Silver dapple gene

The silver dapple gene (Z) in horses is a dilution gene that affects black pigmentation, producing shades of silver, platinum, or gray in the coat while preserving the original black-brown character. It primarily impacts the mane and tail, often leaving the body darker. The gene is linked to Multiple Congenital Ocular Anomalies (MCOA), which includes eye disorders such as cornea globosa, uveal cysts, cataracts, and retinal detachment. These issues vary in severity but are present in all horses carrying the mutated PMEL17 gene.

The gene works by diluting eumelanin, the pigment responsible for black and brown colors, without affecting phaeomelanin, which produces red and yellow hues. This explains why chestnut or palomino horses, which lack functional eumelanin, are unaffected by the silver dapple gene but can carry it.

The PMEL17 gene is critical in melanosome formation and pigmentation. Mutations in this gene have been associated with pigmentation-related issues in other species, such as merle coloring in dogs linked to auditory and ophthalmologic defects. In horses, homozygous silver dapple individuals are at higher risk for severe eye problems due to the Founder effect, where a genetic trait is inherited from a single ancestor.

In summary, the silver dapple gene alters coat pigmentation while causing significant ocular health risks, highlighting the importance of understanding its effects on both coat color and equine welfare.