can also be very white. They can be mistaken for dark flaxen chestnuts. Silver black with an almost undiluted body and no dapples is also called silver chocolate by some.
This color is characteristic in Rocky Mountain Horses, and according to the Rocky Mountain Horse Association (www.rmhorse.com) it is called chocolate. The color is traced to the founding stallion of this breed: “Old Tobe.” Today more than 40 percent of Rocky Mountain Horses have the color.
The strongest dilution of the Silver gene results in silver dapple, which may remotely resemble dappled gray (Photo 60 and see p. 41). Silver dapple horses may also have a sepia shade, which is most common in Shetland Ponies and American Miniature Horses.
Distinguishing silver dapple horses from gray horses with dapples is simple: the head of a silver horse is either darker than the body or the same color. The head of a gray horse is always lighter. You should also look for other traits we discussed on p. 27, such as light eyelashes and leg webbing. White or light eyelashes are found most often in silver black or silver dapple horses.
American Miniature Horses carrying the Silver gene on the black base are often erroneously registered as having the gray, chestnut, or even palomino color. Silver dapple foals with a black base are born rosy cream, light ashy, or the color of a light biscuit, vaguely resembling palominos with gray muzzles. Dapples are absent at this age. The tail and mane of silver black and silver dapple horses sometimes darken with age. In the winter they become browner and dapples disappear.
Other Colors Determined by Silver
The Silver gene dilutes only areas of hair containing eumelanin, and so it has no effect on red hair. The term silver chestnut is occasionally used in English writings on the subject of horse color to describe horses with a chestnut base that are carriers of the Silver gene. Such horses have the potential of producing silver offspring.
If the Silver gene is present in a horse in combination with other dilution genes, the resulting colors can become difficult to identify when only observing the phenotype. This is especially difficult in the Rocky Mountain Horse, Mountain Pleasure Horse, and the Icelandic Horse breeds, in which many individuals have similar combinations of genes. For example, the mane and tail hair diluted by the Silver gene can be mixed up with frosting, a characteristic of dun colors (see p. 22). In combination with the dun or buckskin, the effect of the Silver gene shows very poorly. For example, a silver dun horse looks very similar to a bay dun but also has pale yellow or whitish guard hair; however, the legs are dark, sometimes with leg webbing. I was lucky enough to discover this color in Byelorussian Harness Horses. The silver buckskin also looks quite ambiguous. The color ranges from sandy to whitish, and the mane and tail are diluted brown or dirty yellow. This color is found in Icelandic Horses, where some time ago it was mistaken for the Champagne dilution (see p. 32). One more rare combination is silver grullo. This horse is externally very similar to grullo—the only difference is an almost-white mane and tail, and sometimes leg webbing.
Silver smoky black determined by the Silver gene plus one Cream Dilution gene appears, on the outside, to be very similar to ordinary silver black with a little more yellowish or brownish shade to the body (Photos 61 & 62). Unfortunately, it isn’t always possible to measure this difference by the eye.
There are additional rare combinations, such as silver dun buckskin, which can occur in the Icelandic breed. The horse is a light, sandy color, the lower part of the legs is light ashy, and sandy-colored hair is mixed with black and ashy in the mane and tail. Primitive markings are brown.
Inheritance of Colors Determined by the Silver Gene
The Silver gene mutation is dominant, located on the sixth chromosome, and the gene is named PMEL17 (pre-melanosomal protein 17). Two dominant alleles of this gene responsible for silver colors are designated “Z,” and the recessive allele that doesn’t influence color has the symbol “z.” There is a DNA test for two dominant Silver mutations.
The action of the gene on pigment is most noticeable on guard hair and to a lesser extent on the body hair. As mentioned already, it does not affect the color of a red horse. The effect of the Silver gene on horse colors is presented in the table below:
Table 5.
| Primary Color | Z |
|---|---|
| Bay | Silver Bay |
| Black | Silver Black or Silver Dapple |
| Seal Brown | Silver Brown |
| Chestnut | Chestnut |
| Buckskin | Silver Buckskin |
| Smoky Black | Silver Smoky Black |
| Palomino | Palomino |
| Bay Dun | Silver Dun |
| Grullo | Silver Grullo |
The hypothesis that silver dapple horses are homozygotes, and silver black horses are heterozygous for this mutation could not be confirmed by DNA tests.
Some silver horses have congenital anomalies of the structure of eyes called anterior segment dysgenesis (ASD). Scientific literature also uses the name multiple congenital ocular abnormalities (MCOA). The phenotypes include:
Small pupils, which do not react to light or react poorly. (Application of dilating preparations does not have an effect.)
Abnormally formed pupils.
Hypoplasia (underdevelopment) of the iris, including its stroma.
Hypoplasia of iris formed by a pigmentary layer on the top edge of the pupil.
Abnormal structure of pectineal ligament.
Cataracts.
Cysts in the vitreous body.
Corneal thickening.
Abnormal arrangement of the eye lens.
Retinal dysplasia.
Abnormal circulation of intraocular liquid.
Defects of the ciliary body, such as small cysts in the junction between the sclera and cornea.
Cysts of the iris and ciliary body sometimes extending to the peripheral part of the retina.
Microphthalmia
