Friday, January 4, 2008

A Dominant White update

A few months ago I posted that a Swiss research team had identified Dominant White in horses. That article is now available online here. If you follow the link to the article and look at the horses pictured, you'll quickly see why some people have objected to the use of the term "Dominant White" for these horses. They look like ordinary sabinos.

The term Dominant White was originally used to designate pink-skinned, white horses with (usually) dark eyes. These white horses were thought to be the product of a single copy of the Dominant White gene. Since no Dominant White horse had proven to breed true, it was assumed that the homozygous form was lethal. What was not mentioned in early articles on the color was that these horses typically had a high percentage of broken-colored offspring. This fact, which was consistent across a wide variety of breeding populations, is what prompted me to write my original paper speculating that Dominant White was really just sabino.

It would seem now the issue is mostly one of semantics. What the Swiss research did not find was a gene that produced uniformly white horses. But at the same time, the genes that were identified (and they found four versions) do not behave the same as the already-identified Sabino 1 (Sb1). Sabino1 is the gene that American researchers found in some sabino whites. With it, white horses are homozygous and the broken-colored horses are heterozygous. Because not all sabino-whites carried Sabino1, it has been assumed that there were likely other (as yet unidentified) versions of sabino out there.

The Dominant White horses in the study were heterozygous. Some were completely white, and all were more than 50% white. It was speculated that, at least in the populations studied, the gene was lethal in the homozygotes. So the color does behave differently than Sabino1. For that reason I suspect the newly identified genes will not be renamed Sabino2, Sabino3 and Sabino4, as if they were other versions of the sabino gene. They will most likely continue to be called Dominant White, even if the horses produced are not exclusively white.

Of course, for those of us who paint horses, we'll probably come to think of Dominant White as just another version of sabino. It would be a version that tends towards the "whiter" end of the sabino spectrum, but aside from that the differences don't seem to be significant enough to matter, at least not for our purposes.

[Edit: I've included the illustration that accompanied the article, since some have had trouble accessing the page. I am often asked to post photos of the oddities that I have come across over the years, but I am unsure if what we do qualifies as "fair use" for educational purposes. So I'll include these photos, but be aware that they'll get pulled if someone objects.]

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