One of the big questions about patterns is the outer limits of their expression. That is, how much or how little white can there be in any given pattern? Some patterns, like those in the W-series (“dominant white”), seem to show a lot of variation both in the extent of the white and the overall look of the pattern. Other patterns have a little more consistency in their expression. Consistency is what makes a pattern like tobiano so much easier for the casual observer to identify. But even relatively consistent patterns do have a range, and individuals that deviate quite dramatically from what might be considered typical. Animals at the extreme ends are particularly interesting because they tell us where the white most likely originates for that pattern, as well as how smaller-effect modifiers (and possibly other patterns) interact with it.
Classic splash – that is, SW1 in its homozygous form – is one of the patterns that is prone to a fairly consistent expression. That was why it was fairly easy to identify the presence of that specific pattern in breeds prior to the advent of genetic tests. Homozygous SW1 horses had a consistent look, and that look was pretty distinctive. Even so, there were horses that fell along the more minimal end of expression, particularly among some of the rustic pony breeds where white markings tended to be minimal or absent altogether. These horses skirted the edge of the loose rules I had for identifying the classic splash pattern: four white legs, some white on the lower body, white on the tail tip, white face, two blue eyes. Some tended to blur the line between what I thought of as a sabino-boosted heterozygous SW1 and other splash-like patterns (what turned out in many cases to be SW2). Testing has now made it easier to determine when horses from that latter group really are carrying SW1, but I did wonder what testing would tell us about how far thea “rules” could be bent – or if they could be broken altogether. Tobiano testing confirmed that pattern’s biggest rule – four white legs – could be broken. I wondered if what I thought of as the biggest rule for homozygous SW1 – two blue eyes – was also negotiable.
So I was thrilled when Pauline Högdahl sent a photo of Stenbydal’s MJ, the Gotland Pony colt pictured in this post. His owner, Emmelie Nilsson, had him tested and he is in fact homozygous for SW1. To date there have been a number of whole-colored (ie., no white markings whatsoever) heterozygous SW1 horses, but this fellow is the most minimal homozygous SW1 that I have encountered.
He does have white on all four legs, though the marking on the right front is pretty minimal. His tail end is white, and he does have a small patch of white on his belly.
But remarkably enough, he does not have blue eyes. He does have a small blue segment in one eye, but the eyes are otherwise dark. So while very white faces and blue eyes are often considered the defining characteristic of a splash white horse, and homozygous SW1 horses display the classic form of the splash white pattern, having two blue eyes is not an absolute, unbreakable rule.
While Stenbydal’s MJ may answer the question of whether or not homozygous SW1 horses have to have two blue eyes, he poses new questions. What exactly causes a pattern to be so minimal? Is it all random chance? Or is there some genetic component that tamps down the white, just as there are genetic factors that amp it up on some horses? The latter does seem more likely, especially given that minimized patterns tend to run in families. Here is a picture of MJ with his dam, who would presumably be heterozygous for SW1.
His sire, Skotte 492, would also carry SW1, but only has a few white hairs on his forehead. He has at least one other tested homozygous SW1 that is quite minimal. That is Ängdalas Spirello. He also has a son, Zidane Zojvide, that has what appears to be a classic pattern but with one dark (or at least primarily dark) eye, but I am not aware of any test results on him. (More photos of Zidane Zojvide as a young horse, where the contrast on his pale palomino coat is a little better, can be seen here.)
This question of factors that suppress patterns, or boost them, ties into some of the new research on white markings. That is a subject I’d like to cover, especially in light of the new paper that was published just recently. To do that, however, I want to give some background on past studies of white markings, including what was already known, and the system that was developed to begin assessing white. Watch for those posts in the upcoming weeks, but I have at least one more splash white post to make first.