A while back, I was doing some reading on equine color genetics when I came across information about the silver dapple gene.  I’m always interested in learning new things and when I started to read about this dilution gene, I immediately perked up.  The silver dilution gene acts on black points of the body and lightens them to varying degrees.  This can be expressed as either “black silvers” (white mane/tail, seal brown body, often dappled) or “bay silvers” (chocolate body, silver mane/tail that is darker at the roots, mottled silver legs).  I believe that Tiny may have the bay silver gene, which would explain her unusually light-colored legs and the silver in her mane. 

As I continued learning about the intricacies of equine coat color genetics, I discovered that the silver dapple gene is sometimes associated with ASD – Anterior Segment Dysgenesis.  ASD is an abnormal development of the eye and can result in impaired vision.  ASD is not restricted to horses with the silver dapple gene, but it is thought that there is a connection between the two conditions.  Horses are born with ASD – it is a congenital condition with varying degrees of severity.  There are several signs that a horse has ASD:  a ”pop-eyed” appearance, cysts, cataracts, or abnormal dilation of the pupil.  The retina is also occasionally affected.    So what should you do if you suspect your horse might be affected?  First, have him examined by an equine opthalmologist that has the necessary diagnostic equipment.  It is important to remember that ASD is not progressive and does not result in blindness, and that most affected horses continue to have normal vision throughout their lives. 

Further Reading:

A missense mutation in PMEL17 is associated with the Silver coat color in the horse.  2006.   Brunberg, et al.

http://www.horse-genetics.com/silver-dapple.html

http://www.silverequine.com/

http://old.cvm.msu.edu/research/mregl/asd.htm