This post first appeared on August 9, 2011
I have time for just one more post before I go on my internet sabbatical, so I thought I’d leave off with some musings about white markings.
My friend Caroline Jones shared this photo of a partbred Suffolk with me recently. She is interesting because her sire is a solid chestnut (“whole-coloured”) Suffolk, and her dam was a bay Shire. She has a large star and what looks like some white hairs on her nose. Although they are not visible in the photo, she had completely solid black legs.
The two breeds are interesting because modern Shires are uniformly marked with white, and are likely homozygous for some kind of sabino. British Suffolks, on the other hand, have been heavily selected for the absence of white on the legs. Even in the early stud books (late 19th century) leg white was rare and usually pretty minimal, but over time the preference for solid legs eliminated even that. They can have face white, and some even have blazes, though solid faces or small stars are the most common. The Shire and the Suffolk are therefor at the opposite ends of selection for markings.
What is an interesting question is why the absence of markings won out, at least in this case. If sabino is dominant, then a homozygous sabino should stamp all its foals with some type of sabino marking. Is this mare, with her large star and dark legs, a sabino? And does this mean that the absence of markings might also be more than just that? Do Suffolks carry something in addition, something that limits the white? Chestnut is considered quite a ‘leaky’ color when it comes to white markings, so much so that chestnut outcrops from unmarked black or bay breeds often have white that their darker relatives never displayed. It makes sense that to create an (largely) unmarked chestnut breed like the Suffolk, breeders would have inadvertently concentrated white suppressing genes.
The idea of white suppressors has long interested me. I first came to wonder about them when I ran across a splash overo crop-out (reconstructed) Tarpan. At the time I wondered how such a loud foal could come from two minimally marked parents. The sire had a small snip and a white coronary band on a hind foot, and the dam had a tiny star and small snip. Was something tamping down the white on the parents? Or was something boosting the white on the foal? Since then I have come to believe that, in the case of splash, the parents were typical of heterozygotes in breeds without ordinary white markings. Although it might not have been relevant to the Tarpans, I have come to suspect that the answer to my original question about boosting and suppressing is that both happen.
So is Susie (above) a suppressed sabino, or a boosted star-marked horse? And just what do those suppressors and boosters do to other patterns? How many of each kind are there? Is there a separate suppressor for faces and for legs? (We know there are breeds uniformly marked with sabino-like face markings, but no leg markings, for instance.)
When I return from my break, I’ll get back to dominant whites (and their connection to the zebra topic).