Bay (color)

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For the community Bay Horse and any other uses of the phrase, see Bay horse (disambiguation)

A "blood" bay Chilean stock horse

Bay is a color of the hair coats of horses, characterized by a reddish brown body color with black "points." Black points are the black coloring of the mane, tail, and lower legs. There are usually darker hairs also surrounding the muzzle and tip of the ears. Bay is a favorite color among many horse enthusiasts, and is one of the most common coat colors in many horse breeds.

Bay horses range in color from a light copper red, to a rich red blood bay (the best-known variety of bay horse) to a very dark red or brown called either dark bay, mahogany bay, black-bay, or simply brown. (Though even some breed registries use the term to describe dark bays, Liver chestnuts are also sometimes called "brown" as well, making "brown" an ambiguous term for describing horse coat color)

Image:Holsteiner.jpg

A dark bay Holsteiner, sometimes called "Mahogany bay," "black bay" or "brown." Such horses can be distinguished from black by the presence of lighter hair around the eyes and/or muzzle, as shown in this photo.

[edit] Genetics

For background on colors generally, see equine coat color genetics. For description of other coat colors, see Equine coat color.

Unlike other types of "point" coloring, such as that seen in Siamese cats, the black points characteristic of bay coloring are not produced by a dilution or albinism gene. While equine coat color genetics are complex, a simple explanation of bay color begins with the understanding that "red" color, such as that of the chestnut horse, represented by the "e" gene, and black color, represented by the "E" gene are the two most basic coat colors genes.

A bay horse carries both the Extension gene (E) and a suppression gene known as the agouti gene (A). The extension gene, also sometimes called the "black" gene, adds black color to the coat and the agouti gene limits or suppresses the black coloring to black points, allowing the underlying red coat color to come through.

Simply put, if a horse lacks the agouti (A) gene, it is completely black. If a horse lacks the extension (or black) gene (E), it will be chestnut or "sorrel", but may produce bays when bred to black or bay horses.

If a horse carrying the extension and agouti genes additionally has a heterozygous dilution gene, also called the "cream gene," it will have the buckskin color, which is similar to bay except that red or brown areas are lightened to a yellow or gold shade. If the horse has a homozygous cream gene, it will have a perlino color, where the black points become reddish, and red areas appear nearly white.

Because the extension (E) gene and agouti (A) gene can be either heterozygous or homozygous, the extent to which a bay passes on its color varies widely from one horse to another depending on its genotype and that of its mate:

Ee Aa bay × aa chestnut: 25% bay, 25% black, 50% chestnut
Ee Aa bay × Ee black: 37.5% bay, 37.5% black, 25% chestnut
Ee Aa bay × EE black: 50% bay, 50% black
Ee Aa bay × Ee Aa bay: 56.25% bay, 18.75% black, 25% chestnut
Ee Aa bay × EE Aa bay: 75% bay, 25% black
Ee Aa bay × Ee AA bay: 75% bay, 25% chestnut
Ee Aa bay × EE AA bay: 100% bay
Ee Aa bay × aa chestnut: 25% bay, 25% black, 50% chestnut
Ee Aa bay × aa chestnut 25% bay, 25% black, 50% chestnut
EE Aa bay × Aa chestnut/black: 50% bay, 50% black
EE Aa bay × Aa bay: 75% bay, 25% black
EE Aa bay × AA bay: 100% bay
Ee AA bay × AA chestnut: 50% bay, 50% chestnut
Ee AA bay × Ee bay/black: 75% bay, 25% chestnut
Ee AA bay × EE bay/black: 100% bay
EE AA bay × chestnut/bay/black: 100% bay

In many breeds all of these genotypes are fairly common, making bay breedings unpredictable without a prior blood test for color-related genes.

The following grid shows the two-gene Punnett square for a crossing of two Ee Aa bays, just one of the many possibilities outlined above. Each of the sixteen squares is equally likely. Nine of the results carry both genes (bays), three carry extension but not agouti (blacks), and four do not carry extension (sorrels):