Silver Gene (PMEL)

The silver dilution of black pigment (eumelanin) in the coat of horses is caused by a variant of the PMEL gene. This effect is most pronounced in the mane and tail, where black hairs are lightened to a silvery-white to gray color.

The PMEL gene encodes a protein that is important for the proper formation and storage of pigment in melanocytes. The c.1849C>T mutation disrupts the production of black pigment without significantly affecting red pigment (pheomelanin). Therefore, the silver coloration is expressed only in horses carrying black pigment, while chestnut horses may be genetic carriers without any visible change in coat color.

The variant is inherited in an autosomal dominant manner. Horses carrying at least one copy of the mutation exhibit silver dilution of the black pigment. In chestnut horses, the presence of the variant has no visible phenotypic effect because their coat contains only red pigment.

In addition to its effect on coat color, the PMEL variant is also associated with MCOA (Multiple Congenital Ocular Anomalies) syndrome, a group of congenital developmental abnormalities of the eye. Heterozygous horses may exhibit mild changes referred to as the Cyst phenotype, characterized by the formation of cysts in the iris and ciliary body, occasionally extending into the peripheral retina. Affected homozygous horses exhibit fully developed MCOA, including severe ocular abnormalities that may lead to impaired vision or blindness. For this reason, when planning breeding, it is recommended to avoid mating two horses carrying this mutation.

Knowledge of the PMEL genotype makes it possible to predict the coat color of offspring, identify hidden carriers among chestnut horses, and take into account the potential risk of congenital eye abnormalities when selecting breeding pairs.

.

References:

Brunberg, E., Andersson, L., Cothran, G., Sandberg, K., Mikko, S., Lindgren, G. :A missense mutation in PMEL17 is associated with the Silver coat color in the horse. BMC Genetics 7:46, 2006. Pubmed reference: 17029645