It is possible to construct a rational system for naming and describing pinto patterns. Here is an attempt given what is currently known.
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.
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 quite 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.
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.
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.
For someone interested in producing pinto-patterned sport horses, advertising a horse as Sabino makes good sense, because most pinto breeders do not 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.
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!