Gray Copy NumberQuick Summary
Click here for Price and Turnaround Time
Phenotype: 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.
Mode of Inheritance: Autosomal dominant
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
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.
*In some cases, it is not possible to distinguish between horses that are N/G3 and G2/G2 and these will be reported as Unresolvable - with four copies detected.
Sample Collection
Horse DNA tests are carried out using cells from the roots of a hair sample (roughly 20-30 hairs).
1. Grab about 10 hairs at the base.
2. Wrap the hairs around your finger and give it a quick pull.
3. Check the ends to make sure the pulled hairs have roots.
4. Repeat the process until you have collected about 20-30 hairs with intact roots.
5. You can choose different places on the mane or tail. NOTE: For foals, we recommend pulling all hairs from the tail only.
6. Tape the hairs to the submission form and fold the form along the dotted line to protect the sample. Do not use ziploc bags as they can cause condensation that allows mold to grow on the hair.
7. Place the folded form containing the sample in a paper envelope and mail it to the laboratory.
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).
Gray is found in many breeds and is the predominant color of the Lipizzan breed. Using data generated at the VGL, researchers found that the G3 allele is much more prevalent across horse breeds, which may be the result of strong selection for the all-white phenotype. The slow graying G2 allele was specifically identified in 8 different breeds: Andalusian, Connemara Pony, Miniature Horse, Mangalarga Marchador, Mustang, Quarter Horse, Tennessee Walking Horse, and Welsh Pony.
Testing for these gray variants can help owners identify homozygous horses (animals with two copies of any gray allele), which will always produce gray offspring. Testing can also assist in determining a foal's coat color, as gray causes a progressive loss of pigmentation. Testing also allows for selective breeding towards the G2 allele, which causes graying with a lower risk of melanoma.
Note: Horses with 4 copies of gray as determined by this test may be G2/G2 or N/G3. This test is not currently able to distinguish between these two possible genotypes in horses that have 4 copies of the gene.
