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Anterior segment dysgenesis (ASD) is a genetic eye defect of the horse. Affected horses may have a variety of abnormalities of the eyes, including lesions and cysts, with defects in the iris, cornea, lens, and eyelids (Ramsey et al, 1999, Ewart et al, 2000). They do not respond normally to light, but affected horses don’t usually seem to have significantly impaired vision. Vets had noticed that anterior segment dysgenesis is more common in horses with the silver dapple gene than it is in other horses.
Ewart et al (2000) performed a study to determine wether anterior segment dysgenesis was inherited (Ewart et al., 2000). They studied the eye phenotypes and pedigree information from 516 horses in an extended Rocky Mountain Horse family. They found that ASD was inherited in a codominant manner in these horses. They concluded that it was being caused by a single gene, with cysts expressed in heterozygous horses and complex lesions expressed in the homozygous horses. Only in homozoygous horses was the disorder potentially a problem. Such horses should be bred to non Taffys to avoid passing the condition on to their foals. Nonpenetrance of the cyst phenotype appeared to occur in a small number of heterozygotes, and was associated with a particular lineage (i.e. not all heterozygotes showed eye abnormalities).
It would appear that either the silver gene can cause eye abnormalities (i.e. it has pleiotropic affects), or that (in some Rocky Mountain Horses) it is tightly linked to another gene that causes them. Horses with the palest manes and tails were most affected, possibly indicating that homozygous horses are also more diluted in color than heterozygous ones. Anterior segment dysgenesis does occassionally occur in horses of other colors. This might well mean that the gene for it is separate from the silver dapple gene (but closely linked to it). However it is also possible that another gene is involved in these cases.
We now know that there are probably two or more causative silver mutations in the Pmel17 gene. It is possible that one or other of these, or another mutation in the same gene, is associated with ASD.
Brunberg E, Andersson L, Cothran G, Sandberg K, Mikko S and Lindgren G. 2006. A missense mutation in PMEL17 is associated with the Silver coat color in the horse.BMC Genetics 7:46.
Ramsey DT, Ewart SL, Render JA, Cook CS, and Latimer CA. 1999. Anterior megalopthalmos and megalocornea of Rocky Mountain Horses. Vet Opthalmol 2, 47-59.
Reissmann, M. Bierwolf, J. and Brockmann, G. A. 2007. Two SNPs in the SILV gene are associated with silver coat colour in ponies. Animal Genetics 38: 1-6.
Ewart SL, Ramsey DT, Xu J-F, and Meyers DA. 2000. The horse homologue of congenital aniridia conforms to semidominant inheritance. Journal of Heredity 91, 93-98.