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This horse has one copy of the frame mutation. Horses with two copies of the mutation are not viable.

In the previous post I talked about how the physical location of a mutation can limit the possible pattern combination. There is another potential limitation, which is viability of the organism.

Those of us that like horse colors, particularly the white patterns, are accustomed to thinking of colors as something that is added to what would otherwise be a horse of ordinary coloring. So the horse above has white markings on his body in addition to his chestnut coloring. That is certainly how a lot of artists would approach painting such a horse.

But from a genetic standpoint, that’s not what has happened. Generally speaking, white patterns result when one of the genes involved in pigmentation is impaired. Something prevents the normal function of the gene, and as a result pigment is not distributed in the normal fashion.  That is what we see most clearly, because changes to coloration are really obvious. But those same genes do not just regulate color, and those other functions may be effected as well. Hampering coloration is largely cosmetic, but altering the function of the gene can have more serious implications.

That’s why horses with two copies of the frame mutation are not viable. With just one impaired gene, the horse is not completely pigmented (ie., it has white patches) but is still functional. The horse still has one non-mutated copy of EDNRB, the gene involved with the frame pattern. It can “pick up the slack” for the necessary functions that gene performs. When the horse inherits two copies of the mutation, there is no backup and the gene cannot perform its function in the development of the embryo. In this particular case, no pigmentation occurs, which is why the resulting foals are white, but more importantly the colon is incomplete which means the foal cannot survive.

Lethal White Syndrome is probably one of the best known problems with color because it involves the heartbreak of a live birth of a foal that must be humanely euthanized. Other colors, most notably the various forms of Dominant White, are also thought to be lethal when homozygous. Like the frame mutation, two copies impair the function of the gene to the point that the embryo is no longer viable. The difference between Dominant White and Frame Overo is that the embryo is lost early enough that no foal is born. This may explain why programs centered around breeding white-born horses in the seventeenth and eighteenth century were often plagued by infertility issues.

At one time, roan was also thought to be a homozygous lethal. (Photo from Wikimedia Commons.)

 

In the past, before tests were available, lethal conditions like this were determined by analyzing production numbers. If the ratio of mutated to non-mutated offspring was off, and if true-breeding individuals could not be found, the trait was suspected of being lethal when homozygous. That was why roan was assumed to be a homozygous lethal for so long. Initial studies of production records showed that the ratios of mutated offspring were like those of a homozygous lethal, rather than a simple dominant. Proven homozygous roan stallions have since been identified, so it is clear that two roan genes are not always lethal, at the very least.

So what does this have to do with the KIT mutations? In the comments section, there was speculation of the last post about whether or not mutations could crossover, resulting in a single gene with two separate mutations, rather than two separate genes with one mutation on each. Not asked, but an equally valid question, is whether or not a gene that already contained a known mutation could mutate again. If either were to happen, the next question would be could the situation result in a viable embryo? Would the added layer of impairment change the coloring, or would it damage or even destroy the organism? Have we not yet seen a horse with three KIT mutations (one on one gene, two on the other) because the statistical chances are infinitesimally small, or because the function of some gene is too compromised to result in a viable embryo?

I have wanted to bring up a more technical aspect of horse color for a while, but have struggled with the best way to present the information. Part of the problem is that the way we talk about horse color is misleading. For this to make any sense, I will have to clarify some terms.

We often talk about horse colors as if they are genes. We say, then, that a horse like the one pictured above has one copy of the “sabino gene” and one copy of the “tobiano gene”. It is true that the “torn tissue” look to his pattern is very typical of what a horse looks like when it has both Sabino1 and Tobiano. He is a Spotted Saddler, so he would likely test positive for each color. Saying he has the Sabino1 gene and the Tobiano gene is a simple way to get that idea across.

The trouble is that there is not a specific Sabino1 gene. There isn’t a Tobiano gene. Sabino1 and Tobiano are mutations of an existing gene. When we say that a horse has the “tobiano gene” or the “not-tobiano gene”, what we really mean is that the gene that was there from the start is either mutated (tobiano) or not mutated (non-tobiano). This makes sense when you think about it. Why would an organism carry around a gene that is essentially the absence of a trait?

This might seem like semantics, except that some of what we think of as separate colors occur on the same actual gene. They are different mutations, but they share a location. In the case of Sabino1, the mutation occurred on a gene known as KIT. Other mutations found on or very close to KIT are tobiano, true roan and dominant white. This might not seem important until you remember that an animal has two copies of any given gene, one from each parent. It can only give one to any individual offspring. If a horse only has two KIT genes, then it can only carry two mutations – one on each copy of the gene. That means you only have two slots to fill with KIT mutations. A horse could be homozygous for tobiano, but then he could not also carry Sabino1. His two KIT slots are already filled.

This probably makes more sense when it is understood that most color mutations are one-time events that happened a very long time ago. Sabino1 has been documented in Siberia in the early Bronze Age, so it is at least that old. Horses like the one pictured here descend in an unbroken line from whatever early ancestor carried that first Sabino1 mutation. One of his KIT genes is that same gene with that same mutation. His other KIT gene comes from the whatever horse carried the first tobiano mutation. That pattern has been found in Eastern Europe later in the  Bronze Age, so like Sabino1 it is really old. Were he not a gelding, he could in turn pass on one of those – either tobiano or sabino1 – to his offspring. One, but not both.

This has implications for artists like myself because we tend to mix-and-match the details of different patterns to get certain visual effects. What we have to be careful about is whether or not the limitations of gene locations make something impossible. If a horse can only carry two KIT mutations, and true roan and tobiano prove to be on KIT or linked to KIT, then is a homozygous tobiano roan possible? Is a roan tobiano with cat track markings – a trait closely associated with homozygosity in tobianos – accurate? And what about the other colors and patterns that have not been mapped to a specific location? What conflicts will become apparent when more mutations have known locations? We know, for instance, that the leopard complex gene (varnish roan) is not located on KIT, but what about the patterning genes that work with leopard complex to make the more vivid appaloosa patterns? It is often assumed that all combinations are possible, though they might be so rare that actual living animals cannot be found with them. That is probably a mistaken assumption, with some combinations not possible because of location conflicts.

This also has implications for people who study horse color. Homozygous tobianos are an interesting example because they obviously have two KIT-related mutations. Still a high percentage of homozygous tobianos have face markings. The commonly accepted wisdom is that tobiano by itself will not place white on the face, yet KIT is often assumed to be involved in ordinary face markings as well as the sabino patterns. Does the fact that many homozygous tobianos have broad blazes suggest that some sabino patterns are not, in fact, located on KIT? Or does it suggest that in its homozygous state, tobiano does start to place white on the face?

It is also important to breeders, who may find that attractive combinations do not necessarily breed true. Many Paint Horse breeders have already noted this situation with roan tobianos. Roan has not yet been definitively mapped, and it is thought to be close to KIT rather than on KIT. Still genes that sit close to one another tend to travel as a package, and that is definitely the case with roan and KIT. Roan tobianos typically have a roan parent and a tobiano parent, and they usually pass along either roan or tobiano to their foals, but not both.

Gene location is pretty technical stuff, but the information has a lot of practical uses.

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.

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.

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.

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.

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)

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.

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.

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.

(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.

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.

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.

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!)

 

In the last post, we had a horse with both the tobiano pattern and a sabino pattern, and looked at what the horse might look like if patterns simply overlaid one another. That’s not, of course, how things work.

Here is one version of how those two patterns might interact.

Here the sabino pattern has added white to the face, but has also broken the original tobiano pattern into smaller pieces.  The pattern still retains a great deal of tobiano character with the rounded spots, but there are more of them and they are smaller.

This type of interaction is visible on these horses.

With each of these, sabino has taken the areas of the tobiano pattern that should be dark, and made them both smaller and more numerous.  Sabino often influences other patterns in this way.

Compare the patterns on those horses to the more simple pattern on these.

Reducing size and increasing frequency of spotting is a consistent feature of sabino influence.  To get a better sense of this, assemble a set of tobiano Paint Horse references, and group them into two piles – horses with face white, and horses with nothing more than a star or snip.  Comparing the two groups, the patterns with the sabino influence should appear more complex, with the dark areas more broken up than those without it.

Obviously there are other patterns that put white on the face, but for that breed the numbers are in your favor that most face-marked horses carry some form of sabino.

Sabino does this with other patterns, too. Appaloosas with sabino markings – that is, those with blazes and white feet – often have smaller spots broken up with larger areas of white than appaloosas with solid faces and no white on the legs. The type of pattern many associate with the Appaloosa stallion Prince Plaudit is a good example of this kind of interaction. That is actually what made me think that the earlier Pato horse  might be a manchado despite the fact that the spots across his hindquarter were not as large, nor as rounded, as the ones I had seen on more obvious manchados. That kind of change is something I might expect to see when a pattern is combined with sabino.

In the next post I’ll cover the other way that patterns are often changed when they are paired with sabino, which is laced edges.

Slide from the 2002 BreyerFest presentation

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.

An Example

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 travelling 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.

The full horse from the previous post, showing a rather impressive variety of pattern edges all on one horse.

"My face doesn't look like it belongs with my butt!"

The recent discussion of the possible sabino-manchado horse has had me thinking about the topic that has consumed much of my attention for the last few years, which is pattern interaction. That was the subject that I began to explore in a series of articles for the magazine published for the (now sadly gone) Realistic Equine Sculpture Society. I had touched upon it before in presentations, but only in the most superficial way, because exploring the ways that the different patterns interact is speculative. We cannot test for most of these patterns, and to make matters worse we already know that some of what we call a pattern (like sabino) is actually a catch-all phrase for a group of patterns that may in fact prove to be quite different from one another. When teaching about horse color, it seemed less confusing to stick with what was actually known.

But just as my friend Sarah Minkiewicz-Breunig pointed out in a recent blog post about the difference between anatomical charts and living, breathing animals, and how important that is for anyone wishing to convey life in their sculpture, so too is there a difference between the rules and categories of coat color genetics and the living animals we encounter. Much of what is said about horse color is simplified. It has to be; that is the first step to understanding it. But once those concepts are clear – once a person understands that this is a frame and that a tobiano and that a sabino – then the next step is exploring the far more complicated way that color presents on individual horses.

And one of the biggest influences on that is the way that the different patterns interact. Someone questioned the use of “portions” of the photos in the previous post, but that is exactly what pattern interaction is about. When there are two (or more) patterns, which portions remain? Which are lost? Which get changed so that they look different from either of the original pattern?

In the next few days I am going to try to reformat some of the information that appeared in the RESS articles, and hopefully from there start exploring the topic further.