As we have previously discussed, horses have two basic color genes, black (B) and red (b). Bay horses happen because a special modifier gene called the agouti gene makes the black coat color fade into brown, leaving the points black. But this only covers the more basic colors. What about the others?

Each color is created by some form of modifying gene. These genes can be layered on top of each other, so that you can wind up with a buckskin horse that has pinto markings that fade to gray. To keep things simple we will look at several of the most common color modifiers one at a time.

A color modifier that is especially popular is the cream gene. This is the gene that causes horses to be buckskins or palominos. The cream gene is a dominant gene. Not only is it dominant, but it also expresses itself differently according to how many copies there are of the gene in a horse.

Let's start with a chestnut horse. Genetically the horse is (bb), which give it the characteristic red color. Now let's add a single copy of the cream gene. Now the horse is (bb Cc) which means that it is a diluted red color which is palomino. If you add a second copy of the cream gene (bb CC) the dilution is stronger and the horse is a cremello with pink skin and blue eyes.

When you add the cream gene to a bay horse you wind up with a buckskin. With a second copy of the cream gene the horse is a double dilute called a perlino. Blacks are affected a bit differently. A single copy of the cream gene makes a smoky black which visually shows very little difference to a regular black. However, when the cream gene is homozygous it expresses itself full strength and produces a creamy black.

Color breeders particularly covet the cremello or perlino horse for their breeding programs. This is because no matter what color of horse you breed to, the foal is guaranteed to express some form of the cream gene. This is why you will often see cremello stallions advertised with a 100% color guarantee.

Other genes that dilute the color of the coat include the champagne gene, the dun gene and the silver gene. A horse with the champagne gene can be "gold" (on chestnut), "amber" (on bay) or "classic" (on black). The dun gene causes the black stripe you sometimes see on horses' backs. Silver horses have light colored manes and tails caused by a dilution of the expression of the black color, but not of the red/brown shades.

Another color modifier that is frequently seen is the roan gene. Like the cream gene, the roan gene is dominant. If a horse is not roan it cannot have a roan foal. A true roan will have fine white hairs throughout their coat with the exclusion of their face and legs. While they are often mistaken for grays, especially in the case of blue roans (roan on black) they do not fade the way a gray will over time.

An interesting fact about roans is that there is no such thing as a homozygous roan. This was first discovered when the conception rates of breedings where two roan horses were crossed together were decidedly lower than that of other horses. In fact there were 25% fewer live foals from these breedings. From this they were able to deduce that when an embryo carries two copies of the roan gene the combination is lethal and the embryo dies. If you take a look at the following chart, where (R) signifies the roan gene and (r) signifies no roan gene, you will see why the ratio of lethal foals was 25%.

(R) (r)
(R) (RR)
No foal (Rr)
roan
(r) (rR)
roan (rr)
plain

Now that we have been able to isolate the roan gene this has been proven in DNA studies.

Possibly the base of color breeders is the gray gene. This highly dominant gene will wash out any effects seen from other color modifiers. A horse with the gray gene is often born without any sign of graying. As it ages the gray begins to show. Some horses will grey following a "fleabitten" pattern while others have a "dapple" pattern. The two genes are different, but both have the same overall result. Eventually a gray horse will loose all color from their coats and will be white in color. Because they are born with dark skin these horses are not true whites, instead they are called grays.

There are many other color modifying genes. Some have been mapped out in DNA studies. Others are still a mystery. By studying the traits of each color gene you can have a reasonable way to guess what your foal may turn out like once it has arrived. Certainly there are still many aspects that will be unknown, but at least you can have a reasonably good idea of what color your baby could be.