Sometime around 1992, I wrote an article about my belief that “dominant white” horses could be homozygous for sabino. I had become convinced of this after I acquired the first nine volumes of the Tennessee Walking Horse studbooks. The first genetic tests were still years away, so coat color research was done using studbook records.
I knew that both phenotypes in these two pictures were present in Tennessee Walking Horses and that they were once more common. Until I had access to the books, I did not know that most of the early Walking Horses were sabinos and that there were hundreds of white horses. The books had detailed descriptions of markings, so it was possible to discern a clear pattern of inheritance. I was pretty sure I figured it out!
I was not the first to make a connection between the two phenotypes. Early twentieth-century studies noted that white horses sometimes produced spotted offspring. One article about the white horses of the Fredriksborg Stud by C. Wriedt (1924) included the picture below. The term “dominant white” was used to describe the color, but it was known that some of the horses were not entirely white. Geneticists studying white spotting in mice used “dominant white” and “W” for both white and white-spotting, so researchers were following an established convention.
I did not know at the time, but Dutch researchers had already theorized that there were two different types of white horses in the 1970s. Like me, they found that some all-white horses came from white-spotted parents and that they, in turn, produced spotted (but not white) offspring when crossed with non-spotted mates. They called these horses “sabino.” This distinguished the pattern from “dominant white,” where white horses produced their own color (more or less) in half their offspring.
That distinction became the dividing line between what were thought to be two separate types of spotting patterns. The researchers who identified the mutation for Sabino-1 in 2005 explained it this way:
The pattern [Sabino-1] is unique and distinct from similar phenotypes caused by the W series of alleles in that sabino alleles do not produce a homozygous lethal (WW) as W does. For this reason, sabino was given its own symbol, SB, rather than using the W symbol as suggested by precedence in mouse research. (Horse Genetics, 2013)
So are dominant white and sabino really two different things?
Sabino-1 behaved in a way that was different from the white mutations that had been formally identified up until that point. It was obvious enough that I saw it clearly in studbook records. There were alleles in the W series that had a sabino phenotype, but their pattern of inheritance fit the criteria for dominant white. Using “white” in this context was confusing to the public – sabino was in widespread use for the phenotype by then – but it was consistent with the way the patterns were discussed in prior studies.
Then W20 was identified, and things got complicated. The allele did not align neatly with either definition. Paired with some of the other patterns from the W series, W20 produced an all-white horse. Unlike Sabino-1, homozygous W20 horses were not white; W20 needed a second W allele to give a white phenotype. Unlike the alleles of the W series, W20 was clearly not a homozygous lethal.
Then in 2017, a paper identifying an all-white foal homozygous for W15 was published. Using the above definition, that would mean that this particular mutation belonged in the SB group. It is impractical to rename a testable pattern, and this points to the problem with the distinction between the two groups. The situation with Sabino-1 was recognizable because the allele was ancient. That made a cross between two Sabino-1 parents more likely, and in some populations – like the Walking Horses in the 1940s – it occurred on a regular basis. With the more recent alleles, crossing two horses with the same pattern is less likely, especially if breeders are concerned about the risk of a homozygous lethal. It would be hard to assign a newly-identified KIT allele to the SB series when it might be a number of years before the viability of homozygotes was known for certain. It is not surprising that W became the default designation for the group.
At the same time, Sabino-1 is different from some of the patterns in the W series in name only. Some W alleles have now been shown to produce a sabino phenotype in their heterozygous state, and an all-white phenotype in their homozygous state. That means their pattern of inheritance is the same. Visually, many of these patterns are indistinguishable from one another. I agree with Sponenberg and Bellone (Equine Color Genetics, 2017) that these patterns “have overlapping phenotypes that defeat any attempt to neatly separate them out from one another” and that “discussion of them goes best when they are considered together.”
That is what I plan to do, but first I want to talk about another phenotype that often gets called sabino. I use a different term for it, and I’ll introduce that and then – hopefully! – talk about how to consider them all as an interconnected group of patterns.