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Locus Informationen: G Locus (STX17 gene)

Inheritance: autosomal dominant

→ Horses with one or two copies of the grey mutation (G/n or G/G) will be grey.

General Information:

  • Grey horses are born normal colored and slowly “lose” color over time – they first go grey, and eventually white.
  • At birth, foals that will go grey are often very intensely colored (hyperpigmented).
  • Grey is actually caused by a malfunction in the pigment cells. They produce intense pigment for a period of time, and essentially “burn out” over time and stop producing pigment.
  • Horses with two copies of the grey variant (homozygous, G/G) progress more quickly than horses with one copy (heterozygous, G/n).
  • Hairs have less and less, and at the end, no pigment. Horses will therefore go through different versions: dapple grey, flea-bitten, etc. These are all part of the progression towards white.
  • Different breeds and different horses “grey out” at different speeds.
  • After the greying process has started, it is often not possible to determine the original base color of the horse by looking at the remaining pigmented hairs.
  • The STX17 (grey) gene is involved in cancer pathways. The duplication that causes the grey color also predisposes grey horses to developing melanoma. It is estimated that approximately 85% of grey horses develop melanomas by the time they are 15 years old.
  • Horses with two copies (G/G) of the grey variant will usually develop melanomas earlier in life than those with one copy.

Possible Genotypes:

Genotype: Effect:
n/n No Grey mutation
G/n One copy of G → the horse will progressively go grey and finally completely white.
G/G Two copies of G → the horse will progressively go grey and finally completely white. Because of two copies this process will progress more quickly than in horses with only one copy (G/n).


Test information: This test detects a large duplication in the STX17 gene. The test done by CAG is not able to determine if a horse has one or two copies of the grey variant.

Rosengren Pielberg, G., Golovko, A., Sundstrom, E., Curik, I., Lennartsson, J., Seltenhammer, M.H., Druml, T., Binns, M., Fitzsimmons, C., Lindgren, G., Sandberg, K., Baumung, R., Vetterlein, M., Stromberg, S., Grabherr, M., Wade, C., Lindblad-Toh, K., Ponten, F., Heldin, C-H., Solkner, J., Andersson, L. : A cis-acting regulatory mutation causes premature hair graying and susceptibility to melanoma in the horse Nature Genetics 40:1004-1009, 2008. Pubmed reference: 18641652. DOI: 10.1038/ng.185.

More information can be found on the webseite: Online Mendelian Inheritance in Animals.

“Dapple grey” is an intermediate stage of the greying process.

Test #: H225

Sample: Blood, Hair

Price: 35 € (net)

Time: 10-14 days

Two examples of grey horses: one with “flea bites” (left), and one that has completely “greyed out” to look nearly white (right).




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