Tag Archives | KIT

A family of diverse colors

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It was perhaps a bit rotten of me to bring up the tobianos and dark-headed roans when talking about the confusion about what to call horses with sabino patterns. Instead of saying, “No, this really is pretty simple,” I opted to point out that it is even more complicated. Now that I am feeling a little less mischievous, I probably should attempt to clarify things a bit.

I began this (meandering) train of thought with a post about the change from Mendelian genetics to molecular genetics. That is, a shift from analyzing colors using visual identification and statistical analysis to understanding colors based on the changes to the genetic code. Using the first method, colors were grouped a certain way that has become familiar to many horsemen. When the colors are grouped according to the gene where the mutation occurs, however, they sort a little differently than expected. Colors that look quite different – colors that aren’t even thought of as belonging to the same basic category of modification, like tobiano and dark-headed roan – can be mutations to the same gene.

The technical term for this situation is allelic heterogeneity. In plain English, what that means is that there are a number of different options for one gene. In the case of these particular colors and patterns, the gene where they are found is called KIT.  Tobiano, roan, sabino and dominant white are all alleles at KIT. As different as they look from one another, they can be thought of as belonging to the same family. This may seem a bit esoteric, but it has a couple of implications for breeders.

The Spotted Saddle Horses pictured at the top of this post display a type of tovero pattern that is very common in their breed. The ragged, torn outline of their spots is typical of what happens when tobiano is paired with Sabino1. It is a compound heterozygous pattern. That is, both copies of the KIT gene have a mutation, but they are different alleles. If that same horse had two copies of tobiano (two of the same allele), we would call him a homozygous tobiano. Instead these horses have one tobiano and one Sabino1 (two different alleles for the same gene). A horse has two copies of a given gene, but they only get to give one of them to each of their offspring. So like the homozygous tobiano, if they were bred to a solid horse all their offspring will be pintos, but only half will be tobiano. The other half will be sabino.

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Bred to solid mates, half the offspring of the toveros above should have this kind of pattern – Sabino1.

Allelic relationships like this are important to breeders because it means that under most circumstances, the patterns that result from combinations of alleles are not going to breed true. That might not be important if all that matters is that the resulting foal have a pinto pattern, because a compound heterozygote is going to produce a patterned foal 100% of the time. But if a breeder wants to duplicate the original combination, that might matter quite a lot. And if the other “pattern” is something that would not qualify as a pinto, like dark-headed roan or one of the more minimal versions of sabino, then the 50/50 nature of the inheritance might be a problem.

Breeders have noticed that some combinations, like tobiano roan, are difficult to get consistently. That is because this same splitting of the two alleles occurs; the horse can only give one but not both, so the only way to repeat the combination is for the other parent to contribute the second allele. The fact that some of these alleles look so different from one another makes the relationship between the colors less obvious. Knowing why Sabino1 toveros do not produce their own color when bred to a solid mate allows breeders to pick crosses that stack the deck in their favor. (A cross to the same Sabino1-tobiano combination, for instance, would give the desired pattern 50% of the time.)

The connection between these seemingly different colors might also make it easier to understand some of the quirks within some of these patterns. One of the most common questions I get from breeders of tobianos is about roan patches, or roaning in the colored areas of the coat. It is a relatively common occurrence in tobianos, and it often causes breeders to inquire if their horse might carry some kind of sabino pattern. In many cases, it appears that the roaning is just part of the tobiano pattern itself.

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Dexter has diffused roaning throughout the dark areas of his coat, with somewhat greater concentrations of white hairs around the borders of his spots

RoanPatch2
Here the roaning is mostly limited to one patch, though colored specks remain inside the roaned area

When tobiano is understood to be a mutation to the same site as both roan and sabino, irregularities like these seem less surprising than when tobiano is thought of as something wholly separate. Likewise, the idea that tobianos might be more prone to white on the face than solid horses seems less outrageous. Tobiano is related to a whole group of patterns that can quite rightly be described as doing just that, after all! (For newer readers, more on my scandalous views on tobiano face white can be read here and here.)

In fact, knowing that these colors are alleles of the same gene is useful because it encourages us to think about them in a different way. If we know that KIT mutates frequently, giving a surprising number of white and sabino variations, what about roan? Roan has proven problematic when it comes to testing, which suggests there is more than one version of the color. It is also true that there are quite a few instances of spontaneous “roans” in a variety of breeds. These have been dismissed in the past as not “true roan” because they came from non-roan parents. But what if they are just one of many roan mutations? And what about the various forms of white ticking, like rabicano and salpicada? Are they roan variants on KIT, too? Given what is known about the white mutations, that seems like a reasonable theory.

Taken as a group, many of these colors and patterns blend together with a lot of overlapping traits. Which brings me back to the original question, which is what to call them all. I’ve skipped over the more pressing problem of sabinos and dominant whites in this post, but I wanted to highlight the connection between these different colors and introduce the idea of compound heterozygosity. It is an idea that is pretty important to the situation with the sabinos. I had hoped to wrap this subject up with just one more post, and start posting some less in-depth topics, but it is probably obvious why I have avoided posting about this before. It is not a subject that lends itself well to brevity! So next up, the other group of KIT mutations and some ideas about what to call them. I promise, eventually I will get back to some easier topics!

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New developments

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I think I need to work on final book edits every spring, because it seems to guarantee that a paper will come out within that time period. Animal Genetics has a short communication out with three new KIT mutations and one new PAX3 in horses. There are also two papers out on KIT patterns in dogs, which is new. Before this, the piebald patterns in dogs had been mapped to MITF, which in horses is the other site for splash white. The picture above is the German Shepherd that carried the de novo mutation. (I must give a special thank you to her owner for allowing me to include it here.) I had intended to put together a longer post on this pattern, known as panda, because it touches on the subject of blue eyes in KIT mutations. She obviously has blue eyes, and her owner confirmed for me that some of her descendants have had a blue eye or blue segments in their eyes. This follows the pattern that I have seen in the historical records of some suspected Dominant White horses (also presumed to be KIT mutations). Blue eyes do seem more common in the originators, and then appear to occur sporadically – often in a less pronounced degree – in the descendants. Interestingly enough, the MITF mutations are not associated with blue eyes. In fact, this family of Shepherds was the first instance I noted where blue eyes in dogs were linked with a form of white spotting so I was particularly happy to see the mutation formally identified.

What is interesting about the new equine discoveries is that they really do not fit neatly into existing naming categories. The KIT mutations have been assigned numbers in the “W” series, but at least one appears to be subtle white-booster rather than a true dominant white. I had been urged by a couple of researchers, as I got close to my publication date, to avoid the use of the term “sabino” and just use “white spotting”, and I see now why. I suspect this will become more complicated as time goes on and more mutations are identified. One thing does seem clear, and that is that several of these sites mutate often – apparently in ways both large and small!

Just as I did with the book last year, I’ll be updating the text to reflect this new information for the new full-color supplemental book. I had feared there might not be much new information, but this coupled with some of the things I have been researching in the last few months should make for a lot more content as well as more abundant (and colorful) images!

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More on face white and tobianos

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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.

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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.

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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.

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