Bay

Appearance

Bay horses have a reddish coat with black points. Splashed white patterns tend to express more loudly on black/bay-based horses.

Foals		Bay foals have a reddish coat with black points, though their legs often appear lighter or greyish in color. The soft parts usually have a paler shade and the mane and tail can have lighter hairs in them as a result of foal camo.

Bay horses have black points on their legs that go up to their knees and hocks. They have a reddish coat, and their shade can vary from a very light copper bay to blood bay.		Adults

Bay shade variations

Bay horses come in many different shades. Two commonly used terms for these bay variations are seal brown and wild (low-point) bay. The exact genetics behind these are still unknown and often debated.

Seal Brown		Seal brown is a dark variation of bay. Horses can sometimes appear nearly "black", but the soft parts are still red colored. Seal brown foals are born with their darker color.

Wild or low-point bay can be recognised by the low black points (up to the pasterns or fetlocks) and usually a lighter, more yellow shade of bay. The knees and hocks may have some dark/black smudges on them. Since the points of a foal can take years to fill in, wild bay can't be seen on foals or young horses (~ <5 years).		Wild bay

Mimics

Other colours can look similar to bay.

Genetics

The base colour bay is controlled by the extension gene and the agouti gene.

The extension gene controls the production of red and black pigment. Horses need at least one copy of the dominant E-allele (E/E or E/e) to be able to produce black pigment.

The agouti gene controls the distribution of black pigment. The dominant A-allele causes black pigment to be restricted to the points of the horse. As a result, horses need at least one copy of the a-allele (A/a or A/A) to be bay.

Genetics of Bay variations

For some time, seal brown was believed to be caused by the A^t allele of agouti (dominance: A > At > a). A DNA test existed, but no scientific papers were published, the results were inconclusive, and the test was withdrawn. This theory is no longer supported.

Similarly, wild bay (or low-point bay) was theorised to be caused by the A⁺ allele (dominance: A⁺ > A > a). This idea has also been discarded.

You can still find these older theories and shade names used in many places, which is why I like to mention them here. It is also how bay genetics work in our game, Horse Reality. At the time of development, the A^t and A⁺ theories were still alive. Even so, for any color we don't know the genetics of yet, we have to come up with something to make it work in-game. Which means working with just observations and theories.

Currently, we have some theories as to what actually causes the different shades of bay. One study¹ found that E/E A/a bays tend to be the darkest and are typically seal brown in color. Another study² confirmed that E/E bays tend to be darker. They also found a DNA region, close to the agouti gene, that possibly controls bay shade. That's all we know for now, but it seems like the extension gene acts "incomplete dominant" in some way. We'll have to wait for more research to come out about the shade-region.

Presence in horses

Bay dun (E/E A/A) is considered the wild type or "original" horse color. All other coat colors are the result of mutations. Unsurprisingly, bay-based horses are widespread and common in many horse breeds.

Read more:
Chestnut | Black | Buckskin | Perlino
Extension | Agouti

Articles

1. Corbin LJ, Pope J, Sanson J, Antczak DF, Miller D, Sadeghi R, Brooks SA.; An Independent Locus Upstream of ASIP Controls Variation in the Shade of the Bay Coat Colour in Horses; Genes (2020); Doi: 10.3390/genes11060606
2. Druml T, Grilz-Seger G, Horna M, Brem G.; Discriminant analysis of colour measurements reveals allele dosage effect of ASIP/MC1R in bay horses; Czech J. Anim. Sci. (2018); Doi: 10.17221/105/2017-CJAS
3. Rieder, S., Taourit, S., Mariat, D., Langlois, B., Guerin, G.; Mutations in the agouti (ASIP), the extension (MC1R), and the brown (TYRP1) loci and their association to coat color phenotypes in horses (Equus caballus); Mammalian Genome (2001) Doi: 10.1007/s003350020017

Seal brown references

• PetDNA Services of Arizona - Understanding Equine DNA and Agouti - Archived copy
• PetDNA Services of Arizona - At testing - Archived copy

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