Recently a discussion of Dominant White came up on another forum, and as often happens the question of the naming of the pattern was raised. When the first study was published, many felt that the pattern was part of the sabino series and should have been named Sabino2. (Sabino1 had been identified a short time prior.)
In that discussion, someone suggested that Dominant White was disproven in an article I wrote many years ago. Because I have encountered that comment before, and because I hate to contribute indirectly to the confusion around the two patterns, I’m going to condense my response to that statement here. I had intended to do a revision to the article shortly after the first Dominant White paper was published, but I never found the time, so this will have to do for the moment.
The article was originally written sometime around 1992 and updated again in 1997. It was intended to question the current thinking (that is, early to mid-1990s thinking) about Dominant White. Back then I wasn’t sure that I was right, because there were so many gaps in the information. From the article:
This is not to say that dominant white horses do not exist, but I am personally skeptical, having never found one I could prove wasn’t more likely a sabino white.
Given the breeding records of the aforementioned white horses, I have come to wonder if Dominant White really does exist. But it is clear that even if it does, the majority of white-born horses are probably sabino whites.
My assertions then were based on the picture I had, but I knew I wasn’t looking at a complete picture. And like almost all researchers who look at a partial picture, my analysis was skewed by the specifics of the group I had to study. When I began assembling information on white-born foals, the most common horse used to illustrate Dominant White was the Tennessee Walking Horse. They were mentioned, and their photos almost always appeared in anything about Dominant White. I had access to not only the stud books, with all their elaborate detail, from when the breed had a high proportion of whites, but I also lived just a short ride from the registry headquarters where I had been given free use of their archive materials. I could get as complete a picture as might be possible fifty or sixty years after the fact, and that picture said that each and every white Walking Horse had two sabino parents.
In fact, it said that each had a highly marked sabino, because I had an elaborate numbering system for weighing how much white the sabinos had. Some of the assumptions behind those numbers were wrong, but it happened to give me accurate results for the wrong reasons! (Lesson learned there about the limitations of stud book records.)
That specific conclusion was accurate. What I was looking at in the Walking Horse records was not dominant white. It was an incompletely dominant form of sabino. Sabino1 is a really old mutation, as the study done on colors in ancient remains showed a few years ago. It’s spread out through a variety of breeds, but there probably isn’t another one that has such a concentrated population of them as the Walking Horse, particularly as the breed was at its founding.
I moved from the Walking Horses out to other know cases of whites, but if you read the paper that information is much more sketchy. I didn’t have access to the kind of pedigree and production records I had with the Walking Horses. What I had was a lot of anecdotal evidence that white-born horses in other breeds produced what looked like sabino.
But even so, there were weak spots in the theory that *all* these horses were sabinos. There were horses that only fit the pattern if you allowed for a really liberal classification of sabino. That didn’t fit what I saw with Walking Horses, where the parents of whites were scoring high on my “how white is the horse” scale. At the time, I wondered if linkage to chestnut was hiding sabino, which I mention in the article. I intended to explore that idea more thoroughly in a future paper, which never got written – and probably just as well, since I fundamentally misunderstood linkage! (Lesson learned about the limitations of studying phenotype without a background in the molecular end of genetics.)
Those exceptions did bug me. In fact, I had them all tagged in my own research files as oddities. I have often said that as a researcher, where you are wrong is where you learn. Because the weak spots in a theory are usually where new information is going to come, I tend to flag them in my files. The horses that were flagged as not fitting the theory of sabino white where: R Khasper, Cigale, Mont Blanc, Patchen Beauty and Sneuwwijte.
Three of those are, of course, among the families later identified as Dominant White. Mont Blanc was not part of that study but he certainly fits the same profile.
The last one isn’t as well-known. Sneuwwitje (Snow White) was a Groninger that appeared in the Wiersema book in the 1970s. I had a copy when the article was written, but I could not translate it. The pedigree and color information was there, and I could see that she didn’t fit the profile. If I had access to the sort of automated translators we have now, I would have found at least part of the puzzle that I was missing. That’s because Wiersema outlined in detail the exact theory I proposed; that is, that sabino x sabino crosses produced white foals. (I go into detail in the upcoming book about the different Sabino1 families in the Groninger, and the breeders who were using them to produce white horses.)
Then he discusses the white-born Sneuwwitje, who did not fit his pattern. She was from solid parents, and then when she was bred to different test mates, she produced 50% white or colored foals. That is, of course, exactly what would be expected from a Dominant White. Solid parents (ie., she came out of nowhere) and a 50% production rate of white or pied foals. (Where she a sabino white, you’d have sabino parents and a production rate of 100% pied foals.) He then states that there must be two different types of white-born foals, which of course is exactly what the dominant white study proved almost forty years later.
Since that time I’ve been able to translate not just his book, but a number of papers from other early twentieth century German researchers that were making similar observations. They also studied the “white-born” (weissgeboren) horses and were finding horses like Sneuwwitje. In fact, knowing the behavior of the two different genes, it’s pretty easy to tell which group (W or Sb1) a given researcher might have been studying.
In closing I do think that calling the two different genes separate names is a good idea. Yes, they do look indistinguishable from one another in many cases. I certainly wouldn’t blame anyone for advertising a more patched variety of dominant white as sabino, since that is what most horsemen are going to call the horse based on the appearance. But when talking about the actual genetics, the two different names help clarify the fact that these horses that look alike don’t actually breed alike. The ways that they are different – like whether they can appear out of nowhere or not, are able to produce all pintos or not, are lethal or not – have important implications for breeding decisions, so anything that highlights that is probably helpful.