Sabino


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The blog here continues to be somewhat silent while I work on the next book, “Equine Tapestry – An Introduction to Colors and Patterns“. What began as a reissue of the front portion of Volume I, Draft and Coaching Breedsface in color has (predictably) taken on a life of its own. The original text touched on the as-yet-unidentified pinto patterns, with an attempt to classify the different categories of sabino-like patterns visually. Since that time, quite a few more pieces of the puzzle have fallen into place, and that has lead me to do a major overhaul on that material.

One of the things that became clear to me with this new information was that the way I personally organize my files was leading to blind spots in my understanding. I have mentioned before that a huge part of my research involves massive notebooks with images and pedigrees. Because I am a visual learner, sorting things this way helps me make connections that I might otherwise miss.

Notebooks

I have organized information in notebook pages like these for more than 20 years. Soon after I began, I started sorting the information by breed. Because so much of my interest centered around which colors were present in which breeding population, this made sense. What I began to suspect, working on the new book, was that I needed to rearrange some of my notes by color groups, rather than by breed. The previous structure was great for seeing how some of the louder sabino patterning arose in the Arabian breed. Seeing how that worked convinced me that the louder horses were some kind of new mutation, and not just a more extensive expression of the existing “flashy white” in some lines. Laid out in a breed-centered notebook, it was clear how these louder horses – horses like Rhocky Rhoad – might come from flashy families (like Khemosabi), but their (numerous) relatives did not generally look unusual, while their own descendants most certainly did.

But this same structure made it harder to make connections about the colors themselves. The sheer volume of information – I have thousands of horses on file, and many times that number waiting to be included – made those connections more dependent on my working memory. Writing more has meant less time for musing, and with it the chance that I would make those mental jumps. When recent papers made it clear that my hunch about horses like Rhocky Rhoad were correct, I decide it was time to set up a parallel set of notebooks for the pinto pattern categories, starting with the W-series (W1-W20). I was hoping this would give me better insight into how the patterns within that group – the group previously called “dominant white” – worked. With luck, I might stumble upon the best way to present this very varied, and not-entirely-helpfully-nameed group in the new book.

I now have all twenty of the known W mutations, along with images of every known or suspected carrier, in a single notebook. The first thing that jumped out at me was something that has been a bit of a hobby horse for me for some time now. That was how many of these lines involved blue eyes. Of the twenty families, six have blue-eyed individuals. In some cases, like the W5 family member Sato (above), just have a blue segment. Others have a full blue eye, or even two blue eyes, thought that last is actually pretty rare.

This is not surprising to anyone who has looked at historical records of white-born horses. Blue eyes are not infrequently mentioned. They are mentioned in connection with some of the old European studs that previously bred white-born horses. Early researchers also comment on their occasional presence. Nowadays, a search on the internet will turn up any number of commenters that will tell you that this “obviously” means the horses carry a splash pattern. You can even find those who will assert that no KIT mutation ever produces blue eyes in a mammal.

That was part of why I included the image of the panda German Shepherd a while back. She has a newly-identified KIT mutation, and she most certainly has blue eyes.

FrankieSM

Interestingly enough, most white patterning in dogs has proven to be caused by mutations to MITF – not KIT. In horses, MITF is the gene associated with splash patterning – and with blue eyes. For those familiar with dog coloring, the “extreme piebald” found in many sporting breeds, the “color-headed white” pattern in Collies and Shelties, and many forms of “Irish Spotting” have all been mapped to MITF. None of these patterns is associated with blue eyes in dogs. There is a MITF mutation in dogs – the one that produces white in Boxers – that produces blue eyes on rare occasions, to this is not an absolute, but generally speaking these MITF mutations are not associated with blue eyes. (For more information on the different MITF mutations in dogs, this a good site.)

The common theory in horses is that these W-series horses must have a splash mutation as well. And they may. Certainly there are far more mutations for white patterning than previous expected. I have long thought that the numbers of blue eyes on the dominant whites, particularly among the founder horses (ie., the horse that carried the initial mutation) were just too high for them all to happen to have a splash mutation as well. I did not have an exact number, though – just a sense that it was high. But the new sort gave me a number – six of twenty. That’s a lot, especially with breeds where there is no evidence of the presence of the one splash pattern (classic splash, SW1) known to remain cryptic in its heterozygous state. The other “new” splashes are dominant mutations, and a good bit more obvious in terms of phenotype.

What is interesting is that the one horse that is often asserted by online commenters to “surely be a splash” is the well-known Arabian stallion Khemosabi. This is based on the fact that he has multiple blue-eyed descendants. What became clear as I resorted these records was that yes, he does have a number of blue-eyed descendants. However, all those in my files are also members of the two W-series mutations that occurred in his line. One would expect, if Khemosabi carried a splash mutation (at least, as we currently understand the pattern), it would appear in more than two lines. He did, after all, sire over 1,250 foals. Finding it in two lines, which also just happen to be those that have formally identified white spotting mutations, seems to suggest that the blue eyes are part of the pattern and not some additional inherited trait.

There are caveats to this, of course. Blue eyes are notoriously underreported. When I began to suspect that the louder sabino expressions in Arabians were new mutations, I printed out the markings files for the families where they occurred. So in my files are the marking diagrams for all the first generation Khemosabi descendants. It is quite possible that there are horses in that group with blue eyes that were not part of the registration records. Blue eyes in Arabians are still considered a serious fault, so there is some incentive to overlook it when filing a description, especially for an eye that is not completely blue. (I am always looking for images and records of blue eyes in Arabians, if anyone has them, by the way!)  I cannot also be sure that some of the other lines – the fourteen that are not included as having blue eyes – might not also have blue. In many cases, eye color is not mentioned at all,  and there I have defaulted to the assumption that the eyes were dark. That is not the same as knowing the eyes are dark, though.

My next task is to assemble the even larger group of suspected dominant white horses into one notebook. When that task is complete, I should be able to do another post about the status of the blue eyes in that group as well.  Well, that and get a little closer to a finished book!

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Markings1

In 1989, a horse I was riding lost his footing along a steep incline. The fall broke my wrist and shattered my humerus from elbow to shoulder. I was fortunate to have an employer that provided temporary disability, but confining a technical illustrator to a traction bed without the use of her dominant hand for several months is particularly cruel. A friend took pity on me, and suggested that I get a modem for my computer so that I could “chat” – one-handed, hunt-and-peck style – with people on the local computer “bulletin boards”. So by accident, I became an early adopter of online communication. Mostly I got to talk to young, nerdy guys about gaming (the old-fashioned kind, with polyhedral dice and character sheets) and computers. It would be three more years before the technology matured and spread enough that I could find people to talk about what I really loved, which was horses. Horses and their colors! Once that was possible, I filled countless hours chattering on that subject. (Some things do not change.)

One of my favorite topics at that time was pinto patterning, or more specifically, sabino patterning. Crop-out pintos were the focus of a lot of attention, and a number of articles and papers about “overo patterns” appeared around that time. I had also acquired complete, or nearly complete, sets of Walking Horse and Welsh Pony stud books, both of which included very detailed descriptions of markings. I began assembling data, using a numerical grading system based on the amount of white, trying to see if I could figure out why some horses ended up body-spotted. I became convinced that pinto patterns did not just pop up unexpectedly. I believed that the way we were looking at markings and patterns was mistaken, and that the horse world had drawn an artificial line between “pinto” and “not-pinto” – one that probably did not reflect the underlying genetics. Horses like the one below were universally recognized as pintos, but I was pretty sure you could take a lot of that white off and still have something that was genetically a pinto of some kind.

Markings4

I was so taken with this idea that I spent a lot of time online suggesting that maybe what we saw as markings were just part of a continuum that ended with a white horse. “Minimal” sabinos might look like an ordinary horse with ordinary markings. “Maximum” sabinos might be entirely white.

Having read a lot more papers (including some that others have kindly translated for me from their original Dutch or German), I know now that what I was suggesting was hardly new under the sun. Others had come to similar conclusions over the years. But it was rather unconventional thinking among horsemen at the time I began writing, and I got the message quite clearly from some that perhaps I was a little touched in the head. (I am sure that being overly enthusiastic about color, and sure of what I knew in a way that only seems to happen when you are twenty-something, did not help!)

At one point, in one of these online conversations, an older and more experienced horseman well-versed in Thoroughbreds finally asked, “Are you saying that all markings are really just patterns, minimally expressed?!” His implication was clear: this was crazy talk. I had not actually entertained that idea, but I had wanted people to consider that markings might occasionally work like a continuum. But his comment did make me think. Could that actually be true? Was it possible that all white markings were just minimal expressions of one of the patterns? I did not think that was the case even then, but it was an intriguing question that stuck with me over the years.

Nowadays, the concept of “minimum” and “maximum” patterns is pretty widely accepted. What I find interesting, however, is that taking that idea to its most extreme conclusion – that all white on the face and legs represent some minimum pattern – is also very widespread. What was once used to illustrate how ridiculous an idea might be, now has a certain following. In fact, if you spend enough time in forums and blog comments, you will soon find this “fact” being used as a verbal cudgel against someone “uninformed” about patterns. It seems we have come full circle in the world of online horse color discussions.

So is that the case? Are all white markings just patterns? Always?

The answer depends somewhat on how you want to define “pattern.” In genetics, piebald and “marked with white” are synonymous terms. Most horsemen, however, see a significant difference between a pattern (ie., something that may extend the white past acceptable or desirable levels) and markings (ie., something that will not extend past the extremities). Because that distinction can have serious implications in many breeds, perhaps putting it in those terms is most helpful. Does every horse with white on the legs or face have the potential to produce something body-spotted?

Or are there genes that, at their maximum expression, never extend past the lines that generally qualify  a horse in a pinto registry? Is is possible to produce this much white, say, and no more?

Markings2

What if it never goes even that far? What if you have a breed population where a horse like this represents the maximum end of the white patterning, even counting factors like base color?

Markings3

And it does look like there are marking genes that do not, in themselves, produce pinto patterns. The assumption in the literature has been that there is a gene – or genes – that controls face and leg markings. The most current study was done in the Franches-Montagne breed, and was published in 2008 (the link will take you to the full article).  After analyzing 23,019 horses, the study found that:

Our data support the segregation of a recessive single gene accounting for 20–80% of the total heritability for the traits under study (head, forelimbs, and hindlimbs markings).

Our association analysis indicated that the putative major gene for white markings is located at or near the KIT locus. However, further studies are necessary to prove that the KIT gene indeed is the putative major gene for white markings. Our association analysis indicated that the putative major gene for white markings is located at or near the KIT locus. However, further studies are necessary to prove that the KIT gene indeed is the putative major gene for white markings.

This and the earlier Woolf studies on Arabians confirm what I have seen over the years while looking for body-spotted sabinos. Like the splash-like horses that never seemed to pan out when it came to what I was looking for (what turned out to be the homozygous SW1 pattern), not all families or breeds with markings pan out when looking for body spots. In fact, some families that consistently produce what most would call sabino face and leg markings only rarely produce body-spotting, and then often just a bit of white on the belly. This phenomenon was noted by Dutch researchers looking at sabino in their breeds, too. In my book, I referred to that particular pattern as “Flashy White Sabino”. I have wondered if whatever causes this type of pattern is simply interacting with (boosting) the marking gene proposed by the team that investigated the Franches-Montagne horses.

But the fact is that while it is commonly accepted among researchers that there is a separate “marking gene”, we do not have a clear picture of the relationship between those markings and patterns. We do have much better tools now, not only with a wider arrange of genetic tests, but also online databases that pair photos and marking diagrams with entries. Just this past week I was combing through one of those resources in hopes of unlocking some mysteries about a breed that consistently has extensive face white without leg white. But the time investment in this kind of research is huge, especially when the pay-off is just clues. Lab work would still be required to get final answers. Until more work is done, though, absolute statements about what is “always” or “never” true should be treated with appropriate skepticism.

My previous post about white on the faces of tobianos, made before I left for a trip to Boise, generated a lot of discussion both here and on the Equine Tapestry Facebook page. I thought it might be helpful to expand a little on the subject.

Before researchers had the ability to examine color mutations at the molecular level, what we had was analysis of phenotype (how the individual horse looked) and production records. In older articles on color genetics, those were the tools that were used. Analysis of phenotype is still very popular among people who discuss color on the internet, but the appearance of an individual horse – or even just that horse and his parents – only tells part of the story. Extended production records are needed to get a more complete picture. These can show patterns of inheritance across a broad portion of the population, and that can give clues about the nature of the colors and patterns involved.

Looking at these broad trends requires a lot of data, and one of the biggest limitations is that the kind of information needed is not always retained, or if it is, it is not always easy to access. When I wrote an article in 1997 speculating that some horses being identified as dominant white might actually be “maximum” sabinos, it was because I had noticed trends in the early Walking Horse stud books. Unlike many other books from that time, the entries there listed markings (and eye and point color) in detail. Perhaps even more important, at the time was doing the research behind the article, I lived a short distance from the registry where I was given access to records and archived materials. With extensive family records for hundreds of white-born Walking Horses, I was a lot more confident that what I was seeing was a form of sabino.

On one of my visits to the registry, I ran into a breeder doing research on what would eventually be known as the champagne dilution. In the course of explaining what I was there to find, I mentioned that the phenomenon of white foals did not seem to occur in Clydesdales, even though they were uniformly sabino and many of the patterns looked quite similar to those on Walking Horses. The breeder asked if I had Clydesdales, too. When I explained that I just had an aged Walking Horse and a small pony of unknown origins, she expressed confusion about why I had a set of Clydesdale stud books. The reason was that in the pre-internet era, stud books were one of the few ways to obtain information on whole families of horses. Each breed, and therefor each set of stud books, offered a different “control group” to study different patterns. If Clydesdales, for instance, could be assumed to have sabino but not to have frame, then all the patterns in the breed represented what was possible with sabino alone.* In Paints, where frame was common, the possibility that frame was influencing the pattern was always there so until tests were developed it could not be ruled out as causing white on any given horse.

These control groups were not perfect, since the records could contain errors or omissions, but it did make it possible to identify trends. It might not be possible to prove something, but it could suggest useful avenues for testing ideas.

So what does this have to do with white on the faces of tobianos? Well, the suspicion that some tobianos had face white unrelated to sabino, splash or frame came because it was happening in breeds that were my most reliable control groups for “pure tobiano” because the solid members rarely had white markings of any kind. These were Old World breeds with long-closed stud books, so frame was not likely to be present. Sabino (as we currently understand it) did not appear to be present, and my hope for proof that splash was involved was coming up empty. Why then did so many tobianos have white faces? Why were quite a few quite oddly marked on the face, or blue-eyed? Was it not a coincidence that so many homozygous tobianos – in all breeds – had white faces?

Unfortunately for those of us who live in the United States, it is harder to gather information directly because most of our breeds have markings of some kind, and sabinos of all types are extremely common. The horses in this post, and the horse in the previous post, are all American Paint Horses. Finding a Paint Horse that looks “pure for tobiano” is difficult, and even then it is quite possible that he carries the gene (or genes) for ordinary markings. Those are currently believed to be caused by a recessive mutation to the KIT gene.

That means that this guy, who appears to have only tobiano and no significant white on the face, might carry that mutation and produce offspring that have white on the face.

What was intriguing was not just that white faces seemed to appear on these tobiano ponies, but that an increase in white on the face of the tobianos did not seem to translate into an increase of white on the non-tobianos. That is what might be expected if the tobianos had a separate mutation creating white markings, either the previously proposed KIT mutation or something new. Were they separate but linked? Or was it simply a part of the pattern itself? Was it both, and if so which forms were caused by each?

Or was I misreading the situation based on limited data? What role was selection, both by breeders and by owners, playing in this?

That is why I found the situation with the Polish Hucul so interesting. Because there are conflicting interests, and because patterns can often create strong opinions on the part of breeders, it is hard to know how to weigh claims that the presence of markings on the tobianos threatens the unmarked nature of the solid population. But the question about whether white on the face might be intrinsic to the pattern is a valid one, as is the question about whether or not an existing KIT mutation (like tobiano) predisposes the resulting foals to new (de novo) KIT mutations that add further white. These questions also tie into the larger questions about the nature of white markings and their relationship to the different white patterns.

* Sabino is now understood as a category of patterns, rather than the one pattern it was believed to be then.

One of the most interesting aspects of white patterns, at least for an artist, is whether or not they overlap one another or interact with each other (and other modifiers). Perhaps because most techniques involve adding – or sometimes removing – layers of color, it is only natural that artists tend to assume a layering relationship. What is appears to be true is that patterns typically have a complex set of interactions. For example, there is good reason to believe that some of the sabino patterns (like the one seen on the horse above) interact with whatever mechanism produces ordinary white markings by amplifying the white in those areas. Meanwhile both the markings and the sabino patterns appear to interact with the base color.

Simple layering seems to be far less common. That is one reason why some forms of snowflaking (like the one in this post from a few days ago) are so interesting, because it appears that some types do overlap existing patterns. I suspect that overlapping white spots are behind the really unusual “jigsaw” leopard Appaloosa mare Dazzling Vision Spot. More common, though, are overlapping dark spots on the face. In dogs these are sometimes called Blenheim spots, for the red and white pattern in Cavalier King Charles Spaniels where the markings are considered desirable. One is visible inside the blaze of the red and white spaniel in this nineteenth-century copy of a Landseer painting.

The horse at the top of the post has a similar spot in the top corner of his blaze. In horses these inset spots more often appear offset rather than centered on the blaze. This can give the the top of the blaze a scalloped look. Since the one in the picture is set a little further inside, it bisects the blaze so that there appears to be an irregular section of disconnected white. That white is probably not a separate marking, but rather the remainder of the blaze that the overlapping colored spot did not cover. The tinted areas in this photo show how this works.

I have shaded this Paint Horse’s broad blaze blue, and the imagined overlapping spot bright pink. The area where the colored spot overlaps the blaze is purple. On this horse, the disconnected white this leaves is much smaller. I have called these colored patches occluding spots, since they cover (occlude) part of the white face markings. If the spot is large enough, the result is what is often called a badger face.

Some badger-faced horses have spots large enough that the only thing left of the blaze are small, detached segments of white. Here are some good examples of that: Akhal-Teke1, Akhal-Teke2, Paint Horse, Thoroughbred and another Paint.

The separate nature of occluding spots is even more apparent when they occur when white patterning has completely removed the color from the face: Paint, Gypsy Horse, and another Paint.

I suspect that the horses pictured so far have something similar going on with their face markings, with just slight variations in scale and placement. I do not believe every dark spot inside a face marking is an occluding spot (or minimized badger-face marking, if you prefer). This guy is perhaps a good horse to look at what are probably two different things altering his face marking.

The most obvious change to his blaze is what looks like two or more occluding spots breaking it into two pieces. If you look further down his face, though, his blaze begins to break apart into round spots right behind his nostril. Because the scale and character are rather different, it is my suspicion that there are different causes for the two changes. What is happening close to his nose looks a bit like the way sabino degrades the edges of markings. This Belgian has a more roany version of what is probably the same type of thing.

Here is another horse that has what I think are two different things going on as well. Along the upper edge of her blaze a mid-sized occluding spot has cut off most (but not all) of the corner of her white marking.

Then there are small spots of color inside her blaze. I suspect that is Belton patterning. Since first posting about the possibility of dark ticking separate from the actions of the different patterns, many readers have sent leads on horses with this. That will be the subject of the next post. (Previous posts on Belton patterning, for those missed them, can be found here, here and here.)

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.

Another kind of roaning that is often attributed to the sabino gene is the kind seen on this chestnut tobiano pony, Dexter. This has a softer look than the “laced” edges that Dexter’s sabino-tobiano stablemate Splash has.

What makes Dexter unusual, though, is that he has a solid face.

He does have a white patch on one side of his chin which does not reach up to his lower lip, which can just barely be seen in this picture. (Because it is really under his chin it is hard to get a good image.)

It may be that modifiers are suppressing the sabino gene to such an extent that his chin patch is all that is left. It’s also possible that this type of roaning is itself some kind of modifier, and that the white on his chin is unrelated. The commonly accepted rule is that tobiano by itself does not create white on the face, though both myself and others have had reason to question the absolute nature of that rule.  (Because that statement is nigh upon heresy to many horse color enthusiasts, elaborating on that probably merits a separate blog post at a later date.)

But it is pretty clear that this is different from true roan. Here is what true roan, when combined with tobiano, looks like. (The photo comes from Reasontobecrazy stock photography.)

Here is a close-up of another roan tobiano.

Notice how the roaning is evenly distributed across the spots. Now compare that to a close-up of Dexter’s hip.

It’s also different from the roan patches that are sometimes seen on tobianos, particularly homozygous tobianos like the one below. Those tend to be rather random, whereas the roaning on horses like Dexter are concentrated around the borders of the dark patches.

Here is a close-up of roughly the same area on Dexter.

Here is another horse showing the same kind of softly roan edges,  although he has the white face markings that Dexter lacks. (The photo comes from Citron Vert Stock.)

 

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