Tag Archives | Sabino1

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

Continue Reading

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

Continue Reading

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!

Continue Reading