by: Tina Lewis of the Lewis Stallion Station
They say that you can’t ride color but we all seem to admire it, and even try to duplicate it, anyway. If you know you absolutely don’t want a particular color of horse (or if you know that you positively do), breeding to a stallion, or owning a mare, that is homozygous, may be the way to go. Homozygous simply means that the horse carries two copies of a particular color gene or color modifying gene, instead of just one of each (heterozygous). Here is my take on color, in a nutshell.
There are only two basic colors of a horse, red and black. Cream, gray, roan, agouti and dun, etc., simply modify the red and the black gene to produce other colors. These modifiers can be heterozygous or homozygous, depending upon whether or not the horse carries one or two copies of the particular modifying gene. For instance, the sorrel or chestnut horse is a red horse and always carries two copies of the red gene, making it homozygous for red every time. This is something that is visual (a given) and does not need to be tested for individually. The palomino is a sorrel or chestnut horse (with two copies of the red gene, and homozygous for red), with one copy of the cream gene and a cremello colored horse is simply a sorrel or chestnut horse (homozygous for red, remember) carrying two copies of the cream gene (called a double dilute) so in addition to being homozygous for red, it is also homozygous for cream. This means that when breeding, it will always throw one of each, a red, and a cream gene, to it’s offspring. The perlino is a bay horse that carries two copies of the cream gene and a smokey cream is a black horse that carries two copies of the cream gene. When a horse carries two copies of a color or color modifying gene, he/she is homozygous for it, meaning that when it reproduces it has no choice but to throw it to the foal, making it easier to figure out what color genes the foal will get, thereby making it easier to tell what color the foal will be when it is born.
A bay, brown or black horse usually carry one of each of the color genes (one red and one black), but can, occasionally, carry two black genes, making it homozygous for black (remember, if it carried two red genes it would be a sorrel or chestnut and would be homozygous for red). This is where having your horse tested for the red/black gene comes in very handy and there is just one test that tests for both colors. If a horse is homozygous for the black gene it will always throw a black gene to it’s offspring, making a sorrel or chestnut foal impossible since it cannot throw one of the two red genes necessary for making a red horse. Remember, just because a horse is homozygous for black and is black does not mean you will always get a black foal, it just means you can never get a red based foal. This is where color modifiers come into play.
The agouti gene is the gene that takes the black gene in a horse (only if there is a black gene present) and restricts it to points (muzzle, ears, mane, tail, legs) making a bay horse in the absence of a cream gene (neither parent threw a cream gene to the foal), a buckskin with one cream gene (thrown in by one parent) and a perlino with two cream genes (one from each parent). A sorrel or chestnut horse can carry an agouti gene but since there is no black to restrict to points, it would not be able to express itself and therefore be a hidden gene that can still be passed to offspring. Agouti is one of the few color modifiers that can be there and not express itself and then be passed on to offspring (skip a generation). It is important, when trying to get a black foal by breeding to a homozygous black horse, that any red horse involved be tested for this hidden agouti gene. It is not necessary to test a bay horse for agouti unless you want to know if he/she is homozygous for it (carries two agouti genes), as a bay horse’s agouti is apparent in the black points (another given). It is also not necessary to test a black horse for agouti as the black would be restricted to points if he/she carried the agouti gene (once again, a given). If an agouti gene is thrown to the foal along with the black gene, the black will be restricted to points and you will get a bay foal instead of a black one. If you desire a bay foal this information is of equal importance.
Dun is a modifier that is characterized by a dorsal stripe, leg barring, webbing or mottling of dark color on the withers and the forehead, as well as a coat color that is a shade lighter. The dun gene in a sorrel or chestnut makes a red dun, in a bay horse it makes a zebra dun (some call a bay dun) and in a black horse it makes a grullo (sometimes called grulla) which is pronounced “grew yo” and is a true mouse color (each hair is the color of a mouse and not white hair mixed with darker hair in order to give the illusion of mouse color). If either sire or dam throws one cream gene to each of the above dun color combinations we would get a dunalino (palomino dun), dunskin (buckskin dun) and a smokey grullo (grullo with a cream gene). The dun gene can also be homozygous and there is now a test so you can tell if your horse carries two of these dun genes.
Roan in a horse is basically a modifier that adds white hairs, scattered, intermittently, throughout the coat color, making a sorrel or chestnut horse a red roan, a bay horse a bay roan and a black horse a blue roan. This modifying gene can be homozygous as well (a horse can carry two roan genes), and though it was once thought that a homozygous roan would die in-utero, and, therefore, did not exist, this is now known to be false. There is a test for homozygosity in roans which puts to bed, once and for all, that old wives tale and many horses are now known to be homozygous for the roan gene.
The gray gene is a modifier of color in horses similar to graying in humans and comes with age. Some horses (as do people), just gray earlier than others, which is why some gray horses turn white at an earlier age than others. It is a gene though, and a horse can be homozygous for it (carry two of the genes) and must throw it to it’s offspring in order for the offspring to turn gray. It is quite possible for a horse to be sired or born from a gray parent and never turn gray. Gray is in addition to the base color of the horse and the base color is usually just as apparent as a youngster as any colored horse would be but then almost non-existent in older gray horses as they usually, eventually, turn all white. In the absence of a photograph of a gray horse as a young horse, DNA testing for the black/red gene is recommended in order to determine what base color he/she will throw to the offspring in addition to possibly throwing the gray gene. You must have one gray parent in order to get a gray foal as the gene is always expressed if it is present. A horse having a gray sire or dam will not give them the ability to produce a gray foal as they must be gray in order to throw a gray gene. Gray and roan, in years past, were often confused and sometimes thought to be the same color, making some registries inaccurate with regard to their records and the color of the horses within those records.
Tobiano and Overo are an example of coat patterns that create a paint or pinto horse and are not coat colors. They are in addition to the base color of a horse and are possible in all the above discussed coat colors with all the same rules. The Tobiano gene is the only one of the two paint patterns above (though there are other coat patterns) that can be homozygous (a horse can carry two Tobiano genes) to ensure that you get a paint or pinto colored foal when breeding and there is a test for the Tobiano gene. The Tovero, which is a combination of the two coat patterns, Tobiano and Overo, can also be homozygous for Tobiano and a horse can carry two of the Tobiano genes and carry the Overo gene as well.
Understanding the basics of color genetics and how colors and color modifiers work with regard to breeding and passing them to offspring can make the search for the right stallion a lot of fun but can also be challenging, and many times, still quite a gamble when the color of your foal is important to you. Some mare owners don’t put as much importance on color but a little research in this area, just as in the area of pedigree, conformation and disposition, can go a long way with regard to creating the horse you’ve always dreamed of. Once you’ve found and booked to the stallion of your choice it’s time to consider preparing your mare for a successful breeding experience so that you will ultimately end up with a healthy, beautiful foal. A healthy and breeding sound mare will put you well on your way.







