Tobiano

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May 26, 2012

Leopards, Cat Tracks and Beltons (Part 2)

Posted by The Equine Tapestry under Appaloosa, For Artists, Non-equines and other tangents, Pintaloosa, Tobiano
[3] Comments 

In the previous post, I talked about the two things that cause horses to have small dark spots on a white background. The first was the leopard pattern (Leopard Complex + Pattern1) and the second was the homozygous tobiano pattern. In this post, I want to talk about how dark spots on a white background are different in dogs.

Dalmatian dogs look like leopard appaloosas. It’s the same white background and the same small, round spots of color.  But Dalmatians are genetically very different from leopards. In fact, they have a lot more in common with the homozygous tobianos. That’s because they are “pinto” dogs. They just happen to be missing (or at least mostly missing) their dark patches. In fact, if you can imagine someone starting out with a classic tobiano horse – dark head, large round areas of color on the body – you have a good idea of what the basic piebald pattern is in dogs. In fact, in some countries the name for tobiano and the name for this pattern in dogs is the same: plating. Plattenscheck, platenbont – plate pinto. It makes sense, since tobianos have large “plates” of color on a white background. These dogs do, too. Or at least they started out that way. Here is a popular sire of English Setters from a little over a century ago.

His pattern is very reminiscent of tobiano. But breeders did not care for the patches, so they began breeding away from them.

In dogs, this kind of pattern is often called “extreme piebald”. It is still a “pinto” dog, but it doesn’t have a lot of color left, even on the face. English Setter breeders were not alone in this preference. The Dalmatian breeders were selecting for the same thing. They did not want patches, or even dark ears. They wanted all-over round spots.

Those round spots, which are visible in all three of these English Setters, look a lot like cat tracks to someone familiar with tobiano. What makes them different is that they aren’t actually part of the plating pattern. They are a separate thing entirely. For English Setters and some of the other sporting breeds, that’s the “Belton” pattern. The more technical names for it – ticking and roaning – are unfortunately for us horse people, already taken by very different patterns. So for now we’ll just use Belton to avoid making this any more confusing.

Belton adds dark spots of color to the areas the piebald pattern leaves white. What dog breeders have done is manipulate the scale and spacing of those spots of color. All three dogs at the top of this post have what are believed to be variations on this kind of patterning. The English Setter to the left is of course the original Belton pattern. The Dalmatian in the middle is likewise has a Belton-type pattern, but he also has some kind of modifier that has made the spots larger, rounder and more distinct. (Some of the distinctive nature of his spots are, of course, because he is a sleek-coated dog compared to the setter.)  The Australian Cattle Dog at the end has a Belton-type pattern that was modified to the other end of the spectrum, with spots that have gotten smaller, less round and less distinct. In some breeds, this is what is called Roan. There is some debate about whether Roan and Ticking in dogs are truly separate, or just variations on the same gene. I am not aware of any papers yet published with molecular studies, but it does seem that roan dogs, when outcrossed to non-roan breeds, end up with offspring that look a lot like the Belton setters. Certainly whether these are separate, similar genes or the same gene with layers of modifiers, the end result is that dogs have independent factors that will “recolor” the area that a piebald gene left white.

It didn’t seem that horses had that, at least not until recently.

In 2009 a French sport horse, Vision Morinda was foaled.

Clicking on the image above will take you to the website for her breeder, and her page which has many high-quality photos of her at all ages.

At first glance, it is tempting to assume that Vision Morinda is a tobiano with very loud cat tracking. The problem is that she cannot be homozygous. Her dam is brown. (Note that the mare she is pictured with is a surrogate. Her dam, Scarlett Fontanel, is pictured here.) But perhaps even more intriguing is the fact that her spotting seems to have intensified as she matured. That’s something that is typical of the Belton patterns. As most people are aware thanks to the Disney movie, Dalmatian puppies are born white and develop their spots later. That’s true of the English Setters and the Australian Cattle Dogs. Here is my friend Mary’s (extremely cute) Cattle Dog mix, Volt, as a puppy. (Thank you, Mary, for letting me share your photos!)

As you can see, he looks like a white dog with black patches. He is an extreme piebald. That’s why he has white ears. Well, mostly white ears. He was already starting to show some spotting there. His back and sides, however, looked white. But here is Volt today, as a grown dog.

As you can see, he developed his ticking – the Belton-type pattern – over time.

In a less dramatic fashion, Vision Morinda seems to have spotting that intensified as she matured. (Her breeders even comment on her page about the surprise of getting an English Setter color on their horse.) The spotting on her is also different, visually, from a typical tobiano with cat tracks. The pattern is evenly distributed. The spacing does change somewhat (notably across her shoulder) but it still is pretty consistent across the white areas, rather than clustering into spots or patches. It looks like the ticking you would see on a dog, not a horse.

This raises the question of whether there is some factor in horses that can add ticking – a Belton pattern, so to speak. I have a few more horses to share, all with odd spotting patterns. None are quite like Vision, but all have unexplained dark spots inside white patterns or markings. They all come from my “weird stuff” files. That’s where I put things that don’t make sense, or just seem “off” in some fashion. Sometimes enough of them accumulate – like the odd late greys from a few months ago – that it seems like there might be some thread connecting them all. I am not sure these horses really have a common thread, because they do have some visual differences, but I’m going to start posting them just to see if more turn up. That’s what happened with those greys (I have more that I need to post in the future, by the way!) so maybe sharing them will bring others out of the woodwork!

(Images at the top of the post are courtesy of Wikipedia. Images of historical English Setters come from The Pointer and Setter in America, published in 1911, and Country Life, Volume 22, 1907.)

 

May 26, 2012

Leopards, Cat Tracks and Beltons (Part 1)

Posted by The Equine Tapestry under Appaloosa, For Artists, Non-equines and other tangents, Pintaloosa, Tobiano
1 Comment 

Images of leopard appaloosas with Dalmatian dogs are always eye-catching. Certainly they can look quite closely matched, like this Polish Malopolski and his buddy. Even so, the patterns in the two species are very different in terms of what is really happening to the pigment on the animal. That’s probably off in the weeds for most owners and breeders, but for artists the distinction is actually pretty important.

This touches on one of the reasons why artists who develop an interest in horse color often have such a different perspective. Usually the kind of information a breeder needs is predictive. That is, they need to know what might likely result from crossing this to that, or what they might need to cross if this particular end result is what was wanted. What artists need to know isn’t about prediction nearly so much as it is about possibilities. Not so much what might happen, but what could happen – even far-out-there, not-very-likely, could happen. That’s because artists often want to add something for interest or for composition. For those producing realistic art, that has to be done within the constraints of what is possible. It doesn’t necessarily have to be likely, but it does have to be possible. This unique perspective became apparent to me a number of years ago when I gave my first presentation on horse color. In the question and answer period afterwards, someone in the audience asked if a horse could be both dappled and fleabitten at the same time. It was clear that was not the sort of question my fellow presenter, Dr. Sponenberg, often heard. But it is precisely the kind of question that equine artists ask all the time. Scientists might not notice this kind of detail on an individual horse, but for someone who paints horses, this kind of information – does this happen with this? – has a lot of practical value.

So why do artists need to understand the process behind appaloosa patterns? Spotting is a useful tool, because it breaks up positive and negative space. It makes the horse more visually interesting. If you are particularly clever, it can be used to draw the eye in a way that works with the composition, or to hide flaws. But spotting doesn’t just happen anywhere. It follows rules, and those rules depend with what is happening with the pigment. Understanding the underlying mechanism makes it far less likely that you’ll add some interesting detail that isn’t realistic. When dealing with rare combinations of colors and patterns, it might be difficult to find a reference image to consult. Knowing the process can tell you if there is a reason to bother looking in the first place, because it tells you what is possible. (And when you wing it without a reference, the knowledge will make for more reliable guesses.)

“Trouble”, sculpted by Sarah Minkiewicz-Breunig and glazed by Lesli Kathman.
In the collection of Melissa Gaulding.

This is a ceramic collectible with the kind of spotting (often called cat tracking) seen in homozygous tobianos. It’s a really popular effect. In my normal job (the one I have when I am not trying to get a horse color book to press), I have produced quite a number of these. The problem comes when this gets confused with leopard spotting, and most especially what happens when leopard patterns are combined with the tobiano pattern. That brings us back to the image at the beginning of the post. These three images – the leopard, the Dalmatian, and my ceramic foal –  represent three very different scenarios in terms of the underlying process. I want to take each, one at a time, and explain how they are different despite looking so similar.

This is the typical nose-to-toes kind of leopard. Most people would think of this as a white horse with black spots that have been superimposed on top. That’s not really accurate. From a genetic standpoint, this kind of horse is a two-step process. First she has inherited a gene that progressively adds white hairs to the coat. Those hairs, over time, are going to produce the fairly distinctive pattern known as varnish roan. If the pony in my illustration just had that first gene, she would look like a black version of this pony.

That first gene, known as Leopard Complex, sets things up for leopard but it doesn’t make leopard patterns itself.

That happens when the horse inherits a separate patterning gene in addition to Leopard Complex. In this case, that patterning gene is called Pattern1. What Pattern1 does is take the white from Leopard Complex and amplifies and organizes it. So while our horse looks like she is white with spots, it is perhaps more helpful to think of her as a horse that was roan, but Pattern1 has now taken that mixture of white and dark hair  and reorganized it. Underneath the white hair, what that horse may look like is closer to this.

This is what the underlying skin looks like. She probably does have some truly white skin in the area where a blanket pattern would go. Pattern1 does amplify the white, after all. But under it all she isn’t really a white horse, at least not in the sense that most people would think of as true white. She is more like a roan horse that has been modified a bit. That’s why even nose-to-toes leopards have faces that are shaded much more like a grey than a cremello, because for the most part the face has dark skin, not pink skin. And that is why a pintaloosa looks like this:

The true white areas of the tobiano pattern cover over the appaloosa pattern. The spots from the leopard pattern don’t spread over onto the tobiano pattern because the process with Pattern1 isn’t “add dark spots to the white”, it is “organize the roan into spots.” So the spots don’t happen where the tobiano pattern already took all the roan away. Without the color there in the first place, Pattern1 has nothing to work with.

Of course, if we moved our tobiano pattern out a bit, encompassing more of the dark skin and butting it up close to the “blanket” skin, we could probably get something that looked a bit like the leopard spots migrated over some of the tobiano.

Even so, the spotting is still concentrated in such a way that shows it is an appaloosa pattern with a tobiano pattern layered over the top of it. The spots on the flanks might look like they are in the tobiano white, but really they are just in an area that was already white from the action of the Pattern1 gene. The action is still the same. The tobiano is there adding true, pink-skinned white on the horse, and underneath it Leopard Complex and Pattern1 are just doing their thing.

Even with the tobiano bumping up to the pink-skinned areas of the leopard pattern, it still looks different from the kind of spotting that comes from a horse having two copies of the tobiano gene.

This is a (presumably) homozygous tobiano with cat tracks. Whereas Leopard Complex is a roaning process that Pattern1 takes and organizes into the leopard pattern, this type of spotting is more like a not-entirely-successful attempt to add some more color to a horse that already has large patches of color. Unlike the existing spots, which are large and opaque, these new spots are small and vary in opacity. Some just come through in specks.

Cat tracking tends to cluster around the existing spots to some extent, almost as if these new spots want to occupy the same general area as the existing spots. This is quite different from the spotting on a leopard, which tends to be dispersed across the body.

The exception is the hooves. Tobianos with cat tracks often have a concentration of spots around the coronary band, often turning the hoof completely dark or nearly so.

There are spots on the legs, but typically they are not as numerous as the ones around the feet. The same is true for the face. This is the face that goes with these feet. He does have a few spots in his blaze, but they are not extensive.

So how is this different from the Dalmatian? Well he really is a white animal with colored spots added on top. In dogs, the gene for this is usually called Ticking, but since ticking means something different in horses, I am going to use the older English term for the pattern, which is belton. This post has run really long, so I’ll split that over into a second part. And why delve into the belton pattern in dogs? Because lately there has been a handful of horses that have turned up that just might have that kind of spotting. At the very least there are horses with dark spots inside their white markings that are not tobiano cat tracking and not leopard patterning. More on those will appear in the next post!

 

November 3, 2011

Viability matters, too.

Posted by The Equine Tapestry under Advanced topics, Dominant White, Roan and roaning, Sabino, Tobiano
[6] Comments 

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?

 

November 2, 2011

Location matters

Posted by The Equine Tapestry under Advanced topics, Dominant White, For Artists, Roan and roaning, Sabino, Tobiano
[14] Comments 

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.

 

October 31, 2011

More roaning and ticking

Posted by The Equine Tapestry under Roan and roaning, Sabino, Tobiano
1 Comment 

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.)

The situation with tobiano, (true) roan and sabino all touch on another topic I want to bring up on the blog. I have one more roaning post to make, and then I will jump over to the situation with those genes. It’s a topic I’ve wanted to present here for a while, but it wanders into some of the more technical parts of genetics so I have been weighing how best to present the information. Hopefully I can do that without leaving everyone more confused than when I started!

 

 

August 28, 2011

Appaloosa roaning and pintaloosas

Posted by The Equine Tapestry under Appaloosa, Pintaloosa, Tobiano
[5] Comments 

Christine Sutcliffe shared this guy in the comments section of the previous post, and I wanted to post him here where he’ll be more likely to be seen. He carries the tobiano pattern, as did the others, along with varnish roan (leopard complex) and one or more of the overo patterning genes.

I say that because he has a broad blaze and two blue eyes.

Horses like this one often get misidentified as grey tobianos, especially if the version of their appaloosa pattern lacks spots, or if their tobiano pattern hides the area that would have shown the spots. It is an easy mistake to make, because varnish roans turn whiter over time much like a grey. (I’ve noticed that as my own black near-leopard mare has aged, more people call her a “grey appaloosa”.)

Some greys lose pigment on their faces, which can also confuse the issue. I suspect that is why varnish roan (leopard complex) has remained in some grey breeds (or strains in breeds) even when appaloosa patterns are not considered desirable or are outright banned. If true greys can develop mottled skin with age, then the facial mottling that develops from varnish pattern might not throw up warning flags.

What sets the progressive whitening of varnish roan apart from grey is that it leaves the spots. The dark spots on this fellow’s rump will remain, no matter how much paler his body becomes. It is thought that all appaloosas roan out, sooner or later. That only applies to the body color, though. When grey is added to the mix, it all lightens. The really loud appaloosa Friesian cross Mystic Warrior is a well-known example of this. This link shows his current appearance along with pictures of the loud black leopard he was as a foal.

Because grey eventually erases the spots in a way that varnish roan will not, it is often considered undesirable by appaloosa breeders. Contrast is often the name of the game in breeding for attractive appaloosa patterns, and grey removes it. That’s also why leopards have traditionally been so sought out by breeders; theirs is the pattern that keeps its contrast. Blanket patterns eventually look a lot more like varnish roans over time.

What is interesting about grey and appaloosa, though, is that before it takes the spots away, it tends to skew them. The angled spots on Mystic Warrior show that really well. On appaloosas with dark areas, like those with blanket patterns, it often adds dramatic white spotting that looks like a cross between marbling and dappling. For those that have the most recent edition of the Sponenberg book, there is a part-Arabian with this type of effect. (He is also pictured in the German book Pferde aus Licht und Schatten.)  It seems that not all grey appaloosas get altered in these ways, but it is common enough these are good clues that grey is there.

 

August 26, 2011

Tobiano, the “top dog” pattern

Posted by The Equine Tapestry under Dominant White, Tobiano
[10] Comments 

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.

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.

Here is tobiano overlapping dark-headed frosty roan.

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

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

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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