Silver is an interesting dilute gene, like Agouti, Silver only affects black pigment. A red base horse can carry the Silver allele but not express that they do. The Rocky Mountain Horse is one breed that really expresses Silver boldly, though in that community (as well as other breeds such as the Gypsy Vanner) the colour is referred to as Chocolate. In Australia it is called Taffy. Sometimes it is called Silver Dapple however that tends to cause confusion with people thinking that the coat must be dappled, which it isn’t always. Silver has shown to be an older mutation with horse remains from the Iron Age being tested positive for Silver.
Silver Allele Genetics
There is a test available to the public for the Silver allele. Silver is located at the PMEL17 locus. Silver is a dominant gene however because it doesn’t show on red base horses this can lead to some confusion in breeding with seemingly random Silver horses popping up out of breedings. Silver is written as a Z when writing out a genome.
Homozygous Silver Genetic Defects
Homozygous Silver horses are linked with having eye issues which is referred as Multiple Congenital Ocular Anomalies. It was first only seen in Rocky Mountain Horses but has been noted in Icelandics as well. Most horses have been said that their vision is not altered but this isn’t true for all horses. Most recently (August 2013) studies into the Comtois breed shows a high rate of MCOA, 66 of the 75 horses showed some forms of eye problems. The heterozygous horses showed cysts generally located on the nasal part of the eye. The homozygous horses showed MCOA-syndrome with iridal hypoplasia (100%), cataract (85%), cornea globosa (56%) and lens luxation (8%). Some heterzygous Silver horses in this study showed no eye defects.
Silver dilutes most black to a chocolaty colour, as is the case with every colour, the shade of the colour can range a great deal from very little diluted coats to very diluted coats. Silver also affects the manes and tails of the horses. In some cases there is little flaxen/cream/white hairs to complete manes and tails of the lightened colour, which once again the shade can range largely. Some horses will loose the lightened mane and tail affects as it grows old and the mane and tail will grow more and more true to the body colour. On very pale Bay horses, the black points are lighted so much and the mane and tails are completely flaxen that the horse is confused for Flaxen Chestnuts (or even Palomino!). Silver horses with no other alleles will have black skin and dark brown eyes.
Basic Silver Colours
Below is a chart to help show what the basic colours of the Silver allele are. A basic Silver colour is when the Silver allele is combined with Extension and Agouti colours. On the left we see those basic Extension and Agouti colours and on the right we see what those colours are when the Silver allele is present. You can see the basic colours when combined with Silver listed below, there are many terms with the genome so the most basic names have been used to keep the blog very universal.
Black Silver: Black Silver Dapple, Chocolate, Blue Taffy
Bay Silver: Bay Silver Dapple, Red Silver Dapple, Red Chocolate, Red Taffy
Check out the Silver Horses album to see pictures of Silver horses: