Silver (MCOA)

The gene for silver (PMEL17) is another gene causing a lightening of the basic colour. The silver-gene has no impact on pheomelanin like the genes for cream and champagne and only leads to brightening of black coloured areas. Especially black coloured hair of mane and tail appears brighter because of the occurrence of white and grey hairs. The gene for silver is inherited autosomal-dominant and one copy of the gene is sufficient for the phenotype.

The silver variant in the PMEL17 gene is associated with multiple congenital ocular abnormalities syndrome (MCOA). MCOA is an eye disease, the severity of which is dose-dependent and varies depending on the number of copies of the silver mutation present. Horses with one copy of silver (N/Z) have less severe symptoms (cyst phenotype), which typically manifest as translucent cysts (or fluid-filled vesicles) up to 1 centimeter in diameter. Horses with two copies of the silver mutation (Z/Z) exhibit more severe symptoms (MCOA phenotype) with cysts and additional abnormalities such as enlargement of the cornea, abnormally shaped iris, and/or retina, which can impair vision and lead to blindness.

MCOA shows incomplete penetrance, which means that there is variation in presentation of MCOA among horses with N/Z and Z/Z genotypes. Research suggests that there may be a progressive change in vision of horses with the silver mutation. To avoid producing offspring with the severe MCOA associated with 2 copies of silver mutation, breeders should not mate two horses with silver mutation (N/Z or Z/Z, regardless of base color) to each other. Although the silver mutation has no effect on the coat color phenotype of chestnut/sorrel horses, it can cause ocular abnormalities if present. Chestnut horses from silver lineages should be tested to inform breeding decisions. Regardless of base color, it is advisable that horses with the silver mutation be examined by a veterinary ophthalmologist to determine the extent and severity of ocular condition.