Sabino influence – polygenic traits

The type of influence sabino has on a pattern varies a lot from individual to individual.  Sometimes the only clue will be the white on the face, or perhaps a slightly more complex distribution of dark patches.  In other cases, there can be enough breaking apart and roaning of the original pattern that it can be hard to see that tobiano is present. Still other horses simply have the white that was there from other patterns amplified.

This horse is a good example of that effect.

SabinoBoost

The frame pattern put the white on her neck and side, and she is probably carrying one copy of the splash gene. Sabino has taken those two patterns and “amped up” the white.

That is particularly noticeable with the splash pattern. This type of face marking, with the head entirely white and the eyes both blue, is often thought of as being caused by splash. In fact it isn’t actually a characteristic of heterozygous splash in its pure form. In breeds without the sabino patterning genes, horses with just one copy of the splash gene do not typically have a lot of white on the face. In fact, they often don’t even have blue eyes. To get that kind of face in those breeds, you need two copies of splash. Yet in breeds where sabino is present, this kind of presentation of splash is pretty common. Sabino is boosting the white, although where the white is going is being directed by the tendencies of the other patterns.

Returning to the tovero horse that illustrated the initial blog post on pattern interactions, we can see that these different kinds of sabino effects can appear on one horse – or even differently on different parts of the horse.

scallopedtovero14

The dark areas on his face and neck have a scalloped quality to them, as if the edges were made by numerous ovals layered over one another.

 

In contrast, the hindquarters have a smaller-scale, lacey edge.  Even within this area, parts of the pattern have pronounced haloing, while other areas are not mapped at all.

scallopedtovero17

Looking at these two pictures side-by-side, it’s hard to imagine they come from the same horse.  And yet they not only appear on the same side of one horse, we attribute them to the same pattern – sabino.

EdgeContrast

One of the things that contributes to this broad range of expression in that what we think of a one pattern – sabino – is actually a group of genes.  In genetics-speak, it is “polygenic”. Sabinos vary more than any other pinto because the term is actually a catch-all for what is actually many pattern genes.  Researchers have not yet found the key to this particular puzzle.  At the moment there is one identified gene (Sabino1) and one group of genes (Dominant White) that result in horses that look sabino; they result in strikingly similar horses, as a matter of fact.  Yet they are two separate genes with very different modes of inheritance.  What’s more, very few visually sabino horses test as having either of those genes.  It is likely then that there are many genes involved, most of which we cannot yet identify. This is why sabino-influenced patterns can look so different from one another, and why determining which pattern or patterns a horse has can be difficult.

(wording edited for clarity)

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2 Responses to Sabino influence – polygenic traits

  1. Threnody July 4, 2011 at 8:53 am #

    Thanks so much for the info. I’m really enjoying the series. The only thing I’m seeing that sounds a bit off is Dominant White being called a single gene when there have been 12 separate mutations for it isolated so far.

    Looking forward to the next installment! ^_^

  2. The Equine Tapestry July 4, 2011 at 11:52 am #

    Yes, my wording was unintentionally misleading, because Dominant White has proven to caused by different mutations in each case. I have gone ahead and edited it, because I suspect that wording is not going to help a topic that many find confusing already. I think “one gene and one group of genes” is probably more clear. The DW genes all belong to the same group, so they’ve gotten named that way (W1, W2.. and so on), but they are each separate. Right now I think UC Davis can only test for W10, which is the one found in Quarter Horses.

    What makes DW so interesting is that type of mutation is so frequent, relatively speaking. A dozen have been isolated so far, but there are many more horses that historically fit the profile (or that fit it now and just haven’t been formally identified).