Horse Color Genetics: A Little More on Sabino

Here is the last post I did on overo, but since I have learned much more and would like to do seperate posts, going a little more indepth on each overo type.

Sabino is a part of the KIT gene, which are a set of genes that share the same locus. Roan, dominant white, and tobiano all fall in as some of the others. It is also an incomplete dominant gene, meaning that, whether in heterozygous or homozygous form, the pattern will express itself in different ways.

Sabino is also classified as an overo pattern. However, it has so many different forms that many people don't know how to describe it. The following horses are all sabino:

While the two horses in the middle have very similar markings, the top and bottom one aren't even comparable. That is how much variation there is in the mutation.

The first three horses were all heterozygotes; one dominant copy of the gene. The last one is what is called 'maximum sabino', and it is homozygous for sabino, or SB1. Along with dominant white, maximum sabino can sometimes be confused for albino, but in reality it is, just like dominant white, like a giant white spot that covers up the color underneath. They always have dark eyes, unless combined with other genes which might cause blue eyes.

In the most recent equine genetics book available, The Equine Tapestry, Lesli Kathman talks a little about SB1 and why it has the number on the end. Scientists are anticipating more sabino mutations that are yet undiscovered, but SB1 is the only one that is testable at the moment.

Within the heterozygous range, the pattern can vary immensely, from almost not there, to a fully fledged pinto pattern that can be mistaken as nothing else. Because of this, sabino is hard to breed for. Minimum and maximum form within the heterozygotes is not neccesarily transferred from parent to offspring. For example: a maximum heterozygote could produce a minimum heterozygote, and vice versa.

Note: the difference between maximum sabino heterozygotes and maximum sabino homozygotes is tremendous; the former usually represents itself in the loud pinto form, the latter in the all white form.

Many heterozygous sabinos have some kind of roaning, making the patches less defined than many of the other pinto genes. Some have even been mistaken as plain 'roan' (also sometimes called true roan). Any of the above horses have just the tiniest bit of roaned out bit, but be careful when identifying sabino's, as horses that actually carry both roan and sabino will probably be more defined than you would think.

Sabino can be identified easily in a more maximum form, by the irregular splotchy patches that look like the horse was randomly splattered with paint, and is usually accompanied by some kind of large face marking and stockings. Minimum forms can be easy to mistake, however, for minimum forms of other patterns such as frame or splash. Sadly, I have no rules as to how to differentiate the two, other than the tendency to 'behave' differently. Maximum heterozygous forms can be almost indistinguishable from several of the dominant white mutations, especially within Arabians. The two arabians below have good examples of what some may not recognize as a pinto pattern.

In breeds where pinto colors are not 'available', like Arabians and Clydesdales, sabino is very much a part of. It has been noted, however that Clydesdales do not carry SB1, but almost every single one has some form of sabino. The mutation may be specific to the breed, but that is yet unknown. Notice the large face markings, tall stockings, and belly spots, which all are indicators of some kind of white patterning gene.

If many breeds allow sabino, but not other pinto patterns, the question my mind keeps wandering back to is this: should sabino actually be considered a pinto gene? In most cases, most horsemen don't consider bald face markings, tall stockings, or even small belly spots to be anything of consequence. And homozygous sabinos can barely be called pinto at all.

I would say, in my own opinion, that minimum heterozygous sabinos are much more common than maximum heterozygous sabinos. Even if you look up pictures of the former, there are many more than, because the owners don't see them as anything in particular, aren't labeled as such.

Horse Color Genetics: Dominant White and It's Gazillion Mutations.....

Dominant white is basically a white spotting gene, similar to any of the pinto patterns. HOWEVER (and that is a big however), it is not as simple as all that.

Dominant white is not all that common, and it comes in so many different forms that it is hard to trace. There are 20 identified mutations, but only 3 you can test for. There are probably more since it almost seems like a progressive color, mutating just a little bit every time a horse is bred. That is why it is so hard to trace.

This particular spotting gene can come in all shapes and forms. The easiest to identify is almost all white; about as close to albino as you can get. HOWEVER, the eyes are dark, unlike cremello's and other double-cream horses. Think of dominant white as a huge white spot; whether it covers the whole body, or only parts, depends on the mutation. These can sometimes be mistaken with fully expressed sabino horses.

There is another way it can show up, too. It can also be more spotty, and even look sabino, or even any of the other pinto genes. On the genetics forum, sometimes people will post pictures of their unusual pinto horses that they tested for all the different genes they could test for, and still came back with recessive results. The verdict is that they must be some kind of untraceable dominant white gene. There is never any uniform pattern to partially spotted dominant white horses, unlike all the pinto patterned horses, which is also why it is hard for owners to identify.

The below horse is the founder of Dominant White 3, which is found in Arabians.

What's neat about dominant white is that it is present in quite a few all-solid breeds like Arabian and Thoroughbred, making quite a few look pinto-mix. But dominant white is accepted in those breed registries, wheras pinto is not.

It is thought, but not fact, that all of these dominant white mutations in homozygous form is embryonic lethal, resulting in what scientists think is an embryo which dies after a few weeks. This thought is based on the fact that all of the horses that have been tested for dominant white are heterozygous for the gene. Whether this is within each individual mutation or umbrella for all W genes is beyond me.

More information will be posted as soon as more information is available.  Below are all the traced mutations, but not all of the tests are available to the public yet. Check out W5!

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.

Horse Coat Colors: Roan

Roan is a modifier, or a color shifter, kind of like pinto or appaloosa. It causes the base coat to be evenly mixed with white hairs, lightening the original color. Roans generally have darker manes and tails, darker heads, and dark legs. White 'normal' markings on the face and legs are acceptable.
Some roans have little dark spots. This doesn't mean that they have the LP gene. It is simply a spot that didn't get covered up by white.
Roan is a simple dominant: whether the horse has one or two dominant copies is irrelevant unless you are breeding and want to know. The following pictures there isn't any way to tell if they are homozygous or heterozygous unless you look at their parents or offspring.
A blue roan is a black horse with the roan gene. Sometimes they look a little silvery.

A red roan is a chestnut horse with the roan gene. Sometimes they are called 'strawberry' roan. Please know that there is no difference. In the past, bay roans have also been red roans, but this is not technically correct.

A bay roan is a bay that has the roan gene. Easy. None of these are hard to remember, but honestly, if we just called them black, chestnut, and bay roan, wouldn't it be easier? The first horse isn't a very even roan, but she is still bay roan nonetheless. That second one down may look like a blue roan, but the color of the head gives it away.

And lastly, there is palomino roan. These aren't very common, which is kind of weird (I don't know why). They are sometimes called 'honey roan'. 

And that concludes roan! What is nice about roan is it is easy to identify, and the base color is easy to spot.

Horse Color Genetics: Tobiano

Review: E=black, A=bay, G=gray, Cr=cream, D=dun
Tobiano, for those of you that don't know, is a horse color in which the horse is two different colors in large splotches. The generic term for two-colored (or sometimes three-colored) horses is pinto, but tobiano is a more specific term. Each specific pinto term is defined by different genes.
To define a tobiano: any color combinations with white; the patches will be smooth, with white or color changing across the back between the withers and the top of the tail (like the horse below). Tobiano horses also generally have the darker color across their face and chest, like a shield. Tobiano is the most common pinto color; the amount of white will vary. Markings on the face may or may not be present, but will resemble solid-colored horse's markings on the face (star, stripe, blaze, etc.). If crazy markings are on the face, then another different pinto gene may also be present. Usually white below the knees and hocks (hocks are like the knee for the back legs). Tobiano's may or may not have blue eyes, any horse can have blue eyes.

The tobiano gene (or T, or TO) will show up, whether the horse is homozygous or heterozygous. The amount of craziness going on, or the amount of white, usually depends more on the parent from which it inherited the TO gene. For example: a tobiano horse with not very much white and is homozygous, bred with a recessive TO gene (to), will probably result in a foal with minimal white, like the tobiano parent.

Like I said before, any color can be tobiano. Chestnut, black, and palomino; also bay and buckskin. Bay and buckskin tobiano's are tri-colored horses, because their manes and tails are black, while their bodies are colored and white. However, horses with the cream gene (palomino, buckskin, dunskin) may produce foals that carry the TO gene, but it isn't apparent or is very washed out (a cremello horse's pattern wouldn't show). Sometimes the TO gene will block out other modifying genes. Other stranger, but still possible, colors, are roan and dun. The below horse is a roan tobiano.

Foals that are born with tobiano coloring will stay this way their whole lives. The color may vary a little once they shed out their baby fuzz, but the markings will always be the same. The only exception to this would be if they inherited a G gene, in which they would inevitably gray out, and their markings would no longer be visible unless they never grayed out all the way.

Here is Pepper. He follows all the rules for tobiano: white across the back, white below the knees and hocks, a dark head and chest, and a 'normal horse' marking on his face.