The gray gene causes a progressive depigmentation of the hair, often resulting in a color that is almost completely white by 6-12 years of age, and can act on any base coat color. Horses with G3 will gray faster and have a higher risk for melanoma than those with G2.
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Alleles: N = Normal or non-gray (also referred to by others as G1), G2 = gray gene duplication, two tandem copies of the duplicated sequence causing gray, G3 = gray gene triplication, three tandem copies of the sequence causing gray
Mode of Inheritance: Autosomal dominant
Alleles: N = Normal or non-gray, G = Gray
Breeds appropriate for testing: Many breeds
Explanation of results:
- Horses with N/N genotype will not be gray and cannot transmit a gray variant to their offspring.
- Horses with N/G2 genotype will become gray but more slowly than those horses with G3. The G2 allele is associated with slow greying and lower incidence of melanoma. Horses with this genotype may transmit this gray variant to 50% of their offspring. Matings with N/N genotype will result in a 50% chance of producing a foal who will gray.
- Horses with N/G3* genotype will become gray faster. The G3 allele is associated with faster greying and a high incidence of melanoma. Horses with this genotype may transmit this gray variant to 50% of their offspring. Matings with N/N genotype will result in a 50% chance of producing a gray foal with increased risk for melanoma.
- Horses with G2/G2* genotype will become gray but more slowly than horses with G3. They will transmit this gray variant to all of their offspring.
- Horses with G3/G3 genotype will become gray and show fast progressive graying of the coat. The G3 allele is also associated with a high incidence of melanoma. Horses with this genotype will transmit this gray variant to all of their offspring, who will also turn gray and be at increased risk for melanoma.
- Horses with G2/G3 genotype will show fast progressive graying of the coat. These horses will grey faster than horses with only the G2 allele. These horses also have a high incidence of melanoma. They will transmit a gray variant to all of their offspring. Offspring receiving the G2 variant will show slow progressive graying of the coat. Offspring receiving the G3 variant will show fast progressive graying of the coat and be at increased risk for melanoma.
If gray is tested as part of a coat color panel, results are reported as:
- present which means the gray allele was detected and horse will gray or
- absent in which the gray allele was not detected and therefore the horse will not turn gray.
INSTRUCTION VIDEO: How to pull hair for testing
Please Note: All DNA parentage verification and mandatory health testing results, whether purchased through the EBRA or transferred into the EBRA system, will become permanent record, which the documented owner may access at any time upon request. The EBRA is not responsible for providing non-mandatory DNA test results, therefore the release of any additional testing performed by past or present owners must be negotiated between the private parties at time of purchase.
Gray Copy Number Test
The gray gene causes progressive depigmentation of the hair, often resulting in a coat color that is almost completely white by the age of 6-12 years. Horses that inherit progressive gray can be born any color, then begin to gradually show white hairs mixed with the colored hairs throughout the body. The first signs of gray hair can usually be found on the head, particularly around the eyes.
Gray is inherited in an autosomal dominant manner, meaning that a single copy of either gray allele (G2 or G3) will cause a horse to turn gray. The initial cause of graying in horses was reported by researchers at Uppsala University, in Sweden, as a 4.6 kilobase (kb) duplication in intron 6 of the Syntaxin 17 (STX17) gene. Their research hypothesized that the mutation in STX17 affects the production of melanocytes and, thus, predisposes gray horses to melanoma in addition to causing graying of the coat.
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 horses that are also homozygous for black (a/a genotype at the Agouti locus) have a higher risk for melanoma. Many gray horses show depigmentation of the skin around the eyes, mouth, and anus, but any additional health risks, aside from melanoma, have not been fully studied. Gray horses have also been reported as being at higher risk for ocular squamous cell carcinoma.
A 2024 study led by Dr. Leif Andersson and colleagues at Uppsala University, in collaboration with researchers at the UC Davis Veterinary Genetics Laboratory (VGL), showed that the speed of graying and melanoma risk are influenced by the number of copies of this 4.6 kb sequence, indicating the existence of two different alleles responsible for graying in horses: G2, which consists of a duplication that leads to slow graying, and G3, which is a triplication that results in fast graying and increased risk for melanoma.
The study showed that horses with no duplication (G1/G1, reported as N/N by VGL) did not gray and had the lowest incidence of melanoma. Horses with one copy of G3 (G1/G3, reported as N/G3 by VGL) showed fast graying and increased incidence of melanoma, whereas the fastest graying and highest incidence of melanoma was seen in horses that were homozygous for G3 (G3/G3). Horses with the G2 allele, whether heterozygous (G1/G2, reported as N/G2 by VGL) or homozygous (G2/G2) showed low incidence of melanoma, similar to horses with no duplications (G1/G1 or N/N).