This post first appeared on June 16, 2011.
There is no question that the general level of understanding of horse color among horse people has risen considerably since I first got online in 1989. Back then people were still offering old guides for determining whether or not a horse was a tobiano or an overo (“white does not cross the topline…”). Today many breeders not only know there are many different kinds of overos – many of which do have white that crosses the topline – but can pretty accurately identify a frame or a splash or a sabino.

It is easy, then, to forget that there is still so much that we do not know. I have spent the better part of a decade trying to convince horse lovers that color genetics is really pretty simple, and in many ways it is. But there are still many unknowns.

Blue eyes and their relationship with the different patterning genes is one of those unknowns. It is not uncommon to find online experts asserting that all horses with blue eyes are splashes or frames, and that absent a positive test for LWOS, a blue eye is proof of the presence of splash. We don’t actually know that.  Are blue eyes characteristic of the splash overo pattern? Undoubtedly. Hunting down the splash pattern has been an obsession with me for decades now, and looking for consistent production of blue eyes over multiple generations was one of the ways I tracked down classic splash ‘crop-outs’. Those were horses with this kind of classic splash patterning:

Photo is of Dell Tera’s Long Term, owned by Tegan Leighton Davis.  Thank you, Tegan, for sharing him!)

But a clear link between a given trait and a specific pattern does not prove an absolute and exclusive relationship. The fact that splashes consistently have blue eyes (and classic patterns like the one above seem to always have two blue eyes) does not mean that other patterns cannot have blue eyes, or that we might not one day also find a separate, unrelated gene for blue eyes.

We already know that some of the Dominant White mutations have had one or both eyes blue. Likewise, a number of the sabino white Walking Horses (genetically homozygous for Sabino1) in the early stud books had blue eyes. Were they all also splashes? It seems doubtful, given the production records of the horses involved. Most did not reproduce their own eye color, and none produced the kind of classic pattern pictured above. But it cannot, given what we don’t yet know, be proven one way or the other. With luck, we will one day have a test and time will tell.

And finally, to tie back in with my off-topic posts about color in other animals, here is an example of a separate blue-eyed gene in dogs.

This was a German Shepherd mix from a recent horse show I attended. There are several known causes for blue eyes in dogs, with the most common being the merle gene discussed in the previous post. Often dog people will use the presence of blue eyes as a diagnostic for merle, since the pattern can be cryptic.  This little guy didn’t have any merling that I could find, so I suspect that he had the separate gene for blue eyes. That is the cause for blue eyes in breeds like the Siberian Husky. The popularity of that breed, and most especially blue-eyed Huskies, has led to a rise in blue eyes in the American mixed breed population.

More recently, a spotting pattern in German Shepherds has appeared, and it also produces blue eyes. It is rare that ordinary piebald dogs have blue eyes, though it can happen. The dogs with this mutation (called “Panda” by researchers) seem to have an unusually high frequency of them compared to the traditional piebald pattern.

If dogs can have a gene for blue eyes independent of any patterning or dilution genes, it is worth considering the possibility in horses. Likewise, if multiple patterns in dogs can produce blue eyes in varying frequency, there is no real reason to believe horses are different. But the fact is that until we have more tests, and more information, there is still a bit of mystery to pattern identification