Archive | Pattern interactions

Tobiano, the “top dog” pattern

BFTovero

When it comes to how the different patterns interact, tobiano could be called the top dog. Pretty much no matter what else it gets paired with, the end result still looks pretty much like a tobiano. Sometimes the other patterns add new areas of white, like this tovero here with the bald face and white ear, but visually it is still pretty easy to identify the horse as carrying tobiano.

Here is an pony with both the appaloosa and tobiano patterns. Notice how the white areas from the tobiano just overlay the leopard pattern.

PintaloosaPony1

The lighting for that picture was just right to show the tobiano markings. In bright light, it would be possible to miss it. The outline is also lost as the pattern travels up his hindquarter, when it meets what would likely be the pink-skinned area on a leopard.

PintaloosaPony2

Here is tobiano overlapping dark-headed frosty roan.

RoanTobi

Tobiano even stays intact when inherited by zebra hybrids. (Photo from Wikipedia Commons.)

800px-Zorse

It is tamped down and made more minimal in donkey crosses, but it is still quite obviously tobiano. (Photo by Amanda Slater.)

800px-Mule_Plough_TeamAmandaSlater

Which brings me back to the discussion about white Miniatures from a few days ago. I truly did not think the colt in question was a Dominant White, but rather a tobiano that was rapidly greying out. As a young foal, he looked like a chestnut tobiano. It did lead to the question, though, about what Dominant White might look like paired with tobiano. Would it overlap the pigmented areas (few though they might be in many cases), much like it did with the leopard above? Or would the instructions to make the horse white override the tobiano patterning altogether?

I suspect that the answer lies in the way the two patterns function at the molecular level. I enjoy reading papers about that aspect of genetics, but in many ways that is above my pay grade. As an artist, I am at heart someone who understands the nuances of phenotype (that is, how the horse looks) far more thoroughly than I understand the underlying mechanics. I will need to wait until someone crosses a Dominant White (particularly one of the families that tends towards the “leaky” variety rather than the all-white) with a tobiano to find out.

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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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Sabino influence – lacing the edges

The post from yesterday talked about the tendency of sabino to break dark areas of a pattern into smaller, more numerous pieces. The other thing that sabino tends to do is degrade the edges of the other pattern by making them lacey, roany or ragged.

Using the pattern from yesterday, here is an example of how that might look.

The tobiano pattern is still plainly visible, but the edges have become lacy and irregular.  Looking carefully, it’s obvious that this isn’t an overlay of a loud sabino pattern because the irregular areas are concentrated along the edge of what would be a tobiano pattern, instead of the areas associated with sabino.

Here is a horse with this type of sabino influence.

SabLace3

Notice that while he does have lacey markings along the girth – something you could expect on a sabino – the primary direction is upward toward the withers, following the line his tobiano pattern might take.  Meanwhile, there is almost no white under that front armpit or along the belly, which would be the most likely place to look for white on a purely sabino horse.  So the sabino instruction to “add lacey white” has been redirected to follow the pattern edges laid out by the tobiano gene.

And here is a more extreme version of lacing the edges.

SabLace4

The edges aren’t always lacy, though. Sometimes the edges take on more of a ragged, torn appearance. This horse is a good example of that.

RaggedEdges

(I apologize for the “headless horsewoman”. Since I am only taking pictures for my own reference, I often zoom in to get the horse big in the frame, which means a lop a lot of people off!)

Laced edges are something not only seen when sabino is paired with tobiano, but other patterns as well.  This horse is most likely carrying sabino and frame.

SabLace2

The softened, roany look to the marking on her side is very typical of sabino, yet all four legs are unmarked.  Although not as obvious in this picture, the marking did not extend under the belly either.  The appearance is that of a purely frame pattern with just the outline altered.

And here is a splash overo with laced edges. Notice how the sabino patterning follows the edge here, too.

SabLace1

I should caveat this post with one warning for painters, though. It is true that complex, ragged edges are associated with the presence of sabino and that horses without it have simpler patterns. The edges are more even, but it should be said that even on simple tobianos – and simple splash overos, the other “smooth” edged pattern – that the edges can still have irregularities. Here is a close-up of the brown tobiano mare from the previous post.

UnevenEdge

The overall impression is still that of a large, fairly even patch – but the up-close look will often show this kind of edge.  (I’ll talk about her roaning in a later post, because that’s not necessarily a sabino trait either!)

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