NOTE: Dominant White is know being referred to as White Spotting or simply White because of the discovery of new mutations that don't result in an all white or nearly white horse and are not embryonic lethal.
Mutations in the KIT gene cause white spotting patterns in horses, dogs, cats, mice, rats, cattle, humans, and swine. The frequency of KIT gene mutations vary from species to species but, in horses, it is responsible for more white spotting patterns than any other gene. KIT gene mutations are responsible for Sabino 1, Classic Roan, and all the Dominant White patterns (currently at 20). Tobiano can also be added to this group. Tobiano is caused by a large chromosomal inversion that is very very close (approximately 70kb downstream) to KIT. It is so close in fact, that Tobiano most likely causes it’s white pattern by disrupting KIT gene function. This also means that while Tobiano isn’t technically “on” KIT, it is close enough that it will always be linked with KIT. (More on Genetic Linkage from Wikipedia.) So, while it isn't technically a KIT allele, for all practical purposes it should be treated as one.
So what do horses that have two different KIT gene mutations look like?
Tobiano + Roan: This is the most common combination and also the most easily recognized. It simply looks like a Tobiano with Classic Roan.
Tobiano + Sabino 1: Since Sabino 1 does not have to be loudly expressed this can easily look like Tobiano with “normal” face white. If Sabino 1 is more loudly expressed this can look like Tobiano with roaning and jagged belly spots. The Tobiano + Sabino 1 combination occurs mostly in Spotted Saddle Horses, Miniatures, and Gypsy Horses, breeds where both Tobiano and Sabino 1 are present.
Tobiano + Dominant White: The breeds in which Dominant white has been studied the most do not have Tobiano (think Arabians and Thoroughbreds). Because of this, it is difficult to know for sure how they would express together. However, we can make some educated guesses. If Tobiano was present with a loudly expressed Dominant White mutation, such as W2, Tobiano would most likely be completely masked. You would just have an all white or near white horse. If it was present with W20 (published May 2013) which resembles Splashed White this combination could easily be mistaken for Tobiano and Splashed White. If Tobiano was present with a medium expressed Dominant White, it would probably look like Tobiano with roaning and jagged belly spots (very similar to Tobiano and a highly expressed Sabino 1).
Sabino 1 + Classic Roan: This could look like a well roaned Sabino 1 or a Classic roan with “chrome” depending on the Sabino 1 expression.
Sabino 1 + Dominant White: This would probably look like Dominant White. Depending on the Dominant White mutation it would be all white or it could be look like Splashed White + Sabino 1.
It is also possible that some of these combinations could act together in an incomplete dominant manner. We know that W20 (Dominant White mutation 20) combines with W5 (Dominant White mutation 5) to produce a nearly white horse. It’s possible that Sabino 1 + some of the Dominant White mutations could have a similar effect. It is also possible that some combinations could be embryonic lethal when homozygous. We really don’t know enough to be certain. We do know that horses with Tobiano and Sabino 1 or Tobiano and Classic Roan are perfectly healthy horses. Some of the others, such as Sabino 1 with W2 (Dominant White mutation 2)? Well, that is anyone's guess.
Can there be more than two?So now you know the basics and how things “should” work. The thing with genetics, however, is that sometimes the “rules” get broken. New mutations do occur. Crossover (More on crossover from wikipedia.) does happen. So could a horse have two linked KIT gene mutations that would be passed down together to their offspring? That really is the question isn’t it? For Roan, Sabino 1, and Dominant White the likely result would either be that one mutation would “overpower” the other resulting only in the phenotype of the “more powerful” mutation or the result could be a new phenotype quite possibly all white. In that second cause it would not be considered Sabino 1 + Dominant White, for example, but a NEW mutation. Something similar to this HAS happened before in swine. The all white pig that is used for commercial pork production is white because of a KIT gene duplication. That is right these swine, in essence, have more than two KIT genes. The result is an all white pig.
Ok, then what about Tobiano you ask? That might be possible. Because Tobiano is downstream from KIT (even though it is very very close) it could theoretically be possible for a crossover to occur that would link Tobiano with another KIT gene mutation. This would be a 1 in a billion occurrence. Not only would the horse have to have two kit gene mutations (one from each parent) but the crossover would have to occur in just the right spot such that both the KIT gene mutation and the Tobiano mutation remained intact. That sounds pretty unlikely to me.
Just to make things even more interesting here is a link to a recent conversation I had with members of the Equine Color Study, a Facebook Group. This is a very interesting case of a mare who appeared to pass BOTH classic roan and Tobiano to her offspring. The linkage appeared to last two generations and then was broken. You can read our conversation here.
If you have a horse that has TESTED homozgyous for Tobiano + a copy of any of the other KIT gene mutations or homozygous for any of the KIT gene mutations and also Tobiano please let me know!
NOTE: The horses in the photos are horses that closely resemble what I feel possible KIT gene combinations would look like. They have NOT been tested as having these combinations. References for this article can be found on theReferences Page Under "KIT gene"