September 2012


 

This is a good video explaining chimerism. It deals with cats and human beings, but the general concept is the same for horses. (Thank you, Joanie, for sharing it!)

I sometimes get asked why I do not often discuss brindles. A large part of it is that, as a fraternal twin myself, I find the type of chimerism responsible for brindle in horses – called tetragametic chimerism – deeply disturbing. There are other forms that do not involve the fusing of two embryos into one organism, as the video explains. The video talks about microchimerism, where genetic material passes from the fetus to the mother. There is also blood chimerism, where genetic material passes between twins through a shared portion of the placenta. A certain portion of fraternal twins are blood chimeras. Because I have had a conflicting blood type test in the past, and because my twin had a different blood type, it is possible that I am part of that group. (It is also possible that I had perfectly ordinary blood and a careless individual performing a blood test.)

 

One of the commenters asked for images of the other side of this horse, which appeared in the original tobiano marking post. I was fortunate to get quite a few good images of him. (If only all the classes were held in this particular arena, which is situated just right relative to the morning sun!)

 

You can almost see the random roan patch on his left hip in the side shot, but it is more visible in this one taken from behind.

I liked the way his tail was variegated, so I got a number of shots of it. (Be warned, however, that because Paint Horses can use tail switches, colors on tails may not always be natural to that horse.)

This was a good shot for showing how the color on the head of a tobiano spreads downward towards the jaw, so that in individuals with more white, there is a narrow “V” at the throat where the color on the two sides merge. I am working on a post that goes into more detail about this, and about how color tends to travel on the faces of tobianos, since that is relevant to the discussion on tobiano face white.

I am also trying to pull together some pieces to expand on some of the unusual colors that have appeared in recent posts. I apologize that we have wandered off in several different directions, but threads tend to come back around eventually. Fortunately you are all pretty tolerant when we take the more erratic, meandering path!

 

 

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.

I intended to post another example of reverse dapple roaning, but an interesting development in an obscure pony breed – and an unrelated discussion on an online forum – convinced me it was probably time to air this topic.

For some time I have flirted with an idea that is quite simply heretical in horse color circles. Something so far out there, that only the most uniformed horseman might entertain such a notion. But evidence is evidence. If the evidence does not fit universally held beliefs, then it is time to examine those beliefs even if it does give some people heartburn.

So what is so beyond the pale in the horse color world? What have some researchers noticed, but feared to mention?

The tobiano pattern is sometimes associated with white markings on the face. 

There. I have said it. I have noticed it for some time, as have others that were looking at some of the same breeds, or closely related breeds.

This was not something I was looking to find. What I was looking for was clues that some of the old Nordic or Celtic pony breeds still had splashed white. Most of these are breeds that either did not originally have “ordinary” markings, or where white markings – but not pinto patterns – were systematically bred out of the population. Quite a few of those breeds have classic splashed white (SW1), so it was reasonable to suspect that the horses with white on the face were heterozygotes (Sw1/n). Furthermore, it seemed likely that such a pattern could “hide” in the tobiano populations of the breeds where that was allowed, since few people minded if there were markings on a horse that already had a pinto pattern. The problem was that the expected SW1/SW1 homozygotes – the splashes with the classic pattern – never seemed to materialize. What’s more, when these ponies were crossed on solid mates, the face markings rarely appeared unless the tobiano pattern was also there. Even more tantalizing to someone looking for splash, these same ponies threw occasional blue eyes as well as the face white and the tobiano pattern. (I guess that counts as my second heresy in one post…)

I should clarify that I was looking at pretty unusual breeds. In the United States, it is quite difficult to find a popular breed without white markings of some kind. Most New World breeds not only have white markings, but many have what could be called sabino markings. In these breeds, like the Paint Horse pictured at the top, it is quite reasonable to assume that sabino (or splash or frame) are involved when a tobiano has a lot of white on the face. The question really is whether it is possible to get white on the face of a tobiano without these, or without the basic genes for white markings.

And now that question is being asked quite openly in a controversy surrounding the ancient Hucul Pony.

The Hucul is a primitive pony breed from the Carpathian Mountains. Most are dun, but tobiano is also a traditional color within the breed. Earlier this year, the parent stud book in Poland proposed separating out the tobiano Hucul population into a separate stud book. The reason? Because the tobiano ponies often have white on the face. It was feared that with the growing popularity of the tobiano pattern, the solids were in danger of having markings.

[Polish and Hungarian officials] claimed that piebald Huzuls transmit markings in a greater extent and these markings were undesired. Both recommend that piebald stallions may not cover plain mares. It is proposed to administrate piebald Huzul horses in a separate studbook.

There has been an outcry among breeders of tobiano Hucul Ponies that this fear is baseless because the leg markings on a tobiano are part of the pattern, which is permitted. The face markings, it has been noted, do not seem to pass along to the solids. This has lead to a discussion about the connection between face markings (and blue eyes) and the tobiano pattern. Some of the excerpts from this discussion are interesting:

This is from the paper “Odmiany a srokatosc” (Markings and Piebald) by Anna Stachurska. The emphasis is mine.

Piebald Huzuls have the Tobiano gene, plain coloured Huzuls have not, even if they have piebald parents. Piebald Huzuls may have more markings because the Tobiano gene is located near one important gene for markings and the properties of genes in neighbourhood usually show up together. But this does not matter as piebald Huzuls have white spots anyway. In plain coloured Huzuls, even if they have piebald parents, an absent Tobiano gen cannot influence the appearence of a gene for markings. This means that plain coloured Huzuls with piebald parents need not to have more markings than Huzuls at all.

In clarifying her paper, she also writes this.

I would not write that tobianos have non-piebald head. In American publications tobianos are described as “conservatively” marked on the head or “with minimal extent of markings” on the head. However, in Poland tobiano halfbred horses (e.g. Wielkopolski, Małopolski) have rather big markings on the head, even with a glass-eye, though the markings are not bigger than “normal” (usual).

There has been some research on the subject of markings in recent years, notably one of the Franches-Montagne. From these discussions, it appears that there is more research being done. It is certainly too early at this point to say why tobianos in otherwise solid, unmarked breeds have a higher incidence of white on the face, but it does look like the subject has attracted some attention. Resolving the conflict between the different Hucul stud books may help provide some incentive to research this situation in more detail.

(Huzul group picture from Wikimedia Commons.)