thumb|right|Black silver horse exhibiting strongly diluted long hair with darker roots and flat gray, dappled body color
The silver or silver dapple (Z) gene is a dilution gene that affects the black base coat color and is associated with Multiple Congenital Ocular Abnormalities. It will typically dilute a black mane and tail to a silvery gray or flaxen color, and a black body to a chocolaty brown, sometimes with dapples. It is responsible for a group of coat colors in horses called "silver dapple" in the west, or "taffy" in Australia. The most common colors in this category are black silver and bay silver, referring to the respective underlying coat color.
Mature black silvers typically have sooty white or silver manes and tails with a flat, non-fading, dark grey or grey-brown body coat. The body coat frequently exhibits dapples, rings of lighter-colored hair.
Mature bay silvers retain their reddish bodies, though the presence of small amounts of silver often gives them a chocolate appearance. The mane and tail are usually a sooty silver, darker at the roots, and the legs are usually a flat, brownish-grey mottled with silver. The hair around the eyes and muzzle may also show signs of silvering.
Silver dapple foals can be difficult to identify, but commonly have a pale, wheat-colored body coat, white eyelashes, and hooves with tapering vertical stripes. These characteristics fade over time.
Red-based horses, such as chestnuts and chestnuts with other dilution factors (such as palominos, and cremello) may carry the silver dapple gene, and may pass it on to their offspring, but will not express the gene in their own body color. but this is not true of silver dapples. When actual color is in question, pedigree information or a DNA test can help clarify matters.
Prevalence
thumb|right|Silver dapple foal exhibiting typical wheat-colored coat and pale eyelashes
Many breeds do not possess the silver dapple gene. The coat color is traditionally associated with the Rocky Mountain Horse and the Miniature Horse. Scandinavian breeds and their descendants such as the Icelandic horse, Nordland Pony, Shetland Pony, Welsh Mountain Pony, Welsh Pony, Swedish Warmblood, Estonian native horse and Finnhorse are also found in the silver dapple colors. American horse breeds known to have the silver gene include the Morgan, American Saddlebred, Missouri Foxtrotter, Tennessee Walking Horse, and the American Quarter Horse. European draft breeds such as the Comtois and Ardennais also occur in silver.
thumb|left|Bay silvers retain their reddish body color with black points diluted to silver.
While the role of PMEL17 is not fully understood, the silver dapple gene exclusively produces dilution, or hypopigmentation, of eumelanin. These conditions can be detected via ultrasound examination of the eye. In a 2013 study of Comtois horses and Rocky Mountain Horses, all animals carrying the mutated form of PMEL17 had some eye disorder, though milder problems in animals heterozygous for the allele versus those who were homozygous. None of the control horses of these breeds who lacked the mutated form of PMEL17 had any eye disorder.
thumb|Eye disorders in horses with the silver dapple gene
Similar mutations in other species provide insight into the roles played by the PMEL17 gene. Relatively few such mutations are known, which suggests that the gene is involved in processes critical to survival. Several of the mutations known are related to pigmentation: premature silvering in mice, diluted and white plumage in chickens, and the widely known merle dilution in dogs. The merle coat in dogs is associated with auditory and ophthalmologic disorders, such as deafness and microphthalmia. In Rocky Mountain Horses, the silver dapple color is sometimes associated with Anterior Segment Dysgenesis (ASD) which affects the structures in the face and the front of the eye. Most often, the syndrome presents as benign lesions, though homozygotes may have impaired vision. As ASD is found in non-silver representatives of the breed, it is thought to be physically close to the silver gene and to have come from a silver dapple, ASD-affected foundation ancestor, thus is an example of the Founder effect.