gray horses

Gray horses have an admixture of colored and white hairs over a dark skin. Graying is a process that occurs with ageing so that gray horses are born some other color and eventually look almost white (though they are not genetically white horses, which are actually very rare). The graying gene causes a more or less gradual change in the underlying coat color.

Sometimes people describe gray as being, for example, rose gray, steel gray or dapple gray. Although these may represent accurate descriptions at the time such horses are at a particular stage of the graying process and will change over time. Steel gray horses, for example, may be young gray horses with a black base color. They will change as they mature, perhaps through a phase of dapple gray before going almost white.

The gene which determines whether a horse will be gray has two alleles, which are symbolised G⁺ and G^G. The G^G allele is dominant so that gray horses are of genotypes G^GG^G and G⁺G^G, while G⁺G⁺ horses have coats of some other color (non-grays).

Because the gray allele is dominant heterozygous gray horses can have foals of other colors, depending on their genotype for the other color genes.

For example we�ll consider the probability of a gray foal from a gray mare and a chestnut stallion. The chestnut stallion is of genotype G⁺G⁺ (i.e. is homozygous for �non gray� alleles). The gray mare had only one gray parent herself and so must be heterozygous for the gray allele, i.e. of genotype G⁺G^G.

Genetic contribution from mare:

50% chance of either allele in the egg Genetic contribution from stallion:

Only G⁺ alleles in the sperm

G⁺ 50% chance: G⁺G⁺

Not gray

G^G 50% chance: G^GG⁺

Gray

On average half of the foals will be gray, while the others will be some other color that depends on the underlying color of the mare.

The inheritance of gray illustrates epistasis. The G^G allele is epistatic (not dominant) to E and e at the extension locus, and to alleles at other horse color genes also. Horses with at least one copy of the allele G^G go are gray horses regardless of the genotype at the other genes controlling coat color.

Consider, for example, that we had a gray heterozygous mare and stallion (both of genotype G⁺G^G) who were also heterozygous for the allele e, that causes chestnut when homozygous (i.e. they are therefore both of genotype E⁺e).

The gametes from each horse would be of four types, any of which are equally likely: G⁺E⁺, G⁺e, G^GE⁺ or G^Ge. This di-hybrid cross (illustrated below) gives a modified ratio of 12:3:1 (modified from the standard di-hybrid ratio of 9:3:3:1).

Genetic contribution from mare: Genetic contribution from stallion:

G^GE⁺ G^Ge G⁺E⁺ G⁺e

G^GE⁺ G^GG^G E⁺E⁺
gray
G^GG^G E⁺e
gray
G⁺G^G E⁺E⁺
gray
G⁺G^G E⁺e
gray

G^Ge G^GG^G E⁺e
gray
G^GG^G ee
gray
G⁺G^G E⁺e
gray
G⁺G^G ee
gray

G⁺E⁺ G^GG⁺ E⁺e
gray
G⁺G^G E⁺e
gray
G⁺G⁺ E⁺E⁺
black, bay or brown
G⁺G⁺ E⁺e
black, bay or brown

G⁺e G⁺G^G E⁺e
gray
G⁺G^G ee
gray
G⁺G⁺ E⁺e
black, bay or brown
G⁺G⁺ ee
chestnut

Flea-bitten Gray

Flea-bitten gray horses develop speckles in the original coat color. For example gray horses born chestnut develop chestnut "flea-bites". If this speckling starts soon enough the flea-bites can become quite large and numerous, with the horse developing to look a bit like a leopard spot appaloosa.

"Flea-bites" are thought to be controlled by a separate and probably recessive gene, not linked to the gray gene. It is possible that other colors of horse can have the flea-bitten genotype, but the speckles wouldn't show since they are the same color as the coat!