Multiple Congenital Ocular Anomalies (MCOA)
Summary
Multiple Congenital Ocular Anomalies (MCOA) syndrome is an inherited condition affecting horses with Silver coat color dilution. Horses homozygous for the Silver allele have a wide range of ocular defects, including underdevelopment of the iris, congenital glaucoma presenting as megaloglobus (greatly enlarged eyes), iridociliary cysts (uveal cysts, small cysts in the eye), and cataracts. Horses heterozygous for the Silver allele have less severe defects, primarily iridociliary cysts.
Breeds that have been diagnosed with MCOA include the Rocky Mountain Horse (including the two related breeds Kentucky Mountain Saddle Horse and Mountain Pleasure Horse), Icelandics, Shetland Pony, Exmoor Pony, American Miniature, Belgian Draft, and Morgan.
Date of Last Update: 08/02/2016
Results
Understanding the Results
Results of the genetic test for Multiple Congenital Ocular Anomalies (MCOA) syndrome are presented as shown below.
| Multiple Congenital Ocular Anomalies (MCOA) | ||
|---|---|---|
| n/n | Clear | This horse tested negative for MCOA. |
| n/Z | Affected | Both the normal and mutant alleles are present. This horse is positive for the PMEL-R625C mutation and will develop mild symptoms. |
| Z/Z | Affected | This horse carries two copies of the PMEL-R625C mutation and will develop symptoms. |
Disease Name and Genes
Multiple Congenital Ocular Anomalies (MCOA) syndrome is caused by a C to T substitution in the premelanosome protein PMEL that results in a missense mutation that changes an Arginine (R) to Cysteine (C) at codon 625 (PMEL-R625C).
Inheritance
Multiple Congenital Ocular Anomalies (MCOA) syndrome is inherited as a semidominant trait associated with the Silver coat color dilution trait (PMEL-R625C). The semidominant allele is commonly abbreviated as Z, with the recessive wild-type allele abbreviated as n.
A horse with one copy of the semidominant allele (n/Z) will have mild symptoms. If this horse is bred to a normal horse (n/n), each foal has a 50% chance of having one copy of the semidominant allele (n/Z) and a 50% chance of having two copies of the normal allele (n/n).
If two horses, each with one copy of the semidominant allele (n/Z), are bred, each foal has a 25% chance of having two copies of the normal allele (n/n), a 50% chance of having one copy of the semidominant allele (n/Z) and being mildly affected, and a 25% chance of having two copies of the semidominant allele (Z/Z) and being strongly affected.
If a horse with two copies of the semidominant allele (Z/Z) is bred to a normal horse (n/n), all of the foals will have one copy of the semidominant allele (n/Z) and will be mildly affected.
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