Breeding Two Silver Gypsy Cobs
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All evidence points to the
All evidence points to the eye abnormalities being caused by the silver mutation rather than a closely linked mutation. The evidence for this are recent studies that show that eye abnormalities are much more prevalent in silver horses than previously thought, the existence of eye abnormalities within silver horses of the Icelandic breed (an ancient breed that is not closely related to Rocky Mountain Horses), and the fact that mutations in PMEL17 (were silver is located) cause eye abnormalities in other species. I would therefore be very surprised if the occurrence of these abnormalities were more or less prevalent in one breed than another. The problem with trying to verify this is that only the worst of the abnormalities are apparent without specialized tools. A recent study (jan 2013) has shown that
In 51/73 (69.86%) carrier horses, ultrasound detected ciliary cysts that were missed with direct ophthalmoscopy
This means that the eye abnormalities in silver are much more prevalent than originally thought with the majority of silver horses having abnormalities rather than the other way around. Now, that being said. It is true that in the majority of cases where the horse is heterozygous, these eye abnormalities (mostly cysts) don't cause the horse any visual problem. However, horses that are homozgyous for the Silver mutation display more severe abnormalities in all the breeds studies.
I know that doesn't answer your question directly but I hope it helps a bit.
Here is a list of my sources.
http://onlinelibrary.wiley.com/doi/10.1111/vop.12021/full
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