Let’s talk about color!

Equine color, to be exact.

I’m a big horse nerd, and I like to understand why things are the way they are, so many years ago I started researching horse colors. I thought we just had bays, blacks, chestnuts, grays, and palominos. I really had no idea the incredible variety of colors and patterns that horses could be. Since I started my journey through the equine spectrum, I have learned a lot about how and why our equine friends’ coats are so varied. I hope I can pass some of that knowledge along to our readers.

First, let’s get some definitions out of the way.

(All definitions are from Google or dictionary.com.)

Gene: a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.
Allele: one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
Genotype: the genetic constitution of an individual organism.
Phenotype: the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.
Heterozygous: having dissimilar pairs of genes for any hereditary characteristic.
Homozygous: having identical pairs of genes for any given hereditary characteristic.
Locus: (plural loci) place on the chromosome a gene occupies.
Dominant: gene that produces the same phenotype in the organism whether or not its allele is identical.
Recessive: gene that is phenotypically expressed in the homozygous state but has its expression masked in the presence of a dominant gene.

When we talk about a horse’s genotype, we will use an upper-case letter to indicate a dominant gene and a lower-case letter to indicate a recessive one.
Now that we have that basic vocabulary, we can really talk about color and understand how and why it happens!

Let’s start with the basic coat colors: black, bay, and chestnut (also called sorrel).
There are two loci that affect the base color of a horse’s coat, and all the other colors spring from there. They are called extension and agouti.
Most mammals produce one of two types of pigmentation. Eumelanin, which produces black color; or pheomelanin, which produces a reddish color. In horses, the locus for this basic coat color is called extension (E, e). The gene for black pigment is dominant over red. So, a horse with the genotype EE or Ee will have a black base, and an ee horse will have a red base. Without any other modifying genes, an Ee or EE horse is black and ee is chestnut.
That brings us to agouti (A, a). It acts in concert with extension and restricts black pigment to the points (legs, mane, tail, and ear tips). This results in a bay horse. Bays have the genotypes EEAA, EaAA, or EaAa. Any horse can carry the dominant agouti gene, but since it only works on black pigment only a horse with a dominant extension gene will show it.

macy1.jpgLovely Macy is a black horse. Her entire body and her points are black. Some black horses fade in the sun and look dark bay, but they will always have black hair around their muzzles and eyes.

atlas2.jpgGorgeous Atlas is a classic bay horse. His body is a shade of reddish brown and his points (including his ear tips) are black.

EmiBay.jpgBeautiful Emi is a bay horse, too. Even though his legs aren’t solid black, he is still considered bay. When a horse looks like this, it is called a “wild bay.”

aster.jpgPretty Aster is another bay horse. She is a very dark bay, but you can see that she has reddish cast to her coat. Some dark bay horses are so dark they look black, except for some reddish-brown hairs around their flanks, eyes, and muzzles. They are sometimes called seal brown or brown.

Beau.jpgHandsome fellow Beau is a chestnut. His body and points are all a similar reddish color. Genetically speaking he is ee.

irisEars.jpgAdorable little Iris, Aster’s filly by Beau, is likely a chestnut as well. Even though her mane and tail are very dark (almost black!), we can see here that her ear tips are red. Thanks to this lovely baby we can tell a little more about Aster’s genotype. Since Aster produced a chestnut filly, she must be heterozygous for black pigment.

Now we know about basic genetics and the three base colors. Next time we will cover the cream and dun dilutions, with other dilutions to follow. Once we have the colors covered, we will move on to gray and the white patterns.

Stay tuned as this color thing becomes extremely complex are this series continues.


Thank you to Liz E. for this guest blog for HOP.