Welcome! Since coat colors have always been hotly debated here I’m going to try and smooth out any misconceptions that seem to be floating around about colors and genes. I’m going to start with the basics, and work my way up to the more difficult dilutions and patterns. I will also provide proper examples of each color, and pattern to demonstrate what each looks like (please note that I will not
cover every shade of every color. Therefore, colors may deviate from posted examples). Hopefully you’ll learn something from my ‘essay’, and hopefully I’ll be able to knock out some of those popular misconceptions.
That hardest part about horse genetics is learning that there are only two
base colors. Once you get this down everything else will be easy. Ever hear the phrase “All roads lead to Rome.”? Well in horses “All roads lead to Extension.” Fun Fact: The two base colors Black & Red caused by eumelanin & phaeomelanin.
The first gene in horse colors is “Extension.” This gene is named because when dominant (E) it ‘extends’ the black pigment (eumelanin), and when recessive (e) blocks it causing only red/chestnut pigment (phaeomelanin) to show. (E) Causes black, and (e) inhibits it. A horse with (EE) will never have chestnut offspring. Black – EE, Ee
The horse is black all over. Blacks often fade or bleach when exposed to the sun. Sometimes they may look bay or brown when the sun fading occurs. The fading will go away once the current coat sheds out for the next season. Fun Fact: There is a theory out there that, while genetically the same, fading blacks and non-fading black are indeed different. It is still unexplained, though, as to why some black may fade in the sun, and some will still stay dark as night. Chestnut (Sorrel) – ee
The horse is any shade of red from the brightest orange-red to a near black brownish-red. The points (the legs, mane, tail and tips of the ears) can be lighter, darker, or the same color as the body. A chestnut will never have black points. Chestnuts, also, often express a modifier known as pangaré or mealing, but we will cover this modifier later. Fun Fact: The Friesian breed of horse is bred to be black with no markings, but because chestnut and black are on the same locus (Extension) it is possible, though very rare, to obtain a chestnut Friesian if both parents are (Ee). This color is undesired according to breed standard, and to prevent this all approved stallions are now genetic tested for (E). A chestnut Friesian out of registered parents can be registered, but any future offspring of the chestnut horse cannot be. Chestnut in the Friesian book is also called Fox. To date there are only 4 recorded Fox Friesians. (I could be wrong as the last update on that figure was 2004.)
Agouti is a modifier that restricts (E) to showing only at certain points of the body (ex. Mane, Tail, Legs, Muzzle). (A), dominant, will always produce a Bay when paired with (E), and (a), recessive, will always produce black when paired with (E).
It is also theorized that the Agouti gene has some dictation in the shades of chestnut, but no testing has been done to prove this. Since we believe that Agouti is a black modifier, and there is no black in a red (ee) horse we can assume that this theory is false unless research proves otherwise. Some shades of bay often express a black overlay of hairs which is called Sooty, but we will cover that modifier later. Bay – AA, AAt,Aa
The horse is any shade of red/orange from the lightest orange to a near black brownish-red. The points (the legs, mane, tail and tips of the ears) are always black.
Bay (A) is dominant over Brown (At). Brown – AtAt, Ata
He horse is usually a black horse with tan highlights in areas ( the muzzle, flanks, underbelly, and girth). The darkest shades of seal brown may look black but with tan at the muzzle and flank. The lighter browns may look like a tan horse with an overlay of black covering a good deal of the body, and the very lightest shades may look like some shades of bay. Fun Fact: At one point a theory was stated that brown was the effect of pangaré on a solid black base. This theory was disproved when the test for (a) was developed. No horse of seal brown color that had been tested was (aa), so brown could not be caused by pangaré on black. Wild Bay – A+ (This allele is still only In Theory)
The horse is usually light bay that looks much lighter on the underbelly, but they could get so light they almost look crème with black points. The black on a wild bays legs is much less extensive usually barely surpassing the ankle. An original theory of this color what that the Przewalski's Horse was an example of this gene. We know now that the Przewalski's Horse is actually a dun. Theory still holds though that they couple possibly be wild bay ( A+) plus dun. Fjord’s share similar colorings to the Przewalski's Horse and it is theorized that they to carry wild bay (A+) plus dun dilute . Some also believe that this was also once the most common color in all horses. However, through the selective breeding it took to create today’s breeds the gene has been almost completely bred out of modern horse breeds.
Wild Bay (A+) is thought to be dominant over all other alleles on the Agouti locus. Note: Keep in mind the coloring of a wild bay when I take you though the other modifiers. Do you think that maybe instead of being another allele on the bay locus that this could be a reaction between bay and pangaré? Overview
We learned here that there are only two base colors in horses: Red and Black. We also learned that one gene can display a variety of different shades, but still represent the game genotype. For example: Say we have two horses a Liver chestnut, and a Chestnut with a flaxen. If you were too genetically test then both they would both be (ee). We also learned that a horse homozygous for black (EE) will never produce a chestnut offspring no matter what color its mate might be.
We learned about the agouti locus, and how it can turn a black horse (EE or Ee) into a Bay (E_, A_). The agouti locus also controls the Brown color in horses with a separate allele (At). Remember though Bay (A) is dominant over Brown (At); so even if your horse carries Brown (At) if he does not test positive for (a) he will be a bay (AAt) no matter what.
This is the end of Lesson 1. I was considering a pop quiz, would you guys participate if I posted one? Did any of you really learn anything yet? Please give me input on what you think, though most of this for the color veterans is probably repetitive regurgitation of every other color explanation in the world.
I will try to eventually cover all horse colors including those still under research and those still unexplained like Hanoverian Creme, and Mushroom. -Disclaimer & Credits-
I am by no means a geneticist. I just enjoy horses, and their wide variety of colors. Click here for the lines used for the color demonstrations.