A few years ago, when giving a presentation on horse color, I used the term “pattern hierarchy” to describe what happened when a horse inherited two or more patterns. Often the genetic instructions from one pattern conflict with those of a second pattern. For instance, the gene for the frame pattern dictates that the horse should have dark legs, while tobiano calls for white on all four legs. In that situation, would the legs be white, dark or some combination of both? Pattern hierarchy seemed to be a good term for the rules that governed which instructions took precedence.
In presenting things this way, I was trying to dispel the idea that white patterns are simply layered one on top of another. It is easy enough for an artist, especially an artist painting a three-dimensional figurine, to approach white patterns in this way. It is often how patterns are painted, with white added over the top of the body color. I felt it was important that artists understand that while patterns can layer in this way, their relationships were not strictly additive. Sometimes the genetic instructions not to add white in a given area had priority. An extreme, mostly-white sabino that inherited the frame gene might have a dark topline, because the prohibition against white on the topline was stronger.
But my own observations tell me, and recent genetic studies certainly confirm, that the situation is a bit more complex. What I called pattern hierarchy might more accurately be called pattern interaction. It is true that sometimes that interaction boils down to a simple hierarchy; the white leg rule for tobianos pretty much overrules any instructions about leg color from the other patterns. But sometimes one set does not take precedence, and instead the instructions for one pattern are subtly altered by another.
Of the various pattern combinations, tobiano paired with sabino is probably among the most common. In fact, among American horses it can be pretty difficult to find tobianos who are not also carrying some form of sabino. Most light breeds, and almost all New World breeds, carry sabino to some degree. That makes it a good place to study the interaction of the two patterns because there are so many examples.
This is what tobiano typically looks like when no other patterns (or modifying factors) are present.
The face and chest are dark, as are the hindquarters. The edges are smooth, and while they do not form perfect circles, they do suggest large, rounded shapes. Notice also that there is no visible face white. In most breeds tobiano does not add white to the face, so without one of the patterns that does, the face will be solid or nearly so.
This is what sabino (or at least one of the more common forms of it!) typically looks like when no other patterns are present.
Unlike tobiano, which has a pretty consistent look in its pure form, sabino has a wide range of expression. Nonetheless, the blaze extending over the lips and chin, white on the belly, stockings that rise in the front of the leg and ragged edges are all typical for sabino.
Here is a tinted overlay of the two patterns.
Which, painted one on the top of the other, would result in a horse that looked like this.
Notice the conflicting instructions for the white on the closest hind leg. The tobiano pattern has white traveling up the broad side of the upper leg, as is more typical for that pattern. Sabino, meanwhile, has the white moving up the front edge of the leg.
If patterns behaved as simple overlays, this is how a combination of these two patterns should look. And if patterns had a simple hierarchical relationship – if accurately portraying combinations was just about resolving conflicting instructions – all we would need to know is which version of that leg white might “win out”. Does the white go to the front, or to the side?
The problem with this approach is that it assumes the two patterns remain discreet, separate from one another. What actually happens is that sabino will subtly alter the original tobiano pattern. Tobiano is still going to predominate, of course. In the simpler, hierarchical view of pattern combinations, tobiano can be considered “king”. Whatever else is inherited, horses with the tobiano gene almost always retain a strong suggestion of the pattern. But the effect of the sabino gene will be visible. In the next few posts, I’ll talk about the different ways that sabino tends to interact with other patterns. This ties in with what was going on with the possible manchado discussed a few days ago, because the first interaction we’ll cover is the tendency of sabino to alter spot frequency.