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Breeds: All
Description
Silver is a dominant dilution caused by a variant in the PMEL17 gene, so a horse needs only one copy to show the dilution. Like Agouti, Silver acts only on black pigment and has no visible effect on red (chestnut) horses. The difference is that Agouti controls where black pigment is distributed, while Silver dilutes the black pigment that is there.
The effects of Silver vary considerably. On a black horse, the mane and tail are lightened to flaxen or silver-gray and the body is lightened to a chocolate shade, often with dappling. On a bay horse, the effect is concentrated in the mane, tail, and lower legs, which are lightened while the red body color stays largely unchanged. Silver bays are sometimes mistaken for flaxen chestnuts.
Testing matters because Silver is not always visible. A chestnut horse can carry Silver, show nothing at all, and still pass the variant to its foals, where it will show on any black-based offspring. Testing also confirms Silver in horses whose coloring is ambiguous, and identifies horses carrying two copies.
Silver has been identified in a number of breeds, including the American Quarter Horse, Rocky Mountain Horse, Icelandic Horse, Morgan, Shetland Pony, and Miniature Horse.
Multiple Congenital Ocular Anomalies (MCOA)
The Silver variant is associated with Multiple Congenital Ocular Anomalies, a set of inherited eye defects. Horses with one copy (n/Z) usually have only cysts of the iris, ciliary body, or retina, with little or no effect on vision. Horses with two copies (Z/Z) can have more serious abnormalities affecting vision. Breeders working with Silver may want to avoid producing homozygous foals, which is one of the main reasons to test.
Possible results
| Genotype | Description |
|---|---|
| Z/Z | Homozygous: Horse has two copies of the Silver Dilution variant and will pass one copy to every offspring. Black-based horses will show the dilution. |
| n/Z | Heterozygous: Horse has one copy of the Silver Dilution variant. Each foal has a 50% chance of inheriting it. Black-based horses will show the dilution. |
| n/n | Negative: Horse does not carry the Silver Dilution variant. |
Breeds Affected
American Quarter Horse, Rocky Mountain Horse, Kentucky Mountain Saddle Horse, Icelandic Horse, Morgan, Shetland Pony, Miniature Horse, Paint Horse, Appaloosa, Mustang, Comtois, Ardennes
Reference
A missense mutation in PMEL17 is associated with the Silver coat color in the horse.
BMC Genet. 2006 Aug 7;7:46. doi: 10.1186/1471-2156-7-46.
PMID: 16897109
Related Tests
Testing more than one color trait? The Color Panel includes this test plus seven others for $75.