking of the color itself as a gene. Because we often refer to colors this way—as the “tobiano gene” or the “cream gene”—it is easy to get the idea that something like the cream dilution is an additional gene that palomino, buckskins, and smoky creams have; one which non-diluted horses do not have. The cream dilution is actually a mutation that occurred to a gene, known as MATP, that all horses have. In the absence of the cream dilution, MATP is involved in the normal formation of pigment. So the wild-type for that gene gives a fully-pigmented horse.

Not knowing there is a wild-type makes it seem that the color (Cream) is the gene itself and therefor the starting point. That is why there is a tendency to assume later discoveries are “mutations of the color” rather than alleles for the same (non-mutated, wild-type) gene. So pearl, which is found in the same genetic location as cream, becomes a “mutation of cream” rather than a second, unrelated mutation of the MATP gene. But the starting point is not cream, but the wild-type at MATP. The cream mutation did not have to be present for pearl to occur; it is a mutation like cream, not necessarily a mutation to cream.

But perhaps more importantly, many colors were named before their relationships to other colors were understood. Things that once were assumed to be separate later proved to be alleles of the same gene. At one time we thought, and taught, that the opposite of tobiano was the absence of tobiano. But the “tobiano gene” is not a separate gene. Tobiano is a mutation to the KIT gene, which again is a gene that all horses have regardless of their color. Tobiano shares the KIT gene with a host of other alleles (like Sabino1, Roan and the White Spotting patterns) that have historically been thought of as unrelated. That complex situation is very difficult to explain, especially if someone’s basic understanding of the subject is color-based rather than gene-based, because the relationship between that group of colors is not visually obvious.

Unicorn
Tobiano and roan are both alleles of the KIT gene, which is why the combination does not breed true. The offspring can only get one or the other from the parent.

I know for many who have learned about horse color exclusively in terms of basic colors and their modifiers, focusing on the actual genes is a very different approach. It may seem like it adds a lot of unnecessary complexity to the subject. I certainly can appreciate that point of view, but genes and the importance of using their wild-type as a starting point is the missing piece of the puzzle for a lot of people. When that piece falls into place, color genetics—especially as it is currently understood—begins to make a lot more sense.

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Frosted Buckskins

FrostMan

I am still sorting through some of the old photos I have, trying to remember what has been posted (as opposed to “meant to post but never did”). If I repeat something, please forgive me - though I would imagine that if after three years I cannot remember posting about a subject, maybe readers have forgotten it, too!

Recent conversations about flaxen-maned bays reminded me that I had meant to post these pictures of buckskins with frosted manes and tails. As the photo above shows, the hairs are pale flaxen or white. It is harder to tell because the pulled mane on this Paint gelding is so short, but most of the time the pale hairs are short, which gives the mane a frosted look. Pale hairs are also seen at the tailhead. This next picture shows the distinctive “V” shape that is typical of the frosting on a buckskin’s tail. This shot also shows more clearly how the white on the mane is concentrated at the base of the neck.

FrostTail5028

The frosting on the tail looks quite different from that kind often seen on duns. With a dun, the paler hairs are usually found on the sides of the tailhead, in part because the dark pigment of the dorsal usually runs down the core of the tail.

DunTail

Both frosting on buckskins and on duns looks a bit different from the white “coon tail” seen on some of the white ticking and sabino patterns. With this picture you can see both the paler hairs to the sides of the tail (relative to the deep red dorsal stripe) and the white hairs that are part of the patterning.

DunCoon6701

Frosting is more common in duns than in buckskins, but it is not always pronounced. This dun mare has very little contrast – just a few paler hairs – between the core of her tail and the sides.

DunTailhead

So what causes frosting on a buckskin? Most likely it is the Cream (Cr) gene turning what would be paler red guard hairs to a pale flaxen or white. This photo shows the similarity between the arrangement of the pale hairs on a light bay and those on a frosted buckskin.

CreamFrost

Here is the same bay Paint Horse mare that is pictured above. She has the reduced intensity at the points that is often seen on bays with paler hairs at the base of the mane and tail. If you look closely, you can see the lighter hairs at the base of her mane, too. (Unfortunately she was always on the wrong side for the sun, so none of the photos taken from her other side turned out.)

GuardHairs

I suspect that selecting for this kind of clear bay with reduced black points would increase the contrast on the frosting of both buckskins and duns. That is probably why frosting is so typical of the Fjord. That breed appears to carry almost every factor that might reduce black points.

The downside of frosting on buckskins is that is does not appear to be permanent. As the horses age, they seem to lose the contrast until their manes and tails are black. At least, that has been my observation based a limited number of individuals. Certainly if a reader has an older buckskin that still has pronounced frosting, I would love to hear from them!

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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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