MCOA

Multiple congenital ocular anomalies, MCOA, is the eye picture tied to the same gene that makes a silver coat. The Rocky Mountain Horse Association published the reading we use. Samson is tested ZZ. The foals we plan from him, out of non-silver mares, are not.

The study is Noah Anderson's 2020 genetic survey of the PMEL17 silver mutation, written while he chaired the RMHA genetics committee. The association posted the summary and the full report.

One gene, two doses

PMEL17 is the silver mutation. The shorthand is Z for the mutation and N for the ordinary allele. A horse can carry none, one, or two copies.

Two copies, ZZ, are the horses in which the eye changes are the ones a person can see and a veterinarian can name: changes to the retina, a large cornea, a pupil that does not work in the ordinary way, cataracts, and nearsightedness. The RMHA summary lists those from the earlier clinical work. That is the dose we do not set out to produce.

One copy, Zz, which laboratories often print as Z/N, is the other silver horse. The report says these horses typically have no major eye abnormalities. What has been found is a fluid-filled cyst inside the eye, and the studies have not shown that the cyst changes eyesight. The cysts are treated as benign. In the horse's ordinary life the impact on a horse that is not ZZ is generally not observable.

No copy, and this gene is not in the horse at all. Not every eye problem in a Rocky Mountain Horse is MCOA. The report says that plainly.

Why our book is non-silver mares

Samson is tested ZZ, so every foal of his inherits silver. A non-silver mare does not add a second copy. The foal is Zz. Samsons Legacy and Samsons Silver Mocha are both recorded Z/N, which is that result. A silver mare could add the second copy and make a ZZ foal. We do not take that mating. The breeding contract says the same thing.

The stud listing still prints Samson as Zz. That printing is the error. The test is ZZ. The Samson page keeps both sentences so a researcher can see which one we use.