Horse Color Genetics: KIT Genes and Why KIT Matters

As I've researched genetics, I have heard several references to KIT* genes being in the same locus. I had no idea what that meant, and not really any wish to because I didn't know how it applied to what I already knew. The scenario was similar to when I first started genetics and not understanding homozygous and heterozygous: there was no good basic explanation anywhere.

Until, on the genetics forum, someone referred to KIT genes as sharing bunkbeds. And then my mind clicked, and I understood. Now I can pass this brilliant example on to you, and how it applies.

Roan, tobiano, sabino, and dominant white (which I haven't talked much about because the topic confuses me greatly; it's confusing because there are 20 different mutations) are all labeled as KIT genes. KIT genes share the same locus. Locus meaning where the genes are located.

What in the world does this mean, and why is that important?

To use the bunkbed example, each locus is it's own bunkbed. Extension, agouti, gray, dun, etc. are all their own bunkbeds. Each bunk has either one gene that is dominant or recessive. Now, to apply this, the bunkbed for KIT genes can only have carry, well, two. Generally, bunkbeds are for only two people. That means that one horse can only have up to two KIT genes; three is impossible. The reason being that, if a third one tried to sleep in the top bunk, someone's gonna get shoved off the bunkbed.

I have read a little about a couple horses that somehow inherited three KIT genes. Technically, tobiano isn't exactly a KIT gene precisely, but it is so closely linked with KIT that it works in just about the same way. So the horses with three KIT genes are probably homozygous for tobiano and heterozygous for one of the other genes. I read that there are some general health problems with horses with three KIT genes. Sort of like someone trying to squeeze a third person on the bunk; it can work, in a pinch, but usually it causes problems.

Generally, when showing a color genome sequence, the KIT genes are abbreviated the same way as all the others. Sort of like this: Ee, aa, RNrn, Tt. But it almost would make more sense to combine the KIT genes in the same area. More like this (same example): Ee, aa, RnT. See how I combined the roan and tobiano? Since they are in the same locus, it sort of makes sense. Alas, others don't share my feelings.


How this affects breeding is that the parent in question can only pass on one of those KIT genes. So with the horse above (Ee, aa, RNrn, Tt), it can only pass on roan OR tobiano. But it will pass on at least one.
That is one of the reasons roan with tobiano is rather rare. Tobiano with sabino is more common because generally paint breeders breed paint to paint, if that makes sense, and a lot of paints carry sabino.

Another interesting fact about KIT is that Extension (black/red) is closely linked to KIT. Why this matters is that each individual extension allelle is linked to one of the KIT genes. In the example, the horse is blue roan tobiano, Ee, aa, RNrn, Tt. Those two kit genes are each linked to one of the extension genes. Like, the dominant extension linked to the roan and the recessive to the tobiano, or vice versa. All this meaning that, should you choose to breed, those genes will stay linked together. The resulting foal, if the genes were like my example, would be either blue roan or red tobiano, but couldn't be red roan. See what I'm saying? It is simply a matter of determining which gene is linked to which. If there was one other previous foal, then you could surely predict which one was which because that foal would have been one or the other.

This whole linked-gene concept only matters if the horse is heterozygous black; if it is homozygous, it doesn't matter all the much because the horse will always be black based.

*I can't find anywhere what KIT means! If you know, please comment. I'm also not sure it matters because I probably wouldn't understand it anyway.