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.
