Sabino

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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
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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!

 

 

October 26, 2011

Roaning and white ticking

Posted by The Equine Tapestry under Roan and roaning, Sabino
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Roan and sabino tend to be catch-all terms for horses that have white hairs or white markings. In the next few posts I wanted to share a few horses that a generation ago would simply have been called roan, but that have white hairs or ticking from something other than the true, dark-headed roan gene. The general convention now is to group horses like that in with the sabinos, but their actual relationship to the sabino patterns is not really known.

This American Belgian is a pretty typical roan-like pattern that is probably a type of sabino. Horses like this lack the very distinct dark points of a true roan, which are usually apparent even when roan gets combined with sabino markings.  His legs are dirty, which hide the fact that he does have white feet, but compare his legs to this true roan with flashy white markings:

The upside-down “V” formed by the dark points on his front legs, which is one of the hallmarks of the true roan pattern, is particularly visible in the video. This Belgian does not have those, even though his front socks do not extend far up his legs.

Notice, too, that the roaning not only extends into areas that would be dark on a true roan (like the face and lower legs), but it is quite uneven. His neck, for instance, shows very little roaning. Even more interesting are his hindquarters, which have patches of less intense roaning. For those artists familiar with etching to achieve roaning effects, it looks like the person doing his pattern got bored with the process by the time they reached his bum! These kinds of revertent patches are pretty common in sabino roans.

He also had an unusual roaned patch on the border of one side of his blaze.

I suspect that a lot of the Belgians (of American breeding at least) registered as chestnut roan in the last fifty years are in fact sabinos like this guy.

He isn’t that unusual, as sabino roans go. What is interesting is that it seems possible to get sabinos where there are fewer indicators that the pattern is there beyond the roaning. There are a number of instances where horses from sabino families have body roaning and a blaze, but no other white markings. In some breeds horses with all-over roaning and a blaze (or having even less white on the face) have been tested to carry Sabino1, the only form of sabino that can be tested at this point. Belgians are not known to carry Sabino1, though it’s also true that testing has not been widely done among many of the draft breeds, with the exception of the Gypsy Horse.

What we call sabino is in fact a lot of different types of patterns. Those patterns can be sorted out visually, as I did in the upcoming book, but it may be that those visual categories include different genetic colors that sometimes mimic each other.

 

October 16, 2011

Sabino variations

Posted by The Equine Tapestry under For Artists, Sabino
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As many people have noted, the term “sabino” has become a bit of a catch-all for a lot of visually different patterns. In this way it is a little like “overo” was ten or fifteen years ago. At the time, the term overo was used to mean “not tobiano”. As the different kinds of overo have become more widely understood, many have dropped overo in favor of more specific terms.  But when something doesn’t quite fit tobiano, frame or splash, it usually gets classified – for the moment at least – as some kind of sabino.

In the upcoming book, I attempted to separate out some of the different forms the pattern we call sabino takes. Because only one form of sabino, Sabino1, can be tested at the moment, we don’t actually know if the patterns that are visually different are in fact genetically different. (We already know some things that look virtually the same can be genetically different!)  But it is true that some types of sabino can be found in some breeding groups and not others. It’s also true that some of these variations don’t fit the stereotypical “rules” often assigned to sabino. In fact, quite a number of them are likely to be misidentified as something else.

The pony mare in the picture above is a good example. Many people use high stockings and little or no face white as a clue that the horse or pony has a minimal version of the tobiano gene. (Although it isn’t the best picture, if you look you can see she had some white hairs on her forehead but no other white on the face.)  She’s not a tobiano, though. She’s a Hackney pony, which has not had tobiano in the gene pool since the early 20th century.

The one back leg does give a bit of a clue, since the white goes up the front of the leg rather than the side. This overlay illustration of a tobiano and a typical sabino pattern shows how the difference in the placement of white on the leg typical for each pattern.

That is often a good indicator, though it always has to be remembered that “typical for the pattern” is not necessarily the same thing as “always must be so.” The fact that tobiano is not part of the gene pool is a much more reliable indicator. What’s more, this kind of sabino pattern is not really uncommon in Hackneys. In the book I called these Unbalanced Sabinos, because they break the general rule that flashy white on the legs is usually matched with flashy white on the face. As these types of sabinos go, this little mare is actually pretty minimal. There are Unbalanced Sabinos that have extensive body white and very little white on the face.

Unbalanced Sabinos also break the rule posed by some writers that the sabinos have white on the chin or lips. This was my old Walking Horse gelding, Master.

He had a star and a snip, but no white on his lips or chin. Not only was it unusual to have four white legs without much face white, his leg markings were themselves unusual. The angle on this photo does not show it well, but there was a dramatic difference in the height of his front stockings, which rose in the front to his knees, compared to his hind socks. That was actually what caught my eye when I first saw him, because it is unusual for the front legs to have more white than the hind ones. He also had a belly spot half way between his girth and his sheath.

(And no, he was not a poster child for classic conformational ideals.)

One of the reason sabino variations have been on my mind was this excellent blog post by my friend Sarah Minkiewicz-Breunig. In the post, Sarah talks about the value of taking reference pictures for sculpting. Like Sarah, I am a huge believer in amassing a huge collection of reference pictures. There really isn’t any substitute for looking at hundreds and hundreds of variations of the same pattern. But even more than that, I would recommend the practice of sorting that collection. Nothing helps the eye spot trends like arranging like with like. That was how the different visual categories of sabinos were developed for the book. Stacks of sabinos were sorted into groups that were visually similar. In some cases, I pulled patterns off one body type and transferred it to another. You’d be surprised how often breed type can override your eye when it comes to spotting similarities (or differences) in patterns. Transferring the pattern over to a different body type can force you to really see things. That’s one reason why the illustrations in the book are on generic horse shapes rather than ones specific to the breed or type being discussed.

 

September 14, 2011

Through a glass, darkly

Posted by The Equine Tapestry under Articles (PDF), Dominant White, Sabino
[5] Comments 

Recently a discussion of Dominant White came up on another forum, and as often happens the question of the naming of the pattern was raised. When the first study was published, many felt that the pattern was part of the sabino series and should have been named Sabino2. (Sabino1 had been identified a short time prior.)

In that discussion, someone suggested that Dominant White was disproven in an article I wrote many years ago. Because I have encountered that comment before, and because I hate to contribute indirectly to the confusion around the two patterns, I’m going to condense my response to that statement here. I had intended to do a revision to the article shortly after the first Dominant White paper was published, but I never found the time, so this will have to do for the moment.

The article was originally written sometime around 1992 and updated again in 1997. It was intended to question the current thinking (that is, early to mid-1990s thinking) about Dominant White. Back then I wasn’t sure that I was right, because there were so many gaps in the information. From the article:

This is not to say that dominant white horses do not exist, but I am personally skeptical, having never found one I could prove wasn’t more likely a sabino white.

Also:

Given the breeding records of the aforementioned white horses, I have come to wonder if Dominant White really does exist. But it is clear that even if it does, the majority of white-born horses are probably sabino whites.

My assertions then were based on the picture I had, but I knew I wasn’t looking at a complete picture.  And like almost all researchers who look at a partial picture, my analysis was skewed by the specifics of the group I had to study. When I began assembling information on white-born foals, the most common horse used to illustrate Dominant White was the Tennessee Walking Horse. They were mentioned, and their photos almost always
appeared in anything about Dominant White. I had access to not only the stud books, with all their elaborate detail, from when the breed had a high proportion of whites, but I also lived just a short ride from the registry headquarters where I had been given free use of their archive materials. I could get as complete a picture as might be possible fifty or sixty years after the fact, and that picture said that each and every white Walking Horse had two sabino parents.

In fact, it said that each had a highly marked sabino, because I had an elaborate numbering system for weighing how much white the sabinos had. Some of the assumptions behind those numbers were wrong, but it happened to give me accurate results for the wrong reasons! (Lesson learned there about the limitations of stud book records.)

That specific conclusion was accurate. What I was looking at in the Walking Horse records was not dominant white. It was an incompletely dominant form of sabino. Sabino1 is a really old mutation, as the study done on colors in ancient remains showed a few years ago. It’s spread out through a variety of breeds, but there probably isn’t another one that has such a concentrated population of them as the Walking Horse, particularly as the breed was at its founding.

I moved from the Walking Horses out to other know cases of whites, but if you read the paper that information is much more sketchy. I didn’t have access to the kind of pedigree and production records I had with the Walking Horses.  What I had was a lot of anecdotal evidence that white-born horses in other breeds produced what looked like sabino.

But even so, there were weak spots in the theory that *all* these horses were sabinos. There were horses that only fit the pattern if you allowed for a really liberal classification of sabino. That didn’t fit what I saw with Walking Horses, where the parents of whites were scoring high on my “how white is the horse” scale. At the time, I wondered if linkage to chestnut was hiding sabino, which I mention in the article. I intended to explore that idea more thoroughly in a future paper, which never got written – and probably just as well, since I fundamentally misunderstood linkage! (Lesson learned about the limitations of studying phenotype without a background in the molecular end of genetics.)

Those exceptions did bug me. In fact, I had them all tagged in my own research files as oddities. I have often said that as a researcher, where you are wrong is where you learn. Because the weak spots in a theory are usually where new information is going to come, I tend to flag them in my files. The horses that were flagged as not fitting the theory of sabino white where: R Khasper, Cigale, Mont Blanc, Patchen Beauty and Sneuwwijte.

Three of those are, of course, among the families later identified as Dominant White. Mont Blanc was not part of that study but he certainly fits the same profile.

The last one isn’t as well-known. Sneuwwitje (Snow White) was a Groninger that appeared in the Wiersema book in the 1970s. I had a copy when the article was written, but I could not translate it. The pedigree and color information was there, and I could see that she didn’t fit the profile. If I had access to the sort of automated translators we have now, I would have found at least part of the puzzle that I was missing. That’s because Wiersema outlined in detail the exact theory I proposed; that is, that sabino x sabino crosses produced white foals. (I go into detail in the upcoming book about the different Sabino1 families in the Groninger, and the breeders who were using them to produce white horses.)

Then he discusses the white-born Sneuwwitje, who did not fit his pattern. She was from solid parents, and then when she was bred to different test mates, she produced 50% white or colored foals. That is, of course, exactly what would be expected from a Dominant White. Solid parents (ie., she came out of nowhere) and a 50% production rate of white or pied foals. (Where she a sabino white, you’d have sabino parents and a production rate of 100% pied foals.) He then states that there must be two different types of white-born foals, which of course is exactly what the dominant white study proved almost forty years later.

Since that time I’ve been able to translate not just his book, but a number of papers from other early twentieth century German researchers that were making similar observations. They also studied the “white-born” (weissgeboren) horses and were finding horses like Sneuwwitje. In fact, knowing the behavior of the two different genes, it’s pretty easy to tell which group (W or Sb1) a given researcher might have been studying.

In closing I do think that calling the two different genes separate names is a good idea. Yes, they do look indistinguishable from one another in many cases. I certainly wouldn’t blame anyone for advertising a more patched variety of dominant white as sabino, since that is what most horsemen are going to call the horse based on the appearance. But when talking about the actual genetics, the two different names help clarify the fact that these horses that look alike don’t actually breed alike. The ways that they are different – like whether they can appear out of nowhere or not, are able to produce all pintos or not, are lethal or not – have important implications for breeding decisions, so anything that highlights that is probably helpful.

 

August 9, 2011

Musings about markings

Posted by The Equine Tapestry under Sabino, Splash overo
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I have time for just one more post before I go on my internet sabbatical, so I thought I’d leave off with some musings about white markings.

My friend Caroline Jones shared this photo of a partbred Suffolk with me recently. She is interesting because her sire is a solid chestnut (“whole-coloured”) Suffolk, and her dam was a bay Shire. She has a large star and what looks like some white hairs on her nose. Although they are not visible in the photo, she had completely solid black legs.

The two breeds are interesting because modern Shires are uniformly marked with white, and are likely homozygous for some kind of sabino. British Suffolks, on the other hand, have been heavily selected for the absence of white on the legs. Even in the early stud books (late 19th century) leg white was rare and usually pretty minimal, but over time the preference for solid legs eliminated even that. They can have face white, and some even have blazes, though solid faces or small stars are the most common. The Shire and the Suffolk are therefor at the opposite ends of selection for markings.

What is an interesting question is why the absence of markings won out, at least in this case. If sabino is dominant, then a homozygous sabino should stamp all its foals with some type of sabino marking. Is this mare, with her large star and dark legs, a sabino? And does this mean that the absence of markings might also be more than just that? Do Suffolks carry something in addition, something that limits the white? Chestnut is considered quite a ‘leaky’ color when it comes to white markings, so much so that chestnut outcrops from unmarked black or bay breeds often have white that their darker relatives never displayed. It makes sense that to create an (largely) unmarked chestnut breed like the Suffolk, breeders would have inadvertently concentrated white suppressing genes.

The idea of white suppressors has long interested me. I first came to wonder about them when I ran across a splash overo crop-out (reconstructed) Tarpan. At the time I wondered how such a loud foal could come from two minimally marked parents. The sire had a small snip and a white coronary band on a hind foot, and the dam had a tiny star and small snip. Was something tamping down the white on the parents? Or was something boosting the white on the foal? Since then I have come to believe that, in the case of splash, the parents were typical of heterozygotes in breeds without ordinary white markings. Although it might not have been relevant to the Tarpans, I have come to suspect that the answer to my original question about boosting and suppressing is that both happen.

So is Susie (above) a suppressed sabino, or a boosted star-marked horse? And just what do those suppressors and boosters do to other patterns? How many of each kind are there? Is there a separate suppressor for faces and for legs? (We know there are breeds uniformly marked with sabino-like face markings, but no leg markings, for instance.)

When I return from my break, I’ll get back to dominant whites (and their connection to the zebra topic).

 

July 28, 2011

Sato

Posted by The Equine Tapestry under Dominant White, Eye color, Sabino
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One of the guest horses at BreyerFest this year was the palomino dominant white stallion Sato from Blazing Colours Farm. Sato comes from the dominant white family of Puchilingui.

So far each of the identified dominant white families have been a separate mutation, and in most cases the originator is recent enough that they are known individuals. The version of dominant white that originates with Puchilingui, W5, is known for producing a pretty wide range of sabino-like patterning as well as truly white horses.

With horses like these, it is easy to understand why breeders use the term sabino when advertising their horses, rather than their technical classification. Mare owners looking to produce pinto patterned foals know what a sabino looks like, and dominant whites of this type produce what looks like a sabino. The “proper” genetic term can be, at least in that kind of setting, unintentionally confusing.

In informal settings, it is likely that dominant white horses will continue to be referred to as sabinos. That is what they look like, and that is the term many horse people understand. They are very different though, so in formal settings where genetics are being discussed they deserve their separate names. That topic probably deserves its own post, though.

Another quick note about Sato, though. The close-up at the top of the post shows the blue segment in his eye really well. Although dark eyes are more common, a number of the dominant whites have had blue eyes. This happened with some of the sabino white Walking Horses, too, though it appeared to be less frequent.  (The topic of blue eyes in Walkers is pretty complex, so that will have to wait for the volume that covers light breeds.)

 

July 27, 2011

From the archives… Dominant White

Posted by The Equine Tapestry under Blackberry Lane Archives, Dominant White, Sabino
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Before I talk about Dominant White, it might be helpful to pull one of the archived posts from my studio blog. I still need to finish pulling the posts that deal with horse color over here to the Tapestry blog, but since this one is relevant I’ll repost it now.

(Previously posted on August 30, 2007)

Exciting News in the World of Horse Color

One of the downsides to publishing your theories is that you stand the chance of later being proven wrong. Or at least missing the mark by a bit.

Ten years ago I wrote an article entitled “A Study of White Horses – Not What They Seem“, where I questioned the existence of Dominant White in horses. My skepticism was based on my own research into the backgrounds of as many white-born horses as I could find. From that I came to believe that most of the horses designated as Dominant White were more likely to be extremely marked sabinos. I failed to find horses that fit the profile for Dominant White, so I began to suspect that the color did not exist – or perhaps no longer existed since there were anecdotal stories of horses that had fit the profile in times past.

It appears that my speculation was wrong, at least in terms of whether or not there was such a gene in horses. Not only are there real, live Dominant Whites, but the Swiss team that identified them has been able to test for it. So eventually we will know for sure which horses carry the gene. (It is thought that the gene is extremely rare, so it is likely that many of the white-born horses are still just sabinos.)

Other things are in the works, too. A Swedish team found the mutation for grey, though a test is not yet available. UC Davis has been working on dun, and of course there is the ongoing research on the appaloosa patterns. Pretty soon there won’t be anything left to guess about when it comes to horse colors!

That blog post was made in 2007. The article I wrote ended this way:

Given the breeding records of the aforementioned “white” horses, I have come to wonder if Dominant White really does exist. But it is clear that even if it does, the majority of white-born horses are probably sabino whites.

Much of the research for the original article was done between 1992 and 1995, when I had access to the archives at the Tennessee Walking Horse Breeders’ and Exhibitors’ Association. Looking through old show catalogs and registry records, I found a lot of horses that looked like the horse above, Silver Sultan. (Photo from the TWHBEA archives)  Even more were recorded in the early stud books.

(Don’t worry – the highlighting was done on the image, not the book itself! The only way I mark my books is with post-it-notes, which are visible there on the pages behind this one. Those all mark sabino-patterned or white horses that are recorded as having blue eyes so that I can enter them in my notes. There aren’t any post-its closer to this page because those have already been entered. I am perpetually behind with formal records like these.)

I had a complete set of Walking Horse stud books, which were a gold mine of information because breeders submitted (and the registry printed) detailed color descriptions. Because I was interested in how sabino might work, I developed an elaborate numerical system for evaluating just how much white the horse had, and then set about assessing how horses with different amounts of white produced when crossed with mates with differing amounts of white. In hindsight, it was probably far less useful than the time spent justified! But the one take-away was that the white horses all had two sabino parents, and in almost all cases the numerical value of the parents was pretty high.

The way I scored the horses actually skewed this, because I grew up in the Tennessee Valley and I knew that many horsemen called a horse with indistinct body spotting “roan”. Taking that into account, if the horse had enough points based on markings – if I could be pretty sure it was not a dark-headed roan – then it got extra points if roan was used in the description. Many of the horses probably didn’t have body spots, or at least weren’t as patterned as their number value would suggest, but I was weighing a trait that was tied to the white even if I didn’t know it at the time. That is because the gene that causes sabino-white, Sabino1, tends to produce really roany horses, even when they are not very spotted. This old-fashioned Walking Horse mare is a good example.

Finding that link between white-born horses and sabino in both parents was why I wrote the article. Still, there were horses that did not fit my theory. In the article I speculated that in some breeds sabino might be linked in some way to the chestnut gene. There was no apparent link in the Walking Horses, which inherited spotting without any real difference related to base color that I could tell. I did know, though, that in some breeds (like Arabians) there was a noticeable difference in the amount of white a chestnut horse might inherit compared to a bay or black horse. I wondered at the time if this might play a role in the white-born horses that had parents with low numerical values (ie., little to no white), since in most of those cases the parents were bay or brown.

The biggest limitation was that most of the white-born horses I could find were long gone, and in many cases only partial records remained. I published what I had, and hoped that the article might flush out a bit more information. If your heart is truly in research, you really do want those corrections, because where you are wrong is where you learn.

My correction didn’t come for another ten years, but it did come. Almost all the horses that didn’t fit my theory quite exactly right were found to be Dominant White. R Khasper. White Beauty. Puchilingui.  Those that weren’t in the study fit the pattern for being Dominant White.

Tomorrow I will post pictures of Sato, the palomino Dominant White Thoroughbred stallion from one of those identified families. I was fortunate enough to see him in person in Kentucky, and talk to his owner, April Wayenberg. He is particularly interesting because he is more colored than a lot of Dominant Whites. He is also extremely photogenic, so I took hundreds of photos.

 

July 11, 2011

Rabicano versus Sabino

Posted by The Equine Tapestry under Roan and roaning, Sabino
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Yesterday I use the pattern on this horse to discuss the way sabino can mimic rabicano. If sabino can mimic the rabicano pattern, then what exactly does a sabino-rabicano look like. Is it any different from the sabinos that have flank roaning and coon tails?

One difference might be the way the roaning organizes itself. Rabicano is known for having a brindled effect. That is a bit different from the diffused roaning that is present on the horse above. Contrast his side with this mare:

In the areas where the white is less concentrated, the brindling is visible. (In my experience with my own mare, whose striping is pictured in the blog header, this trait is often less obvious in photos than it is in person.)

It may be that this trait can help identify the presence of rabicano as opposed to sabino roaning or even the other forms of white body ticking. Rabicanos do seem to brindle more often than other types of ticked horses, though it is hard to know if this is an exclusive trait or not, too.

Another interesting difference with this horse compared to the first one is that the tailhead is more completely white.

Could sabino, which often boosts the white in other patterns, be influencing the amount of white on the tailhead? The frequent assumption is that rabicano is adding white tailheads to sabino patterns. This kind of effect, where sabino amplifies the white of another pattern, is consistent with what we already know sabino does.

This all illustrates the problem with identifying patterns using the tools we presently have. We have pieces of the puzzle, but we do not (yet) have a complete picture. The point at which one pattern begins and another ends is not entirely clear. To make matters more complicated, the evidence suggests that at the more minimal end there is considerable overlap. And at t the other end, the amount of white tends to hide the clues!

Both horses posted are good examples. Not only are they roaned and coon-tailed, but they each have high stockings and one blue eye. Here is the head shot from the mare. (I did not have a good, in-focus head shot of the colt in the first picture.)

Does her blue eye come from sabino, which she obviously has? The white high on the broad side of her neck suggests she also has the frame pattern. Did that give her the blue eye? Is it proof of the presence of splash? Right now we simply do not know for sure. When there are more tests, we’ll probably be able to develop a more clear picture of just where the patterns start and end. But for now it really is just guesswork. Sometimes the breed can eliminate certain possibilities, or more clues can be found by examining production records of a given family of horses, but it is still guessing.

 

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