The Extension (Red Factor) and Agouti genes define the Base Coat Color (Bay, Black or Chestnut) of an Arabian horse. A breeder can determine the color odds for a foal, providing these genes are known for the sire and dam.

Definitions
			E = Black Pigment
			e = Red Pigment
			A = Agouti
			a = No Agouti
Rules
			Black (E) is dominant over Red (e).
			Agouti (A) is dominant over Non-Agouti (a).
			The Agouti gene restricts Black pigment to the points (mane, tail, lower legs & ears).
Color
			Bay horses have at least 1 Black gene and at least 1 Agouti gene: EEAA, EeAA, EEAa or EeAa.
			Black horses have at least 1 Black gene and no Agouti gene: EEaa or Eeaa.
			Chestnut horses have 2 Red genes: eeAA, eeAa or eeaa.

	Bay EEAA	Bay EeAA	Bay EEAa	Bay EeAa	Black EEaa	Black Eeaa	Chestnut eeAA	Chestnut eeAa	Chestnut eeaa
Bay EEAA	100% Bay	100% Bay	100% Bay	100% Bay	100% Bay	100% Bay	100% Bay	100% Bay	100% Bay
Bay EeAA	100% Bay	75% Bay 25% Chestnut	100% Bay	75% Bay 25% Chestnut	100% Bay	75% Bay 25% Chestnut	50% Bay 50% Chestnut	50% Bay 50% Chestnut	50% Bay 50% Chestnut
Bay EEAa	100% Bay	100% Bay	75% Bay 25% Black	75% Bay 25% Black	50% Bay 50% Black	50% Bay 50% Black	100% Bay	75% Bay 25% Black	50% Bay 50% Black
Bay EeAa	100% Bay	75% Bay 25% Chestnut	75% Bay 25% Black	56.25% Bay 18.75% Black 25% Chestnut	50% Bay 50% Black	37.5% Bay 37.5% Black 25% Chestnut	50% Bay 50% Chestnut	37.5% Bay 12.5% Black 50% Chestnut	25% Bay 25% Black 50% Chestnut
Black EEaa	100% Bay	100% Bay	50% Bay 50% Black	50% Bay 50% Black	100% Black	100% Black	100% Bay	50% Bay 50% Black	100% Black
Black Eeaa	100% Bay	75% Bay 25% Chestnut	50% Bay 50% Black	37.5% Bay 37.5% Black 25% Chestnut	100% Black	75% Black 25% Chestnut	50% Bay 50% Chestnut	25% Bay 25% Black 50% Chestnut	50% Black 50% Chestnut
Chestnut eeAA	100% Bay	50% Bay 50% Chestnut	100% Bay	50% Bay 50% Chestnut	100% Bay	50% Bay 50% Chestnut	100% Chestnut	100% Chestnut	100% Chestnut
Chestnut eeAa	100% Bay	50% Bay 50% Chestnut	75% Bay 25% Black	37.5% Bay 12.5% Black 50% Chestnut	50% Bay 50% Black	25% Bay 25% Black 50% Chestnut	100% Chestnut	100% Chestnut	100% Chestnut
Chestnut eeaa	100% Bay	50% Bay 50% Chestnut	50% Bay 50% Black	25% Bay 25% Black 50% Chestnut	100% Black	50% Black 50% Chestnut	100% Chestnut	100% Chestnut	100% Chestnut

Besides Bay, Black and Chestnut, the Arabian Horse Association lists Gray and Roan as color options. While both effect the appearance of a horse, neither identify their base coat color.

Gray is a dominant modifier gene which causes the coat to progressively lose pigment. At birth, the base coat color of Bay, Black or Chestnut is apparent and the Red Factor and Agouti rules still apply.

Roan is not a Color, but rather a Pattern of white hair mixed within a base coat color of Bay, Black or Chestnut. It is possible to have a Bay Roan, Black Roan or Chestnut Roan.

ETALON Diagnostics provides DNA testing services for Red Factor, Agouti, Gray and much more.

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