Tag Archives | MCOA

Silver dilution and eye defects


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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The limits of visual identification


As genetic testing has become more common, and there are tests for more and more colors, one thing that has become apparent is that visual identification is not foolproof. Sometimes colors and patterns look alike, but prove to be two different things. The characteristic coloring of the Black Forest Horse is a good example of this.

Many have identified the horse in the picture above as a black silver. He is in fact a chestnut. In his case, there are clues that he is not actually a silver. Although the lighting in the picture makes it less obvious, his coat has red undertones that are more typical of a chestnut than a black-based silver. Even more noticeable is the red coloring at the top of his tail. Visually, it would be possible to guess he is not a silver. There are, however, a lot of Black Forest Horses that don’t give so many clues.


The Black Forest Horse was the subject of a study that determined the stallions currently standing were all genetically chestnut. (Stallions from each of the sire lines can be found here.) Even more interesting, some portion of them carried the rare alternate form of chestnut (ea) previous only known in the Asturcón and other primitive Iberian breeds. There was no correlation between the alternative chestnut allele and the distinctive flaxen-maned liver of the Black Forest Horses, though. Whatever causes the horses to be so dark, and to have such bright, contrasting manes is not currently known. It would be interesting to find out, because the horses have the same look as black silver without some of the downsides. Reports are that the Black Forest Horses keep their contrasting manes and tails over their lifetimes, a claim that is supported by the appearance of many of the older breeding stallions. Silvers, on the other hand, tend to have manes and tails that darken somewhat with age. Perhaps even more appealing, the coloring on the Black Forests is not linked with Multiple Congenital Ocular Anomalies (MCOA, formly known as ASD), an congenital eye defect thought to be caused by a gene residing close to the silver gene.

As more tests are developed, we are likely to find more and more of these look-alike colors and patterns. In the meantime, it is still fun to speculate about what might be found!

(All pictures in this post come from Wikimedia Commons.)

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