Silver is another dilution gene in horses, quite distinct from cream/pearl, dun, or champagne. The silver dapple color is quite common in ponies, especially Shetlands. Body color on these ponies ranges from chocolate to blue, often with quite pronounced dappling and light mane and tail. But the gene occurs as well in many other types of horses, especially the gaited breeds in America and a number of breeds in Europe.
Different breeds and areas have different nomenclatures for horses with the silver gene. In Australia, silver is called taffy. In the Rocky Mountain Horse, it is referred to as chocolate.
In all cases the gene occurs at the silver (Z) locus, and the alleles are silver (ZZ) which is dominant, and wild-type (Z+) which is recessive. It is possible to test for the presence of the silver allele, which is at the PMEL17 locus.
The dilution genes we have discussed so far all have the same effect on black and red pigment, or somewhat more effect on red. Silver appears to have no effect on red pigment, and a highly variable effect on black. Interestingly, the coarsest hairs, whiskers and eyelashes, are most affected, often appearing nearly white. Manes and tails, also coarse, are generally affected more than the body coat.
A genetically black horse may have the body color lightened so little it still looks black. On the other hand, the body may appear blue, chocolate or dead-grass color, but without the reddish cast typical of a chestnut. The mane and tail are generally lighter than the body, and the lower legs may be a little paler near the hoof. The contrast between mane and body color may vary—at one extreme the horse may have a mane only a little lighter than the body; at the other a black horse with a white mane and tail is quite possible. A chocolate silver with light mane and tail may be mistaken for a flaxen-maned liver chestnut.
A genetically bay horse may show little effect of the silver gene aside from the light eyelashes and whiskers, or may have a variable amount of white hair in the mane and tail and a lightening of the black lower legs toward the hoof. The body color stays red, being unaffected by the silver gene. At the light extreme, a silver bay (called a red silver) may be very difficult to distinguish from a flaxen-maned chestnut. Usually the lower legs darken to near-black before lightening again near the hoof, but it may take a gene test to be sure.
Although I have not seen a red silver in person, I have seen what I suspect to be a buckskin silver. Such a horse could easily be the result of at least two types of breeding expected to produce palomino: a red silver misidentified as a chestnut to a palomino, or a chestnut carrying silver invisibly to a buckskin.
Clear chestnut completely hides the presence of the silver gene, though in theory a chestnut with a large amount of interspersed black hair or black eyelashes or whiskers would have that black replaced by interspersed blue or chocolate an the body and white eyelashes and whiskers. Without a magnifying glass and a very careful, hair-by-hair examination, however, this would likely go undetected. Since skin color is mostly due to black pigment, that also could be affected, though the silver dapples I have seen have normal skin color.
Silver dapple has been a rare color in North American horses other than ponies, but this is changing as breeders select for rare and unusual colors.
Some silver dapples, especially those with two copies of the silver allele, do have an ocular abnormality, though it is rare that vision is actually affected. This may be due to a linked gene, rather than the silver allele itself, but it is probably safest to have the eyes of silver animals intended for breeding checked.
Upper photos courtesy of Safyre Sporthorses.