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There is new knowledge about the gray coat color based on the latest research. We will be updating our testing to reflect this knowledge.
As you likely know, the initial cause of graying in horses was reported as a 4.6 kilobase duplication in intron 6 of the Syntaxin 17 (STX17) gene. The new research study that we were involved in determined that in some horse’s gray is caused by the duplication (two copies), as previously published, but in other cases it is caused by a triplication (three copies). The triplication is the most common allele across horse breeds.
You can find more information about this study here and we will have more information on our website when the test is launched.
The previous gray test reported alleles as N (non-gray) or G (gray). The new alleles are defined as G2 (duplication) or G3 (triplication), see Figure 1. Because horses have two copies of every chromosome, they can technically have two, three, four, five, or six copies of this DNA region. Horses with the G3 allele gray at a faster rate and have a higher incidence of melanoma than those with the G2 allele.
To keep our testing consistent with the science we will soon be discontinuing our previous Gray test and will be replacing it our Gray Copy Number test to report the copy number detected and when resolvable which alleles are present. What I mean by when resolvable, horses with 4 copies could be N/G3 or G2/G2 and at this time the technology doesn’t allow us to determine this for every case.
Your contract price for the Grey Copy Number test will be $45 if you choose to add it to your contract.
New results will be:
N/N = "Does not have the duplication or triplication that causes gray. The horse will not gray. The N allele is also known as G1."
N/G2 = "One copy of the duplication (G2) associated with slow graying and low incidence of melanoma. Horse will gray."
G2/G3 = "One copy of the triplication (G3) and one copy of the duplication (G2). G3 is associated with fast graying and high incidence of melanoma. Horse will gray."
G3/G3 = "Two copies of the triplication (G3). G3 is associated with fast graying and high incidence of melanoma. Horse will gray."
4 copies = (N/G3 or G2/G2)= "Unresolvable-four copies detected. The horse either has one copy of the triplication (G3) OR two copies of the duplication (G2). Horse will gray."
This new test will be available as an additional test for the same price ($45) for samples on file, including those that were previously tested for the gray test that will be discontinued.
The gray test that is now a part of our coat color panels will still remain as a presence/absence test and this test will also be available to order on it is own for a contracted rate of $40
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present which means a grey allele was detected and horse will turn gray or
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absent in which neither of the gray alleles were detected and therefore the horse will not turn gray.
Our target date to launch the test is November 7th. If this changes I will let you know.
Rebecca Bellone, Ph.D.
Director, Veterinary Genetics Laboratory
Professor, Department of Population Health and Reproduction
UC Davis School of Veterinary Medicine
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