Tag Archives | dominant white

“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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So what happened to all the sabinos?

SabinoA

As I mentioned in the previous post, sabino was once a catch-all term for a group of patterns that had a pretty broad range of expression. The first use of the term in an English-language book on horse color that I have found was Reiner Geurts’ Hair Color of the Horse, published in 1977. (The original Dutch book was published four years earlier.) Although it appears that Geurts took the term from an earlier Dutch paper written by J. K. Wiersema, he does state that it is an American term. I suspect that  by this he may have meant South American, since it is a Spanish word that does not appear to have been widely used in the United States until the late 20th century. This is how Geurts characterized sabinos:

[The term refers to] a kind of piebald with often indistinct and irregularly bordered white radiating from under the breast, the belly and the extremities, and thence spreading laterally up the ribs and sometimes to other parts of the body. The rest of the coat is often ticked with white, or roan.

In 2005, scientists identified the mutation responsible for one of the sabino patterns. In their paper, the authors’ criteria for sabino was consistent with Geurts’ definition:

Horses characterized as having a Sabino white spotting pattern had three of four of the following characteristics: (1) two or more white feet or legs, (2) blaze (white patch extending the length of the face), (3) jagged margins around white areas, and (4) spots or roaning in the midsection.

The identified pattern was named Sabino-1, because the researchers expected to add more patterns to the series. From the same paper:

Consequently, we anticipate future reports for other genes encoding different Sabino phenotypes (possibly SB2, SB3, and so on)… The variant of KIT described in this study can explain some but not all the phenotypes described as “Sabino.”

That last sentence proved to be true. The resulting test is useful for only a small percentage of horses with a sabino pattern. So what happened? Where are the additional sabino patterns that were predicted?

SabinoB

The simple answer is that many sabinos, like the two Paint Horses pictured above, have patterns that have not yet been formally identified. That is, the mutation that causes them has not yet been found, which is necessary for the development of a test. But it is also true that along the way, the approach to naming the different KIT mutations seems to have changed.

Around the same time that Sabino-1 was identified, researchers began looking at a suspected Dominant White family in the Franches-Montagnes breed. Just like other researchers in the past, they indicated that the mutation did not always produce a completely white horse. The paper on Sabino-1 noted that these horses fit the phenotype for sabino, but that the pattern of inheritance matched the one described in the original studies of Dominant White. In 2007, a paper was published identifying the Franches-Montagnes mutation along with three others. These were named White-1 through White-4. Because these particular mutations consistently produced white or near-white mutations, and because the patterns appeared to be homozygous lethal, it made sense to categorize them as Dominant White and not Sabino.

It should be noted, however, that the original paper outlining those first four Dominant White families did anticipate the possibility that future dominant white (W) mutations might not necessarily be lethal. In fact, it was not clear if all four of the initial mutations were lethal. Nonetheless, some saw the traditional distinction between the non-lethal Sabino patterns and the homozygous lethal Dominant White as significant for determining the appropriate category for a given pattern.

This became important when some of the patterns identified in subsequent studies more closely resembled Sabino than White. This was particularly true of the fifth White family, which originated with the Thoroughbred stallion Puchilingui. Puchilingui had what would have been called a “sabino roan” pattern, but his offspring were often loud, patchy sabinos.

W5

Sato, pictured above, has what many would consider a typical pattern for a horse with the W5 mutation. It seemed that most horses that inherited this particular Dominant White mutation had a sabino phenotype. There were, however, some members of the family that could be described as white. This is the white Puchilingui daughter Shew.

ShewForBlog

With the identification of the W5 mutation, some began to refer to all highly-contrasted, patchy patterns as Dominant White while others (including many breeders) continued to prefer the term Sabino, even for those families with known mutations assigned a number in the W series. The confusion was increased with some of the later additions, particularly the two most recent Arabian mutations, W15 and W19, which did not seem to produce white phenotypes – or did so only very rarely. Would there be any new sabino patterns? Was the single incompletely dominant pattern likely to remain the first and only named Sabino pattern?

This question came up again when it was discovered that the reason some of the Puchilingui horses were white was that they carried two different KIT mutations. In addition to the already-named W5, white horses like Shew had another mutation that was initially overlooked because it was not thought to significantly alter pigmentation. It would appear that on its own, the W5 mutation produced what would have been called Sabino. Loudly marked, but still Sabino. It took adding a “quieter” pattern to produce the white or nearly-white phenotype.

The additional mutation was said to be “common and widely distributed”, and to have a “subtle white-increasing effect”. Of the 145 horses in the study, 52 were found to carry this particular mutation. Although only a handful of tested horses have published photographs, those that have been identified appear to be minimally-marked sabinos of the “flashy white markings” variety. In assigning the name W20, the authors gave the following reasoning:

We termed the new KIT variants W18 – W20 to provide a simple and unambiguous nomenclature for future genetic testing applications.

Some have interpreted this decision to mean that the name Sabino has been abandoned. It is an understandable conclusion, if horses like these two are not going to pick up the numbering in the Sabino sequence.

W20Samples

Yet perhaps overlooked is that in that same paper, the term Dominant White was also dropped. The previous seventeen mutations in the W series are referred to as Dominant White, but that phrase appears in scare quotes. Instead, the patterns are referred to as “White-spotted”. Sabino appears as a descriptive term (“sabino-like”) for one of the mutations described in the paper. The approach appears to be to number the new mutations to KIT consecutively in the W-series, regardless of the phenotype. That means that the “official” notation for the mutation may not correspond with the visual appearance of the horse. Given that diverse colors like tobiano and dark-headed roan have been found to share this same molecular location, it might be helpful to develop additional terminology that communicates visual differences that breeders find significant. From a molecular standpoint, dark-headed roan is one type of depigmentation, and is therefor a form of “white spotting” (and has been referred to in this way by scientists). A breeder interested in a predictable amount of white (either enough or not too much!), however, might have quite a different idea of what is and what is not white spotting.

It may be that phenotypes that fall clearly into existing groups, like roan (Rn), will retain their unique names. Unfortunately the appropriate category for many of the resulting colors – including some of the roan phenotypes as well as those factors that produce “ordinary” white markings – is likely to be less clear. While adding to the list of W mutations might provide “clear and unambiguous nomenclature” for tests, breeders and owners may find that they need additional names and categories to effectively communicate with one another.
KITTobi

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Names, categories and descriptive terms

TobiTypical

FrameTypical

I believe that often a seemingly illogical system makes sense when its history is considered. Because revelations are often gradual, systems build on what came before. In this way you often end up with something that is less ideal than what could have been devised had a more accurate picture been known from the start. I believe that knowing this is essential to understanding where things stand today with the terminology for pinto patterns.

When I first took an interest in horse color, most American horsemen accepted that there were two kinds of pinto patterns: tobiano and overo. Those were the terms used by the American Paint Horse Association, which was the largest registry for pinto-patterned horses. If you read enough horse magazines, eventually you would run across a list of rules for determining whether or not a horse was a tobiano or an overo. Did the white cross the topline? Tobiano. Was one or more legs dark? Overo. Did the horse not follow either set of rules? Then he was a combination of the two – a tovero. The rules were useful if you had a typical tobiano or a pure frame overo like the one pictured above.

They were slightly less useful for minimal tobianos like this one.

MiniTob

But the real limitation came when dealing with any of the other pinto patterns. Splashed white, for instance, actually followed the tobiano rules more closely than those for overo. The truth was that the rules for “overo” could be more accurately said to be the rules for frame overo. The rules described that very specific pattern, but in actual practice the term overo was being used to mean anything not tobiano.

In the 1970s and early-1980s, a number of writers began to include additional pinto categories. Among these were splashed white (which had been described in still earlier literature) and sabino. In the United States, Paint Horses with these patterns were registered as overos, so the convention became that, aside from tobiano, the “new” patterns were different forms found under the general category of overo. Thus there was a frame overo, sabino overo, and splashed white overo. Later, when it was identified and named, there was manchado overo. This convention can be a source of frustration for breeders in other countries where overo has been used pretty exclusively for frame overo. (The fact that frame overo is one of the better-known lethal genes probably adds to the desire not to see the same word tacked on to the back of other pattern names.) It makes more sense if it is understood that this was a way of integrating new information into a widespread, existing naming system. It was easier to tell the many breeders of Paint Horses that they needed to be more specific about which type of overo they had, rather than tell them that they had been wrong for calling all those non-tobiano horses overo in the first place.

Categories of Patterns

That process has repeated itself a few times in the years since. We start out with patterns that have names, and then find out we have unwittingly placed genetically different things together under one name. So the name becomes a category, rather than one specific pattern. This has meant it was necessary to search for a way to distinguish between the patterns within the new category. With the advent of genetic tests, the most common approach has been to number the different variations. So when it became clear that Splashed White was a category of patterns, and not one specific pattern, the variations were given numbers in the order in which they were discovered: Splashed White-1, Splashed White-2, and so on through to (at the time of this writing) Splashed White-5.

Although the speed of new discoveries made it difficult for many to adjust, the system has worked reasonably well, with one big exception. That exception is Sabino.

I have written before about my own involvement in researching sabino patterns. In the 1990s, an article I wrote speculating that many white-born horses were in fact “maximum” sabinos was widely circulated on the internet. While I came to that conclusion primarily using research on the Tennessee Walking Horse, I really was just rediscovering what the Dutch researcher J. K. Wiersema already knew in the 1960s. The pattern we both were studying is now known as Sabino-1. With that specific pattern, heterozygous Sabino-1 horses have flashy white markings with roaning, and homozygous Sabino-1 horses are white. We both also observed, however, that there seemed to be other kinds of sabinos that did not look quite like this. What’s more, those patterns did not seem to have a connection to white horses. We both also noted that there were white-born horses that did not quite fit the profile. In hindsight, it is clear that this should have been a big clue that too many different patterns were being lumped into this one category.

This problem is quiet obvious if you group images of horses with patterns that have been called sabino in the past. Here is a slide I used in a recent presentation that makes this point in a very visual way. All the horses in this image have patterns that would, at one time, have been referred to as sabino.

SabinoSlide

Given the broad range of white patterning in that image, it would not be surprising that many began to consider sabino a pattern category. Within that category, or group, one might expect to find quite a lot of separate patterns. And that was what many expected, after the first sabino pattern (Sabino-1) was identified. The assumption was that researchers had found the first of what was likely quite a large number of them. Certainly it was clear that what was arguably the most common form of sabino – what many breeders refer to as “flashy white” – was not the one that had been identified. Although Sabino-1 was later found to be quite an ancient pattern, in modern times it is found in a rather limited range of breeds. It was expected that some of the more common types of sabinos would soon follow.

FlashySab
Horses like this one, with flashy white stockings, a blaze, and maybe a bit of white on the belly or girth, have long been thought of as the most common form of sabino

That expectation did not come to pass. The next set of patterns that were formally identified were classified as Dominant Whites and given the names White-1 through White-4. Each of these particular mutations resulted in horses that were, in terms of phenotype, either completely white or nearly all-white. Because the idea that white-born horses were “maximum sabinos” had gained a lot of traction in the years prior, there were some that felt those first four patterns should have been part of the sabino category, picking up the numerical sequence at Sabino-2. The truth was that those first patterns did fit the original definition of Dominant White in that they produced their own color 50% of the time, and the mutations appeared to be homozygous lethal. The fact that some of the individuals with those mutations were not entirely white would not have actually come as a complete surprise to earlier authors who wrote about Dominant White. Almost every account of the color mentioned the tendency to throw “pied” foals along with white ones. It did make sense to classify those mutations as White rather than Sabino.

When more patterns were added to the White category, the difficulty with the terminology increased. That was especially true with some of the later patterns that looked far more like what had traditionally been called Sabino. Some breeders working with the bloodlines where the more patchy expression was common referred to their horses as Sabinos in advertising. This stallion, Sato, from the Puchilingui line (W-5) of Thoroughbreds, is a good example.

Sato1

For someone interested in producing pinto-patterned sport horses, advertising a horse as Sabino makes good sense, because most pinto breeders do not actually want a white horse. Indeed, some registries for colored horses will not issue regular papers for white horses; they are considered “solids”. Did breeders know that a horse like Sato was technically White? Yes. And the line did produce all-white horses on occasion. But most were loudly patterned in a way that would, at least at one time, be called Sabino.

Officially, however, these horses were White, popularly known as Dominant White. For the sixteen patterns of this type that were discovered after those initial four, each was named as part of the W series of mutations. It was assumed that any other mutations found in that same region were likely to continue in this fashion, while nothing else would be added to the Sabino series. In this way, Sabino could be said to have gone from a named pattern, to a category of patterns, to an informal term used to describe a specific pattern on a specific horse (much like the way roaning is used). Put another way, sabino became the official name for one specific pattern (Sabino-1), and an unofficial descriptive term for some individual horses with patterns of some other official name.

That was roughly where things have stood until very recently, as reflected in the chart below.

NameChart

That seemed to be the system that would continue, until researchers got to W-20. That is where the naming system became a bit harder to explain, because there have been unexpected twists in the most recent discoveries. Because things get a bit more complicated from here, I am going to break this across to a second post. This one has already run quite long (even for a blog that routinely breaks the “keep it brief” rule of blogging!) and there is a lot to digest. I also want to pull the next part out because a study is currently being organized that touches on these new developments, and I want to give that the space it deserves. With luck, I will get the next part out in a more timely manner than this one!

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