This post first appeared on September 21, 2012
My previous post about white on the faces of tobianos, made before I left for a trip to Boise, generated a lot of discussion both here and on the Equine Tapestry Facebook page. I thought it might be helpful to expand a little on the subject.
Before researchers had the ability to examine color mutations at the molecular level, what we had was analysis of phenotype (how the individual horse looked) and production records. In older articles on color genetics, those were the tools that were used. Analysis of phenotype is still very popular among people who discuss color on the internet, but the appearance of an individual horse – or even just that horse and his parents – only tells part of the story. Extended production records are needed to get a more complete picture. These can show patterns of inheritance across a broad portion of the population, and that can give clues about the nature of the colors and patterns involved.
Looking at these broad trends requires a lot of data, and one of the biggest limitations is that the kind of information needed is not always retained, or if it is, it is not always easy to access. When I wrote an article in 1997 speculating that some horses being identified as dominant white might actually be “maximum” sabinos, it was because I had noticed trends in the early Walking Horse stud books. Unlike many other books from that time, the entries listed markings (and eye and point color) in detail. Perhaps even more important, at the time was doing the research behind the article, I lived a short distance from the registry where I was given access to records and archived materials. With extensive family records for hundreds of white-born Walking Horses, I was a lot more confident that what I was seeing was a form of sabino.
On one of my visits to the registry, I ran into a breeder doing research on what would eventually be known as the champagne dilution. In the course of explaining what I was there to find, I mentioned that the phenomenon of white foals did not seem to occur in Clydesdales, even though they were uniformly sabino and many of the patterns looked quite similar to those on Walking Horses. The breeder asked if I had Clydesdales, too. When I explained that I just had an aged Walking Horse and a small pony of unknown origins, she expressed confusion about why I had a set of Clydesdale stud books. The reason was that in the pre-internet era, stud books were one of the few ways to obtain information on whole families of horses. Each breed, and therefore each set of stud books, offered a different “control group” to study different patterns. If Clydesdales, for instance, could be assumed to have sabino but not to have frame, then all the patterns in the breed represented what was possible with sabino alone.* In Paints, where frame was common, the possibility that frame was influencing the pattern was always there so until tests were developed it could not be ruled out as causing white on any given horse.
These control groups were not perfect, since the records could contain errors or omissions, but it did make it possible to identify trends. It might not be possible to prove something, but it could suggest useful avenues for testing ideas.
So what does this have to do with white on the faces of tobianos? Well, the suspicion that some tobianos had face white unrelated to sabino, splash or frame came because it was happening in breeds that were my most reliable control groups for “pure tobiano” because the solid members rarely had white markings of any kind. These were Old World breeds with long-closed stud books, so frame was not likely to be present. Sabino (as we currently understand it) did not appear to be present, and my hope for proof that splash was involved was coming up empty. Why then did so many tobianos have white faces? Why were quite a few quite oddly marked on the face, or blue-eyed? Was it not a coincidence that so many homozygous tobianos – in all breeds – had white faces?
Unfortunately for those of us who live in the United States, it is harder to gather information directly because most of our breeds have markings of some kind, and sabinos of all types are extremely common. The horses in this post, and the horse in the previous post, are all American Paint Horses. Finding a Paint Horse that looks “pure for tobiano” is difficult, and even then, it is quite possible that he carries the gene (or genes) for ordinary markings. Those are currently believed to be caused by a recessive mutation to the KIT gene.
That means that this guy, who appears to have only tobiano and no significant white on the face, might carry that mutation and produce offspring that have white on the face.
What was intriguing was not just that white faces seemed to appear on these tobiano ponies, but that an increase in white on the face of the tobianos did not seem to translate into an increase of white on the non-tobianos. That is what might be expected if the tobianos had a separate mutation creating white markings, either the previously proposed KIT mutation or something new. Were they separate but linked? Or was it simply a part of the pattern itself? Was it both, and if so which forms were caused by each?
Or was I misreading the situation based on limited data? What role was selection, both by breeders and by owners, playing in this?
That is why I found the situation with the Polish Hucul so interesting. Because there are conflicting interests, and because patterns can often create strong opinions on the part of breeders, it is hard to know how to weigh claims that the presence of markings on the tobianos threatens the unmarked nature of the solid population. But the question about whether white on the face might be intrinsic to the pattern is a valid one, as is the question about whether or not an existing KIT mutation (like tobiano) predisposes the resulting foals to new (de novo) KIT mutations that add further white. These questions also tie into the larger questions about the nature of white markings and their relationship to the different white patterns.
* Sabino is now understood as a category of patterns, rather than the one pattern it was believed to be then.