There are separate but equivalent mutations that can produce visually similar colors. This can complicate terminology and testing.
As I mentioned in the previous post, one of the things that attracted me to the original Sponenberg book was that it proposed a system for categorizing colors. In later books, the author revised his system from one that grouped things by point color to the more familiar base colors and modifiers that most of us use today. Not only did that system make it easier to explain coat color inheritance to others, but it promised a common language so that horsemen involved with different breeds might find it easier to communicate with one another about the subject. Needless to say, a lot of us who talked about the subject spent a lot of time explaining that no, there was no “Arabian chestnut” that was different from, say, “Morgan chestnut”. Chestnut was chestnut. Color from breed to breed was more alike than it was different.
In some sense, that is still true. We know that the vast majority of chestnut horses, like the American Curly Horse above, carry the same mutation to the MC1R gene. Somewhere in their distant past, chestnut horses all trace back to a common ancestor – the founder – that carried that original change. Although this “common ancestor” story has made news lately, especially when it comes to hair and eye color in people, as I mentioned during the discussion about founders, that is actually what would be expected. It would be expected that grey horses would trace back to the one horse that carried the initial mutation for grey, and that tobianos would trace back to the original tobiano. The question is, would this happen more than once? Could there be multiple grey mutations? Is there more than one tobiano?
In the case of chestnut, it turned out there were at least two. In 2000, another form of chestnut (ea) was identified in the Black Forest Horse. Technically the ‘new’ mutation does not cause the horse to produce red hair; horses with this version have the original mutation for red. They just also have another mutation that interferes with the test used to detect the original mutation. What is important, though, is that the two forms are functionally equivalent. That is, horses that carry the common mutation (e) and those that carry the more rare form (ea) do not look different from one another. They both can be light or dark in shade, or have pale or self-colored manes and tails. Whatever variation there may be from one chestnut to the next, it would seem the controls for those traits are to be found elsewhere in the genetic code.
A good example of functionally equivalent mutations can be found in dogs. There are three known variations that produce chocolate in dogs – bS, bd, and bc. Given the isolated nature of the different dog breeds, it is thought that there may be other less common mutations as well. Like the two chestnut varieties in horses, though, these mutations are not thought to produce a visually different outcome. From a testing standpoint, the versions of chocolate are slightly different, but the results are the same. For that reason, test results in dogs are usually reported simply as b, without indicating the specific version. (I should note that as far as I know, the different forms of chocolate in dogs are independently arising mutations that are similar, rather than the mutation-then-another-mutation situation with chestnut in horses.)
The idea that there are equivalent mutations that produce the same visual result makes testing for colors more complicated because it can produce confusing test results. Prior to the discovery of the alternate form of chestnut in horses, it was possible to get test results back on a chestnut that indicated the horse was bay or black. So far the spread of that particular mutation has been fairly limited to some of the European draft breeds and some of the rustic Spanish ponies. (The latter are thought to have the mutation from more recent introductions of draft horse blood.) It is not hard to imagine a situation where an equivalent mutation might be more widespread, however, leading to confusing test results.
Even greater confusion might result if you had mutations that already produced a pretty broad range of outcomes. The visual range of a chestnut horse, or a chocolate dog, is pretty subtle compared to the range of what we have called sabino in horses. Both of the horses below have patterns that have, in the very recent past, been called “sabino”. Not many horsemen would think that two mutations producing patterns of white this different from each other could be thought of as “functionally equivalent.”
To confuse things further, there are mutations to the same gene that have not, by tradition, been put in the same general category. It is likely that this horse, a true dark-headed roan, has a mutation to the same gene as the sabino horses above. Again, this is not something most people could think of as equivalent. Tobiano, the pattern many horsemen think of as the “opposite” of sabino and the other patterns in the overo category, also alters that same gene (KIT). Meanwhile the splashed white patterns, which are thought of a single category, include mutations that occur in two different locations (MITF and PAX3). So things that look completely different can be mutations to the same gene, while things that look alike can be mutations to different genes.
And that is the challenging part about where things stand in color genetics at the moment. Advances in molecular research are expanding our understanding at a phenomenal rate. In just a short time, more than thirty white patterns have been formally identified, and we know there are many many more. We know more about how these different patterns are related than ever before. The downside is that what has been discovered does not exactly match up with the structure that many of us have come to depend upon when explaining white patterns. Just naming the different patterns is a challenge, if you believe that the primary benefit of a uniform naming system – one that is used across breeds and countries – is clarity. This challenge of names and structure will be the topic of the next post.