UC Davis School of Veterinary Medicine Veterinary Genetics Laboratory

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The Gray gene causes progressive depigmentation of the hair, often resulting in a coat color that is almost completely white by the age of 6-8 years. Horses that inherit progressive Gray can be born any color, then begin gradually to show white hairs mixed with the colored throughout the body. Usually the first signs of gray hair can be found on the head, particularly around the eyes. Gray is dominant, therefore a single copy of this gene will cause a horse to turn gray. If a horse has two copies of Gray, all offspring of this horse will be gray. Research indicates that horses with one copy of Gray often retain some of the original pigment while homozygotes tend to progress to almost completely white.  Gray is found in many breeds and is the predominant color of the Lippizaner breed.

Gray horses have a high incidence of dermal melanomas that are commonly seen around the tail and head. Over 70% of Gray horses older than 15 years will develop melanoma. Gray homozygotes are more likely to develop melanoma than heterozygotes. Gray horses that are homozygous for non-agouti (“aa” genotype at the Agouti locus) also have a higher risk for melanoma. Many Gray horses show depigmentation of the skin around the eyes, mouth and anus but there are no health risks associated with this condition.

Researchers at Uppsala University in Sweden discovered that a 4.6 kilobases duplication in intron 6 of gene syntaxin 17 (STX17) produces progressive graying in horses.

Gray results* are reported as:

N/N No copies of the gray gene. Horse will not turn gray.
N/G One copy of the gray gene. Horse will turn gray and approximately 50% of   offspring will be gray.
G/G Two copies of the gray gene.  Horse will turn gray and all offspring will be gray.

*Gray results reported in 10-15 business days.

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

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