“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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3 Responses to “W” is for White-spotting

  1. SuePS March 17, 2014 at 7:19 pm #

    Fascinating, as always! Thank you for all this information — the earth continues to move beneath our feet, and ain’t we got fun!

  2. Sirpa March 18, 2014 at 8:24 am #

    Few questions to get this all clear.
    So, there are at least 20 different versions of the White gene, some result all-white, some not-so-all-white color but with W gene anyway. Some 15 years ago dominant white gene was thought to be lethal when homozygous. You are mentioning “heterozygous Dominant Whites” in this post, so is W still considered or proved to be lethal? Some variations or all of them?
    And if W or at least some of its variations is lethal, how about recognizing it? As some of horses with W gene are considered as Sabinos due to their color so they won’t necessarily be tested for W. If you have discussed this earlier, please tell me so, I haven’t read all your posts, I found this blog only recently.

    What about Splashed White (or Splash)? Is it related to W or KIT gene, or is it totally separate thing?

    • equinetapestry March 19, 2014 at 1:27 pm #

      There are 20 identified mutations that have been placed in this group of white-spotting patterns. There are also a few more mutations to that same gene that have different names, because they were traditionally called something else and are familiar to a lot of horsemen.

      When dominant white was originally studied, it was thought to be a homozygous lethal. It was not understood at the time that there were many different versions of the color, and that conclusion was based on a study of one herd of related horses. When some of the first mutations were formally identified in 2007, it was believed (based on the kind of mutation involved) that at least some of them were homozygous lethals. There are some that might not be, but since the kind of lethality they are talking about is probably early in gestation, it is hard to know for sure. It is not like frame overo (Lethal White Overo) where there is a foal that cannot survive and must be euthanized.

      The patterns in the splashed white group are mutations to a different set of genes (MITF and PAX3), as is frame overo (EDNRB). It is theorized – again, based on the type of change being made at the molecular level – that some of the splashed white patterns may be homozygous lethal. It is clear, however, that some of them can be combined without harming the horse, because horses with multiple splash patterns do occur.