Dapple Grey Coat Color in Horses

White horses are never truly albino. The closest horses come to being completely depigmented (i.e., no melanin deposition of any kind) is when they express the dilution modifiers that make them perlino or cremello. Any white horse you see who has grey pigmentation around the nose and eyes is not truly white, either. Rather, the animal is expressing an age-dependent trait controlled by the gray gene. The dominant form of this allele causes gradual loss of melanin in the hair shaft as the animal ages. A baby grey horse is born with normally pigmented hair (it can be any color).

At the age of 1-2 year, the coat is already starting to lose pigment:

Pigment is lost in such a way that it produces a spotting ("dappling") pattern on the coat.

The progression continues until at the age of about 6-8, the horse is pretty much pure white except for its nose and around the eyes.

Horses homozygous for the dominant allele tend to lose pigment completely, whereas heterozygotes may retain more pigmentation in the hair.

The Gray Gene and Melanoma
Interestingly, dapple gray horses are at very high risk of developing melanomas. More than 70% of dapple grey horses over the age of 15 years develop this type of dermal carcinoma, which is usually highly metastatic to the respiratory and nervous systems. Homozygotes are at greater risk than heterozygotes for melanoma, and if they are also homozygous recessive at the agouti locus (i.e., their pigmentation is uniformly deposited all over the body, instead of just at the points), they are at even high risk.

A team of Swedish researchers recently reported their discovery that the graying trait in horses is caused by a 4.6 kb duplication in intron 6 of the gene syntaxin 17 (stx17).(1)

(Syntaxin proteins mediate intracellular vesicle fusion. How errors in this function relate to cancer risk is not yet understood. But certain forms of human cancer (notably in the liver) are also associated with errors in syntaxin localization in the cell. There's a BIG research project in there...)

Gray vs. Roan
The graying gene should not be confused with the gene that causes roan coloration in horses. A roan horse has a coat combining both pigmented (either eumelanin or phaeomelanin) and unpigmented hairs. The head, legs, mane and tail usually have fewer white hairs, and appear darker than the body. Roan horses are born roan, and--unlike greys--do not progressively become lighter as they age.

Roaning is controlled by an allele (R') of the wild type R locus. A horse with genotype RR will have a fully pigmented coat, a horse with genotype RR' will be roan. It was once believed that a foal with genotype R'R' would be inviable, as one study (2) showed that roan x roan crosses did not produce the proportion of roan offspring predicted by Mendel's Law of Segregation. However, a more recent study (3) by the late Dr. Ann Bowling--done with direct genetic analysis--showed that the R'R' genotype produces viable animals with roan coloration. This was good news for roan lovers.

The Rr genotype can combine with chestnut (ee --) to produce a red or "strawberry" roan, with bay (-- A-) to produce a bay roan, or with black (E- --) to produce a "blue" roan.

References: (1) Pielberg G.R., Golovko A., Sundstrom E. et al. 2008. A cis-acting regulatory mutation causes premature hair graying and susceptibility to melanoma in the horse. Nature Genetics, 40 (8):1004-1009.

(2) Hintz, H. F. and VanVleck, L. D. 1979. Lethal Dominant Roan in Horses. Journal of Heredity, 70:145-146