I have long been fascinated by the varied range of color and beautiful tones created by the magic of genes as they set in to play on a horse's hair coat. Even a genetically plain red horse (one with no color modifying genes) come in such a wide range of colors that names like cherry sorrel, copper penny sorrel, chestnut, liver chestnut, have been created to try to describe the many shades of red. For so many variations to occur in a horse that is the simplest of colors (genetically) it is no wonder that so many of us have trouble understanding how horse color genetics work. I have trouble remembering even the basics and I actually spent some time studying the subject. Rather than try to memorize the correct jargon and true genetic make up of the various colors I instead try to remember in the way I did back in . You can find a link to that post here.
I think of a base color as the primer on a wall you are about to paint. I like to use bay, red and black. So I would paint a primer of one of these three colors on my wall:
Red- which would include any color of sorrel, liver or chestnut.
Bay- which would include any horse with black points and brown on its body (however minimal) including blood bay, dark bay, and brown.
Black would be simply that- any horse that is black with no brown anywhere on its body.
So now I have my primer base. Some horses, the majority actually, don't get another layer of paint. They stay in their primer color. It isn't until you try to calculate what color one of those "primer only" horses will produce that you have to pay any attention at all to how bay, red and black horses vary genetically, but we will leave for today. For now let's pretend that we are in a room and I've painted one wall with a primer of red, one with a primer of black and one with a primer of bay. In the center of the room there are a number of gallons of paint with labels on them that read "cream", "dun", and "roan". I have other cans to chose from like "champagne" and "silver dapple" (the less common color modifying genes) but I am sticking with the more common colors for now.
So I stand in front of my red primered wall and pick up the paint can labeled "cream". I paint cream on top of that red and just like that my wall has turned palomino!
I turn and paint my bay primered with that same can of cream and just like that my wall has turned buckskin!
Finally I turn to the black wall and paint it with that same can of cream. I stand back and see that the cream barely showed up (but it's there) and my wall is now smokey black. That can of cream has a warning label on it that says I have to be careful to only paint one layer of it because if I paint two it will completely cover up my red primer. So if I were to paint my red wall with an extra coat of cream then I will end up with a cremello. If I paint my black or bay wall with two coats I will come up with a perlino. This double coat of paint is really what happens when horse is "homozygous" for a cream gene. Which in the ever day world means that the horses look funky with blue eyes and pink skin and no matter what you breed them to they will always have a baby with a coat of cream on top (palomino, buckskin or smokey black).
Now had I picked up that can of "dun" (instead of cream) I would have ended up turning my red wall in to a red dun, my bay wall in to a dun and my black wall in to grulla. That dun paint is really cool because it always comes with something called "dun factor" which almost always gives us a stripe down their back but this often confuses people because non-dun horse can also have something called "countershading" which looks just like dun stripe and to make it even more confusing sometimes a dun horse can have such a dark body or such a faint stripe that you cant see it at all. Sheesh! Like, get your shit together dun! But most of the time, like almost all of the time, a dun horse will have a distinct stripe down it's back (among other dun factor characteristics.) But enough of that for now! I have painting to do!
Had I picked up "roan" instead of "dun" or "cream" I would have turned my red wall in to a red roan, my bay wall in to a bay roan and my black wall in to a blue roan.
But here is the cool part. Let's say, after I painted all my walls with that "cream" bucket I decided I'd just ahead and pick up the "dun" bucket and paint those palomino, buckskin and smokey black walls with "dun" paint too! I'd end up with a dunalino (dun and cream gene) on my red wall, a dunskin (dun and cream) on my bay wall and a smokey grulla (dun and cream) on my black wall! And then I went totally crazy! I picked up that can of "roan" and threw that on there too! I could have a roan dunalino (red wall), a roan dunskin (bay wall) and a smokey grulla roan (black wall). And it doesn't stop there! There are other cans of paint hanging around. I could throw in some champagne or silver dapple or pearl even. I could end up with all sorts of combinations... but underneath any one of those wild and wacky colors I would still have a base primer of red, black or bay. Sometimes it can be really hard to tell which combination of paint cans created which color, only genetic testing can figure that out.
But wait! I forgot! There are a few more cans of paint stacked by the wall and I hear they do some cool things! Like one called "gray". Gray is more like stucco than paint except it always has to go on top. That wall could have cream, roan, dun and champagne all on top of that red primer but you stick some gray stucco on top of it all after that wall has been painted and it will keep getting thicker and thicker until finally you cant see any of that original color. I actually owned a horse who had a red primer, then a cream layer (palomino), then a champagne layer and finally gray stucco on top. She was the color of oatmeal.
There are some other cans hanging around too. Like "tobiano" and "overo" these guys are also like stucco. They go white (solid white from birth) but they don't cover up the whole wall, only patches. The patches that show through can also be any kind of color like dunalino, grulla or plain old red sorrel.
See, horse color genetics can be easy when you think of it like this!?!?....
The last part to all of this is how it works when you breed on color of horse to another. People really like to think that their plain old red mare has a "recessive" gene for palomino. There is no such thing. When a horse has a gene for cream it must express itself. Must. Always. Whenever I try to explain that the rules of science made that miracle palomino foal completely utterly impossible people still want to insist that they know someone who knows someone who had a horse that was bay that they bred to a horse who was plain sorrel and that horse had a palomino. Ah ha! But you see there is an reasonable explanation besides that some people are stupid (did I say that out loud!?) and other people like to cheat (so they can get more money for their bay by calling it a dun) but mostly because horses LIE!! It's true! That bay looking mare that spontaneously produced that palomino out of a sorrel stallion was lying about her color! She only looked bay. She was really a dark sooty buckskin. Buckskin and dun can be sneaky little buggers. Palomino is not nearly as capable of going incognito but they can still fudge a little.
Just because there are rules to the genetics of horse color, that doesn't make them any less fun. Sometimes the painter of that room has too much wine and goes hog wild on a Saturday night and ends up throwing so many layers of paint on that wall that only the blood work can tell us what the hell happened. As horse people we like to make our own rules and come up with our "colors" and while it drives me batty to see horses registered as a color that they are not (AQHA apparently hasn't read even the basics of color genetics and still allow two sorrel horses to produce a "bay" (genetically impossible) it all really doesn't matter in the end... afterall, a rose by any other name...
Here is a link to a website with a "color calculator" which can tell you what color can be produced from a breeding.
Also, UC Davis provides DNA color testing, find a link here.