The recent posts on eye defects in dogs reminds me that I meant to share a recent paper on eye defects in silver horses. The issue of eye defects first came to light within the Rocky Mountain Horse breed, where it was initially called Anterior Segment Dysgenesis (ASD). That name was recently discarded in favor of Multiple Congenital Ocular Anomalies (MCOA).

Since its discovery, questions remained about whether or not MCOA was directly linked to the silver dilution, or if it was a more recent mutation tied to one of the Rocky Mountain founders. The recent study, “Multiple congenital ocular anomalies in Icelandic horses“, tied the issue directly to the silver mutation.

In this study we have shown that the MCOA syndrome is segregating with the PMEL17 mutation in the Icelandic Horse population. This makes the hypothesis that the MCOA mutation has recently arisen unlikely.

The Icelandic population is significant because it has been isolated from other domestic horses since 982 AD. If silver Icelandics have the same problem as silver Rocky Mountain Horses, then it is far more likely that the silver mutation is involved.

One of the things that makes this interesting is that the silver dilution – which is linked to eye defects in horses – occurs in the same location as the merling gene in dogs. Both are  PMEL17 (SILV) mutations. Both also dilute black, but not red, pigment. Those are interesting parallels between two colors that have such a visually different appearance.

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