Читать книгу Horse Economics - Vera Kurskaya - Страница 42

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 or megalophthalmia (abnormally small or abnormally large eye or eyes).

 Abnormally large eyes.

The majority of the eye anomalies are connected with the forward segment of the eye, and some of them can occur independently, although there are also exceptions—such as the retinal displasia. Heterozygotes for the Silver gene are less affected than homozygotes. The vision of horses with cysts is usually normal. Unlike in normal horses with healthy eyes, the sclera of silver horses is sometimes pink, but not pigmented. Not all horses with MCOA have notable problems with their vision, but there are, of course, exceptions. For example, when cysts of a ciliary body are torn due to an impact, it can lead to the detachment of retina.

Most often MCOA is found in related breeds that originated from horses coming from Appalachia. These include Rocky Mountain Horses, Kentucky Mountain Saddle Horses, and Mountain Pleasure Horses. Similar abnormalities are also observed in other breeds, such as the Morgan, Belgian, Shetland Pony, American Miniature Horse, and Icelandic Horse. Affected horses can display abnormal behaviors due to the problems with their vision. Structural eye abnormalities are the result of a pleiotropic action of the PMEL17 gene. For this reason, since 1998, Kentucky Mountain Saddle Horses are required to pass veterinary inspection for the absence of vision problems before being allowed for breeding (animals with eye anomalies are excluded). The rules are not so strict for the Rocky Mountain Horse. Nevertheless, breeders of Rocky Mountain Horses, including those in Europe, do not recommend breeding carriers of the Silver gene.