“ I do not need to tell you to curry the Godolphin Arabian,” he smiled with his eyes. “Already his coat is the color of honey when held in a jar against the sunlight.”

Marguerite Henry’s description of Sham, the Godolphin Arabian, stayed in my mind from the moment I read the book ‘King of the Wind’ as a little girl. Bay horses: a combination of fiery red, and coal-black have captured the imagination of people for millennia.

Colour in animals serves many useful purposes such as ‘concealment, communication, and regulation of physiological process’ (Corbin, et al, 2020). Horses were domesticated approximately 5,500 years ago and selective breeding resulted in the vast array of horse types and colours we see today.

All horses receive two alleles from each parent that determine the base colour. An Allele is each contribution by parents- denoted as EE (each parent contributed one E), or Ee (one parent contributed E, the other parent contributed e). These alleles are located at specific locations on a chromosome called a loci/locus.

Base colours are red or black. The horse begins with the MC1R, or Melanocortin 1 Receptor locus. This determines black or red and is designated by E.

EE Black
Ee: Black
ee: Red

Next, the locus that contributes to colour is called the ASIP or Agouti Signaling Protein which determines where the black will appear, either all over, restricted to parts of the body, or only the points (lower legs, mane, tail, tips of ears). There are several subtypes (A, A^t, A+).

In order to control black, the horse must have at least one E gene (EE, Ee), otherwise, the ASIP has no effect, so no black appears with ‘ee’ and the horse is chestnut.

AA: Black restricted to points, red body (Bay)
Aa: Black restricted points, red body (Bay)

A^tA^t: Seal Bay (black is less restricted, resulting in a dark horse with reddish areas on the ‘soft areas: muzzle, over the eyes, elbow, flanks, in front of the stifle, back of buttocks).

A^ta: Seal Bay (black is less restricted, resulting in a dark horse with reddish areas on the ‘soft areas: muzzle, over the eyes, elbow, flanks, in front of the stifle, back of buttocks).

aa: Black all over, it turns off all red when EE or Ee is present (ee only results in chestnut colour).

A+A+: Wild Type Bay (Mane and tail are black, may have some red. Black on points is limited to the fetlocks, or has red hair mixed in, not solid black lower legs)

A+A: Wild Type Bay (Mane and tail are black, may have some red. Black on points is limited to the fetlocks, or has red hair mixed in, not solid black lower legs)

A+a: Wild Type Bay(Mane and tail are black, may have some red. Black on points is limited to the fetlocks, or has red hair mixed in, not solid black lower legs)

*Wild Bay are uncommon to rare

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		MC1R	AGOUTI
EEAA	Bay	Homozygous	Homozygous
EeAA	Bay	Heterozygous	Homozygous
EEAa	Bay	Homozygous	Heterozygous
EaAa	Bay	Heterozygous	Heterozygous
eeAA	Chestnut	Homozygous	Homozygous
eeAa	Chestnut	Homozygous	Heterozygous
eeaa	Chestnut	Homozygous	Homozygous
EEaa	Black	Homozygous	Homozygous
Eeaa	Black	Heterozygous	Homozygous

Other coat colour modifications:

There are links to other coat modifications we posted for the Equine Genetic Series (they appear as live links when you wave your mouse over the word).

SHADE: There is a gene responsible for how light or how dark the colour appears. It is not well understood, but researchers found an independent locus close to the ASIP locus. A different gene called the sooty gene can cause a coat to appear very dark, and dilutions such as the Cream or Dun gene lighten the coat of horses. But ALL horses have a base coat of Red or Black or the absence of both (white).

SOOTY: Some horses have the ‘sooty factor’ which can darken the coat of any horse. The genetics are not well understood (link to sooty).

PANGARÉ: Some horses have pangaré also called mealy where the soft areas (muzzle, over the eyes, elbow, flanks, in front of the stifle, back of buttocks) are light, or pale. (Link to pangaré)

FLAXEN: Commonly seen in chestnut horses, this gene lightens the mane and tail to a pale blonde color. Again, genetics are not well understood. Two bay horses can produce a chestnut with a flaxen mane, so it is possible it is present in bay horses genetically, simply not expressed (visible).

DILUTIONS:

• Cream (Cr)
• Pearl (Prl)
• Dun (D)
• Silver Dapple (Z)
• Mushroom (Mu)
• Champagne (Ch)

NONSYMMETRICAL WHITE PATTERNS

• Tobiano (TO)
• Frame Overo (O or Fr)
• Splash White (SW)
• Sabino (Sa)

SYMMETRICAL WHITE PATTERNS

• Appaloosa (Lp)

ADDITIVE WHITE PATTERNS

• Grey (G)
• Roans (Rn)

Bays with Modified Coats

We hope you enjoyed the Bay Horses. Stay tuned for more exciting posts in the Equine Genetic Series brought to you by Karen McLain, Meredith Hudes-Lowder & Equus ferus Wild Horse Photography

References
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; 11(6):606. https://doi.org/10.3390/genes11060606

Gower, J. (1999). Horse color explained: A breeder’s perspective. North Pomfret, VT: Trafalgar Square.    

Henry, M., & Dennis, W. (2017). King of the wind: The story of the godolphin arabian. Aladdin Paperbacks, an imprint of Simon & Schuster Children’s Publishing, Division.

Jacobs LN, Staiger EA, Albright JD, Brooks SA. The MC1R and ASIP Coat Color Loci May Impact Behavior in the Horse. J Hered. 2016 May;107(3):214-9. doi: 10.1093/jhered/esw007. Epub 2016 Feb 16. Erratum in: J Hered. 2016 Sep;107(5):479. PMID: 26884605; PMCID: PMC4885240.

Kathman, L. (2014). The equine tapestry: An introduction to horse colors and patterns. Charlotte, NC.: Blackberry Lane Press.  

Sponenberg, D. P. (1996). Equine color genetics. Ames: Iowa State University Press.