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Gray (Grey) is a modifier that, over time, causes depigmentation of the horses hair.  Horses born with this modifier are born colored but gradually loose pigmentation and can become mostly white in 6-8 years.  The vast majority of white horses are in fact horses that have fully grayed out. The Gray modifier is a fully dominant gene meaning a single copy of the gene will cause a visibly graying effect on the base coat.  Horses homozygous for the mutation (GG) showed an increased rate of graying as well as more evenly distributed effects during the final stages of graying than a heterozygous gray horses (Gg). 

Gray occurs in almost every breed although it is more common among a handfull of breeds. Different breeds of horses that commonly show this phenotype are Andalusian, Arabian, Connemara, Iclandic, Lipizzaner, New Forrest Pony, Shetland pony, Thoroughbred and Welsh.

Gray horses, especially horses that are homozygous for the gene, have an extremely high rate of dermal Melanomas (Melanoma Cancer).  Research conducted in Sweden showed that 70-80% of gray horses age 15 and older have melanomas.  Primary melanomas are generally benign but later metastasize to internal organs.

The genetic mutation that produces graying in horses was located in 2008 by researchers at Uppsala University in Sweden. The gray mutation is caused by a 4.6-kb duplication in intron 6 of STX17.

Rosengren Pielberg G, Golovko A, Sundström E, Curik I, Lennartsson J, (More Authors) A cis-acting regulatory mutation causes premature hair graying and susceptibility to melanoma in the horse. Nature Genetics. 2008 Aug;40(8):1004-9. Epub 2008 Jul 20.

Gray Testing

Animal Genetics offers DNA testing and detection of the gene mutation responsible for Gray and the determination of Gray zygosity.


$25.00 US per sample.

Sample Collection

Collect sample by pulling (not cutting) 20-30 mane or tail hairs with roots attached. It is important that you pull the hairs and confirm that the actual root of the hair is being collected. The root contains the genetic material of your horse that is needed for DNA testing. Therefore, cut hairs do not provide an adequate sample of your horse. Place the collected hairs of each horse in a separate zip-lock bag labeling the bags accordingly with the horses name or identification number. Download and complete a submission form for each sample and send along with payment to Animal Genetics for testing.


Results are given using the following symbolic notation:

The horse carries two copies of the Gray gene and is homozygous for Gray. The horse will always produce offspring that will go gray.
Both the normal and Gray alleles were detected. Horse tested heterozygous for Gray and has a 50% chance of producing an offspring that will go gray as well.
Horse tested negative for the Gray gene.




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