Color Combinations

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Homozygous Colors     Color Combinations     Exceptions

Have you ever seen a horse that appeared to be a combination of two different colors?  Or a horse that had 100% colored foals?  How about a stallion who threw foals of a color that you would never have guessed, based upon how he looked (phenotype) and the colors of the mares he bred? These horses might be examples of horses that carry more than one color-modifying gene.  This might be happening in one or both of two ways:

Heated arguments have happened over horse color.  Many a long-time breeder has made the mistake of calling a sooty buckskin a grullo, or a graying horse a roan.  Knowing what color a horse really is can help you look like you know what you're doing (instead of....well...looking inept), and can also help people (who have unusually-colored breeding stock) understand what to expect in their foals' colors.

It would take a couple chapters of a genetics book to describe all the combinations and the why's of them, but  here's a shortened summary of what might go on with those unusually-colored horses.

Homozygous colors

Horses all are variations of a very few base colors.  Some say all horses are a variation of sorrel or black, while others include bay in those base colors.  To avoid going into more genetics here, I'll include 3 base colors from which all other colors evolve:  sorrel, bay, and black.

A horse that is homozygous for a color means that the horse carries a matched pair of alleles for a certain color, and that all of its foals will receive one of those traits.  Many of those traits can be seen visually.  For example...horses that are homozygous for black will have 100% black-legged offspring.*  A homozygous roan will have 100% roan foals.*  A horse that is homozygous for the creme gene will have nearly all buckskin and palomino foals, and maybe a few smoky blacks.*

In order to be homozygous for a trait, BOTH parents of the horse must have had that same trait.

That means that a homozygous roan must have two roan parents.  A homozygous gray must have two gray parents.  A homozygous black must have both parents with black legs.*  Registration papers can be wrong, but the parents themselves must both carry the same trait to have a foal that is homozygous for the trait.

Why is this important?  It isn't, for most horses.  It is mainly important to those horses that are of breeding quality and who are therefore used for breeding.   Knowing a horse's homozygous status for color-affecting genes just allows you to have some predictability of offspring colors.  Being homozygous for a trait does NOT constitute breeding quality!  But it does help you predict and determine foal colors.

To learn more about what homozygous means, click here.

This table sums up what you might expect from horses that are homozygous for these traits:

Examples of Homozygous Horses:

*Exceptions

Other genes can cover up or alter visual expression in horses that carry certain color genes.  For example, a perlino or smoky creme foal can carry a black gene (or even be homozygous for black) but will still have white/cream legs.  A gray gene will change black legs to gray or white over the years, though the horse still carries the black gene if it received it from a parent.  Combinations with other colors may also change what you see.

To learn more about what homozygous means, click here.

Color Combinations (aka "Composite Colors")

There isn't any genetic law that says that you can't have a horse with multiple color-altering genes.  All horses' genotype (genetic) color stems from their base color (sorrel, bay, or black).  But the phenotype (how it looks visually) can be modified by one or more color-modifying genes.  When horses receive more than one modifying gene, it can really get difficult--even emotional--when trying to determine its color.

Think of it as adding food coloring to water.  If you drop blue food coloring into water, what color will the water turn?  Blue!  But what if you add yellow color to the blue water?  There's no law against adding a second modifying color to the blue water, so do it!  It then changes to green.  You can keep on adding colors, and the water will keep changing for a couple more turns.

Horse colors are similar.  If you add a dun gene to a bay, you will get a dun horse.  But if you add a gray gene to that dun, you will get a "dun turning gray."  (This horse should properly be registered as a gray.)  What if you added roan to that dun instead of gray?  You would have a dun roan.  AQHA would register this foal as a dun, though in the summer, its roan coloring would be easily seen.  Some color combinations you might have seen and wondered about include:

Examples:

There are many, many more combinations of colors.  Some are obvious, and some are not (for example, a grullo, black, or blue roan might also carry the creme gene, but you can't tell unless you test it or learn via its offspring).  But the next time you see an oddly-colored horse, try to pick apart its characteristics and see if you can figure out what modification(s) have been applied to that base coat!

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