Tag Archives | sabino roan

“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”.

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(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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Rabicano versus Sabino

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Yesterday I use the pattern on this horse to discuss the way sabino can mimic rabicano. If sabino can mimic the rabicano pattern, then what exactly does a sabino-rabicano look like. Is it any different from the sabinos that have flank roaning and coon tails?

One difference might be the way the roaning organizes itself. Rabicano is known for having a brindled effect. That is a bit different from the diffused roaning that is present on the horse above. Contrast his side with this mare:

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In the areas where the white is less concentrated, the brindling is visible. (In my experience with my own mare, whose striping is pictured in the blog header, this trait is often less obvious in photos than it is in person.)

Striping

It may be that this trait can help identify the presence of rabicano as opposed to sabino roaning or even the other forms of white body ticking. Rabicanos do seem to brindle more often than other types of ticked horses, though it is hard to know if this is an exclusive trait or not, too.

Another interesting difference with this horse compared to the first one is that the tailhead is more completely white.

coontail1

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Could sabino, which often boosts the white in other patterns, be influencing the amount of white on the tailhead? The frequent assumption is that rabicano is adding white tailheads to sabino patterns. This kind of effect, where sabino amplifies the white of another pattern, is consistent with what we already know sabino does.

This all illustrates the problem with identifying patterns using the tools we presently have. We have pieces of the puzzle, but we do not (yet) have a complete picture. The point at which one pattern begins and another ends is not entirely clear. To make matters more complicated, the evidence suggests that at the more minimal end there is considerable overlap. And at t the other end, the amount of white tends to hide the clues!

Both horses posted are good examples. Not only are they roaned and coon-tailed, but they each have high stockings and one blue eye. Here is the head shot from the mare. (I did not have a good, in-focus head shot of the colt in the first picture.)

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Does her blue eye come from sabino, which she obviously has? The white high on the broad side of her neck suggests she also has the frame pattern. Did that give her the blue eye? Is it proof of the presence of splash? Right now we simply do not know for sure. When there are more tests, we’ll probably be able to develop a more clear picture of just where the patterns start and end. But for now it really is just guesswork. Sometimes the breed can eliminate certain possibilities, or more clues can be found by examining production records of a given family of horses, but it is still guessing.

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More mimicry

tickedsab21

Horses like this one often get labeled as sabino-rabicanos. The high stockings with their irregular edges and the wide blaze that travels under the jaw are typical of horses with the sabino pattern. The reason many people add rabicano to the description is the roaning on the flank and the white on the tailhead.  Here is a better angle to see that trait.

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The trait is sometimes called “coon tail” because the roaning can take on a banded appearance. This horse has banding that is a little more pronounced than the first horse. (It can be even more noticeable on some horses.)

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The problem with this is that a lot of sabinos have these traits, too. Both flank roaning and white at the tailhead do not appear to be exclusive to the rabicano pattern. There is overlap between the characteristics of the two patterns.  We can assume that because were all the horses with roaning on the flanks and white tailheads carrying two separate genes, one for rabicano and one for sabino, then the two patterns should segregate in a certain percentage of the offspring. That is, in addition to producing sabino-rabicanos, they should also have some pure sabinos and some pure rabicanos.

Consider the well-known frame overo Saddlebred, Beau Decision. He has a very typical expression of the frame and the sabino pattern together. Not surprisingly, those two genes segregate in some of his offspring.  Here are a few of the sabinos, and here is a pure frame. Another frame, though probably not the only pattern, is here.  (Beau Decision sadly passed away last year.)

That is why many of the horses being called sabino-rabicano probably aren’t carrying two separate patterns; the two patterns are not segregating in the offspring. They should, if the horse really has two separate pattern genes, especially when crossed with non-sabino mates. That would suggest that in some cases, the roaning and the coon tails are part of the sabino pattern. Sabino can mimic them, just as it did with the white sclera in the previous post.

Here is a sabino with roaning right in the area rabicano tends to concentrate.

baysabino211

Here is a full body shot of the same horse. (The top shot was a little overexposed, and I had to adjust the contrast, so she looks more red there.)

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She did not have any noticeable white on her tailhead.  But this sabino roan does, completely with the banding. (For those familiar with Paint Horse bloodlines, this particular horse had the kind of sabino roan-splash pattern often seen in the Sullivans Heathen horses.)

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This horse had an almost entirely white tail, and white at the tailhead that looked more like specks than individual hairs. (Unfortunately I did not get any good close-us of it.)

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This is the same mare from the side.

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All of these horses have the kinds of patterns that are routinely described as sabino-rabicano, and yet these same types of patterns occur in breeding groups where the two traits – flank roaning and coon tails – are almost invariably found together with sabino-type markings.  Given how common the traits are, one would expect it to be easier to find true, pure rabicanos. That is, if rabicano causes flank roaning and white tailheads, but not leg or face markings, why are horses like this relatively rare? And why aren’t horses like these producing them?

I suspect it is because those traits are not exclusive to rabicano, and that the pattern is being over-identified.

One of the most frequent questions I get asked about the breed color charts is why so many horses have rabicano listed as “not determined”. That is because sabino is believed to mimic those two traits, so the only way to be sure the horse has rabicano is to see a horse with flank roaning and a white tailhead without white markings – or at the very least without obvious sabino markings. Those are harder to find than one might think!

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