Silver (Z)

General Description

Silver (sometimes called silver dapple or taffy) is a variant of PMEL17 that dilutes black pigment. Silver is inherited in a dominant manner, indicating that both homozygous (Z/Z) and heterozygous (Z/N) horses exhibit the phenotype. The original paper suggested that homozygotes may be slightly more dilute than heterozygotes, but no follow up study has been published. The dilution tends to have a more extreme effect on the mane and tail than on the body. Silver foals usually display striped hooves and white eyelashes. Black silver horses are referred to as chocolate palominos in some breeds due to their dilute brown body (sometimes with dappling) and white/cream mane and tail. Bay silver horses will have a normal reddish body colour with grey legs, which has led to misidentification of some horses as flaxen chestnut, and thus has allowed silver to “hide” in some breeds such as the Morgan. There has been no documented effect of silver on the coat colour of chestnut/sorrel horses.

Equine multiple congenital ocular anomalies (MCOA) syndrome, previously described as Anterior Segment Dysgenesis (ASD), is a collection of clinical symptoms affecting the eyes of silver horses. MCOA is inherited as an incomplete dominant trait, with homozygotes displaying a more extreme phenotype. Horses with one copy of the allele (Z/N), whether they are chestnut or have a silver coat colour, display mild ocular cysts. Homozygous horses (Z/Z) are more severely affected and display a range of symptoms, including uveal cysts, cornea globosa, iris stromal hypoplasia, abnormal pectinate ligaments, cataracts and iris hypoplasia.

Genotype and Phenotype (Colour Names)

Black with single silver (E/_ a/a Z/N): black silver with iridociliary cysts

Black with double silver (E/_ a/a Z/Z): black silver with MCOA

Bay with single silver (E/_ A/_ Z/N): bay silver with iridociliary cysts

Bay with double silver (E/_ A/_ Z/Z): bay silver with MCOA

Chestnut with single silver (e/e Z/N): chestnut with iridociliary cysts

Chestnut with double silver (e/e Z/Z): chestnut with MCOA

* Other dilutions, modifiers, and spotting genes can further alter the colour of a horse – for simplicity, we have not named all possible combinations!

Gene Information

PMEL17 is a transmembrane protein that is involved in the productive of black pigment (eumelanin). Mutations in other species have led to a variety of dilute coat colors in other species, including zebrafish, mice, chickens (dun and smokey), and dogs (merle). In both the dog and horse, these coat colors are also associated with abnormalities in the eyes. The Z allele in the horse is a single base change that alters an amino acid, likely affecting the normal function of the encoded protein.

References

Brunberg E et al., “A missense mutation in PMEL17 is associated with the Silver coat colour in the horse.” (2006) BMC Genet. 7: 46.

Andersson LS et al., “Targeted analysis of four breeds narrows equine Multiple Congenital Ocular Anomalies locus to 208 kilobases.” (2011) Mamm Genome. 22: 353-60.

Andersson LS et al., “Equine multiple congenital ocular anomalies and silver coat colour result from the pleiotropic effects of mutant PMEL.” (2013) PLoS One. 8: e75639.