Equine coat color is based on one of two possible base pigments: red or black. The extension gene controls the production of this base pigment (red or black). All of the coat colors we see today, from white to black, sorrel to grey – every single one of them begins with either a red or black base pigment. All horses will have the genes for either black or red pigment, regardless of their physical appearance. There are a number of dilution patterns and modifiers a horse can carry that affects the base pigment of a horse. These modifications cause the wide variety of colorations that are seen in horse coats.
Horses that are bay, black, grullo, buckskin, black/blue roan, or other black pigmented horses carry at least one copy of the black factor (E) allele. The black (E) allele of the extension gene is dominant. This means that only one copy is needed for the horse to have a black base. The E allele causes a black pigmented base in both the heterozygous (E/e) and homozygous (E/E) state. A horse that is heterozygous for Red/Black Factor means that it carries one copy of the black allele (E) and one copy of the red allele (e). A horse that is heterozygous (E/e) for red/black factor can pass on either the red or black pigment to its foals. A homozygous black (E/E) horse means that it carries two copies of the black base (E/E). A homozygous black horse will always produce black-based foals regardless of its mate.
Horses that are chestnut or sorrel, palomino, red dun, red roan, or other red pigmented horses carry two copies of the Red Factor (e) allele. The red (e) allele of the extension gene is recessive and will only cause red pigmentation when the horse carries two copies of this allele (e/e); this is referred to as homozygous red. A red-based foal results when both parents have passed on a copy of the red (e) allele. A red-based horse must be bred to another red-based horse or a heterozygous black horse (E/e) in order for a red foal to be produced. However, if breeding to a heterozygous black horse, there is a 50% chance per foal born that they will inherit the black allele and will be a black-based horse.
|E/E||Homozygous Black: Horse carries two copies of the Black gene and will have a black-based coat.|
|E/e||Heterozygous: Horse is Black based and carries a copy of the Red gene. Horse will appear black-based and can produce red-based horses.|
|e/e||Homozygous Red: Horse carries two copies of the Red gene and will have a red base coat.|