Let's talk basics. There is a great deal of information to absorb when it comes to equine genetics, let alone the absolute basics. So we thought we'd address those in a very detailed way, in hopes that this can be a quick resource of sorts.
If you haven’t read our article on heterozygous or homozygous traits yet, have a look at that article as well, as it will help you in further understanding some of the commentary you'll find in this article.
At the very core, all horses start out as red or black. On top of this pair of genes we can stack any number of genetic markers that create the phenotype1, or observable characteristics, of the horse's genetics, or genotype2.
There is some confusion even when it comes to what is true black and what is "dark brown," or what is sorrel and what is a chestnut? The fact of the matter is that these names are simply descriptors of the phenotype, but may have little bearing on the genotype. A sorrel horse is a red, the same as a chestnut. There is no genetic difference in these two colours at all, and there are many slight differences between each red horse individually. They can be a deep, almost chocolate liver chestnut all the way to copper. However, all of those descriptors are genetically very much the same base. The same goes for dark brown, bay, blood bay, and black-bay. These are all genetically "bay" horses, and very different from a true black. Some very dark brown horses even appear black, apart from a few observable differences that tip us off to their genotype.
How does it work?
Black
Black horses must have at least one black gene (E/E or E/e), and cannot have any Agouti present (A). Therefore, every true black horse will be one of the two combinations: (E/E, a/a) or (E/e, a/a).
Why is this the case? Namely, because if the agouti gene was present (A), it would push the black pigment to the points of the horse (legs, ears, muzzle, mane, and tail) and our horse would then become a bay. With no agouti present (a), the black maintains its position covering the entire body. Note that at least one of the gene pairs for E or e must be E (black), or the horse would be red.
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Red (Sorrel, Liver Chestnut, Chestnut)
Similarly, the red horse cannot have any black (E) present at all, and because Agouti is only dominant over Black, it doesn't matter the variable on that gene pair (A/A, A/a, or a/a). Therefore, all red horses will be (e/e, A/A), (e/e, A/a), or (e/e, a/a).
Why is this the case? The red horse lacks zero black (E) pigment and therefore wipes the slate clean. This means that agouti markers mean essentially nothing, because that gene pair works only on black pigment.
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Bay (Brown, Dark Brown, Blood Bay, etc.)
With the two base colours detailed above, perhaps now it's a bit easier to understand what we have left over, and how that makes a bay horse. The bay horse must have at least one black gene (E) and it must also have at least one agouti gene (A). Furthermore, stallions who are bay can be homozygous black (E/E), without being observably black, due in part to the fact that they can also be agouti carriers (A), forcing the black pigment to their points. All this to say, bay horses can be any of the following (E/E, A/A), (E/e, A/A), (E/E, A/a), or (E/e, A/a).
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What about brown, sorrel, and liver chestnut?
Much like "blue roan" and "grullo" these are simply descriptor words that we have used over time to help identify horses of certain shades between one another. A brown horse is actually a true bay genetically, but from the horse's appearance alone (which for a great many years that was all we had to go by), these are excellent terms. Grullo is a great descriptor for a black dun and does actually have genetic significance in that we know from one word that the horse is a true black + dun factor. Blue roan is also genetically relevant in that it perfectly describes the lovely shade of black roan, and distinguishes itself from red roan or bay roan. Sorrel is a terrific way to describe a copper red horse. Liver chestnut is great for those deep, rich chestnut horses. Are the two latter descriptors genetically relevant? Not really.
Horses of a not-so-different colour.
Genotype: Bay
Genotype: Chestnut/Red
Genotype: Bay
Struggling to decode genetics? Reach out or comment on the post on Facebook and we’ll be happy to answer any questions and discuss with you there!
Definitions
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Phenotype: refers to an observable trait, something that you can see with the naked-eye. You can "see" that a horse is palomino, chestnut, or bay.
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Genotype: the genetic makeup of a horse, the complete set of genes present. The genotype is not always observable to the naked eye, such as a horse who carries certain diseases, or a horse who is homozygous roan (there is no visual marker to distinguish between heterozygous or homozygous roan).
Natashya V.
Founder, StallionCompare.com | Creative Director
Natashya, the founder of StallionCompare.com, is a dynamic force in both the equine and creative industries. As a lifelong equestrian, Natashya’s journey with horses began with Quarter Horses in her hometown in British Columbia, Canada. During her college years she took a brief (and eye opening) hiatus from Quarter Horses to work in the Thoroughbred industry – returning to the AQHA circuit as a young adult. Natashya was also able to reconnect with the quarter horse industry through design. She worked with former owners of Pleasurehorse.com and Show Horse Today on their design needs prior to Robyn Duplisea purchasing the business. Natashya also worked at Horse Canada (HPG) as the Creative Director for many monthly, quarterly, and annual equine magazines. Her passion (read: obsession) for breeding, genetics, and equine pedigrees ran ever-increasingly deep, which lead her to create StallionCompare.com.
Beyond her equine pursuits, Natashya is the Creative Director & Co-Founder of a leading design agency known for its boutique, customer-focused approach. With her expertise, she blends creativity and technology to make a lasting impact in both horse breeding and design.
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