Silver (Z)
Gene or Region: PMEL17
Reference Variant: T (N)
Mutant Variant: C (Z)
Affected Breeds: Many
Research Confidence: High - Strong association in studied population
Explanation of Results: Z/Z = homozygous for Silver, trait expressed Z/n = heterozygous for Silver, trait expressed n/n = no variant detected
General Description for Silver
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 color 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 color 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 color, 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
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 color of a horse – for simplicity, we have not named all possible combinations!
Gene Information
PMEL17 is a transmembrane protein that is involved in the production of black pigment (eumelanin). Mutations 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 color 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.
More Horse Color Genetics
Congenital Stationary Night Blindness & Leopard Complex
Congential Stationary Night Blindness (CSNB) is characterized by the inability to see well in low light and no-light situations. It is linked to Leopard Complex Spotting (LP), where homozygous horses (LP/LP) will have CSNB. Congential Stationary Night Blindness is present at birth and is non-progressive.
Champagne
Champagne (CH) is a dilution that affects all coat colors. Champagne foals are born with pink skin and blue eyes that slightly darken with age. Adult champagne horses have a distinct pumpkin colored skin with mottling in the hairless regions, as well as amber/green/tan eyes. Horses with multiple dilutions can be difficult to accurately identify color without genetic testing.