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


Several labs provide Red Factor and Agouti testing services:

Animal Genetics
ETALON Diagnostics
UCDavis / Veterinary Genetics Laboratory
University of Kentucky / Gluck Equine Research Center


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.


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