Color Genetics: Vol. 2

Dilutions

Introduction

So far we've learned about how black and agouti work together to create our three base colors: bay, black and chestnut. Now, we're going to learn about how different dilution genes can affect those base colors. The main thing to remember about dilution genes is that they dilute a horse's color, but not always in the same way. For example, some dilutions target black and do not show on chestnut, while other genes are nearly invisible on black bases but drastic on chestnut.

Each gene will receive it's own article that goes over how the gene works as well as providing some examples. We will discuss the combination of multiple dilution genes in the summary, as the more diluted a coat color becomes, the harder it becomes to distinguish.

Black & Chestnut

We briefly went over black in the previous section, but it is worth going into a little more depth here. As you know, we indicate the presence of black in horses using the letter E. We also know that black functions as a simple dominant, which means that horses only need one copy of a dominant (E) allele to express black. Horses that have a black allele generally have dark skin and a uniform dark black coloration over their bodies.

Shades of black can vary widely from individual to individual. Some horses remain a uniform, solid shade of black year-round, while others may have areas that fade to a bronzey, reddish tone with sun exposure. This is most often found on the ends of manes and tails. Dark blue-blacks are rarer than warmer-toned blacks, but can be found. Most black foals are born a light silvery-gray that darkens to their true color during their foal shed.

Black horses are most often mistaken for seal bay or seal brown. The easiest way to differentiate is to check whether there is reddish or brown fur at the flank, muzzle or armpits of a horse. If so, the horse is likely a seal bay, which we will talk about more in a different section.


In the absence of a dominant black allele, a horse will be chestnut based. A chestnut horse (ee) can never pass on a dominant black allele to their foals. Depending on the shade of their coat, chestnut horses may also be called sorrel, red, and liver. Their bodies will generally be a uniform reddish brown shade unless affected by other modifiers. Chestnut horses generally have grayish muzzles, and may have a mane or tail that is slightly lighter or darker than their body. Some individuals also have subtle lightening around the backs of their legs and fetlocks as well.

The Agouti Gene

In horses, the agouti gene directly affects how the black gene is expressed. Agouti will not express without a dominant black allele (E) present, which means that it has no effect on chestnut (ee) horses but can still be passed on. Agouti is a complex gene, but for the purposes of this section, we can think of it as a simple dominant. Later, we will go into the intricacies of what makes this gene a little more complicated. When indicating the presence of a dominant copy of agouti, we use A. A homozygous (AA) agouti horse has no phenotypical, or visual, difference to an individual that is heterozygous (Aa).

The way that agouti works is by restricting the expression of black on a horse's coat. If you think of a bay horse, they have black areas, but they are pushed to the farthest edges of the body, or to their points. Points include the lower leg, mane, tail, muzzle, and sometimes ears of a horse. This has to do with the way that a foal develops in utero, with skin cells pushing outward from specific spots on the body. As you can see in the first image, the black cells migrate from points on their chest, head, and flank and 'push' toward the outermost edges of the horse.

In the second image, you can see an illustration of how this works. Each individual is different, but as you can see the black will generally follow the same path away from those origin spots. The rest of the body will be a reddish brown color that varies as much in shade as chestnuts do. This is why you often hear bay horses referred to by the shade of bay that they are: cherry, dark, golden, red, etc. Bay foals are often born with lighter legs and underbellies that darken as they shed, and adults may have extensive countershading that darkens their topline.

It is helpful to remember that it is impossible to tell whether a horse is homozygous or heterozygous just by looking at them. Because of that, sometimes we use a '-' to indicate that we don't know what the second allele is. For example, if we have a bay horse, we know that they must have at least one dominant black (E) allele and one dominant agouti (A) allele, but we may not know if they have any recessives. We would indicate their color genes as E- A-, with the '-' standing in as a wildcard. For this site, we have the ability to choose or see the actual genes so this does not matter as much, but it's useful to know.

So what are base colors?

So the three colors that we have learned about so far (bay, black, and chestnut) are considered the main "base" colors because they are the foundation of every other horse coloration you can imagine. They fundamentally change the phenotype and name of every other horse color. Because of this, Black and Agouti are always written when denoting a horses genetics, even if the gene wouldn't affect the color that is expressed. For example, if you have a chestnut horse, you would still specify their agouti alleles even though they have no black for the agouti to modify.

Another thing that makes base colors different from other horse colors is that they can be found in almost every horse breed. There are a few exceptions of course. Friesian horses are well known for being black, but the rare chestnut has been known to slip in. Agouti has been bred out of the breed, so Friesian horses cannot be bay. Cleveland Bays on the other hand, have been selectively bred for the bay coloration and do not come in other colors. The Suffolk Punch and Haflinger breeds are chestnut based, although they also have other modifiers affecting their coat colors.