Archive | Terminology

The Law of the Soft White Underbelly

WoodMouse

In my previous post, I said that I would be following up with a post about dun. It occurred to me, however, that it might be helpful first to give some background on some terminology. There is no question that among those who like to talk about horse color, terminology can be a minefield. Some of that comes from the limits of the English language—there are only so many words for spotted—and some of it comes from the non-linear nature of discoveries. If only the pieces of the puzzle were found in order, or if we at least got to glimpse the picture on the puzzle box before proceeding, so we knew what the end result was supposed to look like!

But since that is not the case, terms get borrowed and repurposed and stretched to fit things. And in a sense, this is going on within each community focused on a particular domestic species. There is some back-and-forth, but with the rapid pace of discovery (and the ability of non-technical audiences to talk to one another about the subject, often without direct collaboration with the academic community) there is a certain amount of divergence. This can create inconsistencies that can lead to confusion for the unaware.

The conversation surrounding the new discoveries about dun have highlighted an instance of that kind of situation for those interested in horse color. The term that may cause confusion is countershading.

If you frequent online horse color discussions with any regularity, you have probably heard this term used to describe dorsal stripes on horses that were not true duns. But countershading has a very specific meaning when speaking of animal coloration. In fact, it is the subject of a scientific law. Thayer’s Law states that animals display darker coloring along their topline and paler coloring on their undersides as a means of camouflage, because it counters the shadow cast by the sun. My husband once referred Thayer’s Law as the Law of the Soft White Underbelly. It is why so many animals, like the wood mouse pictured at the top of this post, have pale undersides.

Thayer2
Abbott Thayer used this photo of a white rooster against a white backdrop to illustrate how shadowing on the underside of solid colored animal would make it more conspicuous. Meanwhile the pale undersides of the three ibexes in the second picture help to conceal them. 

Abbott Thayer was quite enthusiastic about his theory, and he tended to overstate just how pervasive this type of coloration was in the animal kingdom. It is certainly true that horses do not display this type of coloration in the same pronounced way that many other animals do. Writers have noted that strong countershading is not really a feature of horse coloration.

Externally, horses are recognizable by the long-haired tail; the mane that is both long and thick… and the poor countershading
The Genetics of the Horse, Bowling and Ruvinsky (2000)

Even so, cave paintings suggest that wild horse coloration followed Thayer’s Law at least to some degree.

Lascaux Cave Paintings - horse

We would call the color in the above painting mealy or pangare. In her book Farben und Farbvererbung beim Pferd, this is the color Henriette Arriens associates with the term countershading, and it is the equine color that most closely fits the scenario described by Thayer. It is, however, not a particularly common coloration in modern breeds. Outside of some of the heavy draft horse and rustic pony breeds, this type of pronounced mealy pattern is rather rare. In that it is much like the other primitive equine color, dun!

MealyBelg
It is not hard to imagine that ancient cave painters were looking at horses with this type of coloration.

But the term has also been used by some authors for sootiness. In his book Equine Color Genetics, Philip Sponenberg refers to it as “black countershading”.

…the sooty effect is most commonly expressed over the top of the horse so that the back, shoulder, and croup can almost look black, whereas the horse is redder on the lower body, belly, and upper leg. This effect is called black countershading, or “sooty,” and results in an animal that is dark on top and lighter beneath.

This makes sense since the overlay of black hairs on sooty horses often reflects that same dark-on-top, light-on-bottom configuration. A countershaded bay, then, was a bay with an overlay of black hairs along the topline. This can be seen in some of the examples of sooty dappling on this Pinterest board.

This association with sootiness is probably why, when trying to distinguish the soft-edged dorsal stripes seen on some sooty horses from true dun dorsals, some began referring to markings like the one on the sooty buckskin below as “a form of countershading”. The idea being conveyed was that the marking was not related to dun, but rather was one way that sootiness (“countershading”) expressed on some horses. And since the term was so often used in online forums to argue that this or that horse was “not really dun”, for some the original connection to sootiness (never mind Thayer’s Law) was lost. Countershading was, for them at least, just another word for a non-dun dorsal stripe.

SootyDorsal

So when news came out that researchers looking for the causative mutation for Dun (D) had found an additional mutation for primitive markings “without any dilution”, quite a few people imagined that this second mutation was for “countershading”. There was confusion, however, that term did not appear in either the paper on the new discovery or the dissertation that described the research involved. That is because the scientific meaning of the term has not changed; it is still about a lighter underside countering the self-shadow of an object. Researchers may not be aware that some have given the term this different meaning, but even if they were, an audience of fellow scientists would likely be confused by the change. If the idea is to counteract the shadow cast by the mass of the body, a thin line of darker color along the back of a non-dun horse is not likely to provide much camouflage.

That is why the term is conspicuously absent in the scientific literature on dun. But I think there are other reasons why those of us who are not scientists might want to adopt some different terminology. I’ll talk more about that in the next post on the new not-quite-dun (d1) mutation.

And for those that have an interest in Abbott Thayer and his work on animal camouflage, check out “A Painter of Angels Became the Father of Camouflage.” I have a soft spot for him as another artist whose obsessive nature drew him into science (and the science of animal coloration, no less), but by any measure his work was incredibly influential. You can also access his son Gerald’s compilation of his work Concealing-Coloration in the Animal Kingdom (1909) through Google books.

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“W” is for White-spotting

WhitePrince

The last few posts have focused a fair bit on the two KIT colors that, by long tradition, have their own names: tobiano and (dark-headed) roan. At the moment it is hard to imagine any problems with the terminology used for tobiano. The fact that it involves the same gene as sabino and dominant white has implications for breeders who are dealing with lines or breeds where those colors occur, but in most cases the fact that the horses are tobiano is still pretty obvious. The situation with true dark-headed roan is likewise pretty clear, at least visually. (The more confusing situation with roan genetics is a post for another day.)

The problematic terminology, at least at the moment, centers around the sabinos and the dominant whites. It appears likely that most, if not all, future discoveries are going to be given consecutive numbers in the W-series. Because that is true, it is helpful to keep in mind that the W in the name means “white-spotting” and not “white”. Related to that, the tendency to refer to all the patterns in the series as “dominant white” is probably not helpful, because while KIT mutations are expected to be dominant (that seems to be consistent across a range of species), many are not dominantly white. In fact, some are not white at all. Like the older category of overo, the white-spotting category may not really tell breeders what they need to know about a color; it is not specific enough.

From a practical standpoint, there are really three different categories that these white-spotted horses fall into. The first are the truly white – or at least nearly-white – horses. The horse above, White Prince, is an example of this type of pattern. In fact, his particular mutation (W2) is one that produces all-white horses consistently. There are others that produce a bit of color (usually on the ears or topline, often in the form of dark ticks or spots), but the overall impression is still that the horse is pretty white. A good example of a family with this near-white expression is that of the Arabian, R Khasper (W3).

The second category are the obvious pintos. These are the horses that are clearly broken-colored, with extensive white on the body. The average horseman, even one pretty familiar with color, would not call these horses white. Instead, they are most often called sabinos. The Thoroughbred family of Puchilingui (W5) and the Arabian families of Rhocky Rhoad (W15) and Fantasia Vu (W19) fall into this category.

SatoSide
Sato, Thoroughbred stallion with the W5 mutation

And finally there are the horses that would be thought of as having white markings. Not only are horses like this not obviously white, but many would not recognize this as a pinto pattern. The recently-discovered W20 mutation falls into this category.

MonaLisaGF
Mona-Lisa GF, German Warmblood mare heterozygous for W20

I say these are practical distinctions, rather than genetic distinctions, because most often breeders have a preference for one of these three phenotypes. Breeders of white horses, for instance, have struggled with the tendency their lines have for producing what they thought of as pintos. Meanwhile many breeders of pintos often want to avoid a horse like White Prince, which many colored horse registries view as “solid”. And patterns that produce flashy white markings, but that do not consistently produce white on the body, can be problematic in both solid and pinto breeds. That is why it is not necessarily helpful to categorize all the W mutations as “white”, nor for that matter to call anything that puts white on the horse “white patterning” – even if it is genetically accurate to do so.

The challenge, however, if that while those really are the categories that interest breeders, the genes themselves do not all fit neatly into just one of them. The relationship between the three basic phenotypes can only be called complex. In fact, there are intermediate colors that make the borders between the categories pretty fuzzy. Patterns like this one, for instance, sit somewhere between the white (and near-white) horses and the sabino patterns. For most horsemen, this is still very much a pinto pattern even though from a visual standpoint it is quite different from the pattern Sato has. Some of the draft and pony breeds, either by informal tradition or by actual rules, penalize horses with white on the body, but this kind of pattern might fly under the radar as “roan”. (If you nudge the contrast down a bit, that is even more likely to be true, and many individuals have less contrast than this particular horse.) Paint Horse breeders, meanwhile, would not only consider this a pinto pattern, but they would think of it as a fairly loud and obvious one. There are different traditions (and incentives) that play into how these pattern varieties are perceived.

ApacheLineSabino
A Thoroughbred from the Airdrie Apache line, which is widely rumored to have a mutation that has (to date at least) evaded detection

Here again are the images that given an idea of the range that white-spotting (W) can take: white, sabino-roan, sabino, and white markings. As pointed out, it can be difficult to draw a hard line between these four categories because one tends to blend into the next. But what makes the situation difficult to simplify is that there are different combinations that can produce the same color. Horses that look the same can be the product of a different genetic “recipe”.

KITrange
(Click to enlarge this graphic)

To illustrate this, let’s take a hypothetical all-white horse of unknown parentage. There are a number of ways, within the known mutations, to get that color. However, if she was then bred to an unmarked, solid stallion, the expected outcomes would be very different depending on why she was white. Here are some of the “recipe” options for her color and what they might mean for the resulting foals:

  1. She could be homozygous for Sabino1. In that case, she would produce 100% sabinos but no whites. Most would probably run towards the roany end of sabino expression.
  2. She could be heterozygous for a Dominant White variant that was inclined to be more truly white. She would then produce 50% white or near-white, and 50% solid.
  3. She could be heterozygous for a Dominant White variant that tended to “leak” color a bit more. She would be expected to produce 50% white, near-white, and sabino (probably more to the sabino roan end), and 50% solid
  4. She could be a compound heterozygote for two white-spotting genes. She would be expected to produce one of those patterns 50% of the time, and the other 50% of the time, but no whites like herself. If the other white-spotting pattern was something like W20, half the offspring might have white markings, but might not be readily identifiable as pinto-patterned.
  5. She might have a combination of patterns unrelated to the KIT gene which, when combined, give an all-white foal. In this scenario, she might produce quite a range of patterns either singly or in combination.

That means that without knowing what a white horse carries, you might have a horse that never produced its color, or sometimes did. Likewise it might never produce a truly solid horse, or it might sometimes do so. In the case of the fifth option, there could be entirely unexpected outcomes. It is not surprising then that white breeding programs established in Europe during the Baroque era typically ended in failure! Without a way to tell visually identical colors apart, the results would have seemed very unpredictable.

For many, it probably still seems this way. The fact is that what is true for one particular white-spotting mutation might not be true for the others. Each mutation requires its own explanation. With twenty different named white-spotting mutations, and many many more believed to exist, it is not surprising that many find that prospect discouraging. Because this is an area of ongoing research, it seems the most pragmatic approach is to not expect a perfect system for categorizing and naming these colors – at least not yet, when the picture is still incomplete.

In the next post, I want to focus on the effort to fund a study of the newest of the white-spotting mutations, W20. That study has the potential for expanding our understanding of these types of patterns, so I want to give it the attention it deserves.

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A family of diverse colors

SabTobHalf

It was perhaps a bit rotten of me to bring up the tobianos and dark-headed roans when talking about the confusion about what to call horses with sabino patterns. Instead of saying, “No, this really is pretty simple,” I opted to point out that it is even more complicated. Now that I am feeling a little less mischievous, I probably should attempt to clarify things a bit.

I began this (meandering) train of thought with a post about the change from Mendelian genetics to molecular genetics. That is, a shift from analyzing colors using visual identification and statistical analysis to understanding colors based on the changes to the genetic code. Using the first method, colors were grouped a certain way that has become familiar to many horsemen. When the colors are grouped according to the gene where the mutation occurs, however, they sort a little differently than expected. Colors that look quite different – colors that aren’t even thought of as belonging to the same basic category of modification, like tobiano and dark-headed roan – can be mutations to the same gene.

The technical term for this situation is allelic heterogeneity. In plain English, what that means is that there are a number of different options for one gene. In the case of these particular colors and patterns, the gene where they are found is called KIT.  Tobiano, roan, sabino and dominant white are all alleles at KIT. As different as they look from one another, they can be thought of as belonging to the same family. This may seem a bit esoteric, but it has a couple of implications for breeders.

The Spotted Saddle Horses pictured at the top of this post display a type of tovero pattern that is very common in their breed. The ragged, torn outline of their spots is typical of what happens when tobiano is paired with Sabino1. It is a compound heterozygous pattern. That is, both copies of the KIT gene have a mutation, but they are different alleles. If that same horse had two copies of tobiano (two of the same allele), we would call him a homozygous tobiano. Instead these horses have one tobiano and one Sabino1 (two different alleles for the same gene). A horse has two copies of a given gene, but they only get to give one of them to each of their offspring. So like the homozygous tobiano, if they were bred to a solid horse all their offspring will be pintos, but only half will be tobiano. The other half will be sabino.

Sb1HeteroHalf
Bred to solid mates, half the offspring of the toveros above should have this kind of pattern – Sabino1.

Allelic relationships like this are important to breeders because it means that under most circumstances, the patterns that result from combinations of alleles are not going to breed true. That might not be important if all that matters is that the resulting foal have a pinto pattern, because a compound heterozygote is going to produce a patterned foal 100% of the time. But if a breeder wants to duplicate the original combination, that might matter quite a lot. And if the other “pattern” is something that would not qualify as a pinto, like dark-headed roan or one of the more minimal versions of sabino, then the 50/50 nature of the inheritance might be a problem.

Breeders have noticed that some combinations, like tobiano roan, are difficult to get consistently. That is because this same splitting of the two alleles occurs; the horse can only give one but not both, so the only way to repeat the combination is for the other parent to contribute the second allele. The fact that some of these alleles look so different from one another makes the relationship between the colors less obvious. Knowing why Sabino1 toveros do not produce their own color when bred to a solid mate allows breeders to pick crosses that stack the deck in their favor. (A cross to the same Sabino1-tobiano combination, for instance, would give the desired pattern 50% of the time.)

The connection between these seemingly different colors might also make it easier to understand some of the quirks within some of these patterns. One of the most common questions I get from breeders of tobianos is about roan patches, or roaning in the colored areas of the coat. It is a relatively common occurrence in tobianos, and it often causes breeders to inquire if their horse might carry some kind of sabino pattern. In many cases, it appears that the roaning is just part of the tobiano pattern itself.

Dexter2
Dexter has diffused roaning throughout the dark areas of his coat, with somewhat greater concentrations of white hairs around the borders of his spots

RoanPatch2
Here the roaning is mostly limited to one patch, though colored specks remain inside the roaned area

When tobiano is understood to be a mutation to the same site as both roan and sabino, irregularities like these seem less surprising than when tobiano is thought of as something wholly separate. Likewise, the idea that tobianos might be more prone to white on the face than solid horses seems less outrageous. Tobiano is related to a whole group of patterns that can quite rightly be described as doing just that, after all! (For newer readers, more on my scandalous views on tobiano face white can be read here and here.)

In fact, knowing that these colors are alleles of the same gene is useful because it encourages us to think about them in a different way. If we know that KIT mutates frequently, giving a surprising number of white and sabino variations, what about roan? Roan has proven problematic when it comes to testing, which suggests there is more than one version of the color. It is also true that there are quite a few instances of spontaneous “roans” in a variety of breeds. These have been dismissed in the past as not “true roan” because they came from non-roan parents. But what if they are just one of many roan mutations? And what about the various forms of white ticking, like rabicano and salpicada? Are they roan variants on KIT, too? Given what is known about the white mutations, that seems like a reasonable theory.

Taken as a group, many of these colors and patterns blend together with a lot of overlapping traits. Which brings me back to the original question, which is what to call them all. I’ve skipped over the more pressing problem of sabinos and dominant whites in this post, but I wanted to highlight the connection between these different colors and introduce the idea of compound heterozygosity. It is an idea that is pretty important to the situation with the sabinos. I had hoped to wrap this subject up with just one more post, and start posting some less in-depth topics, but it is probably obvious why I have avoided posting about this before. It is not a subject that lends itself well to brevity! So next up, the other group of KIT mutations and some ideas about what to call them. I promise, eventually I will get back to some easier topics!

KitComic

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