Quick Summary
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Phenotype: Multiple congenital ocular anomalies (MCOA) is associated with the silver dilution and is characterized by ocular cysts, enlargement of the cornea, abnormally formed iris/retina, and additional abnormalities.
Mode of Inheritance: Incomplete dominance
Alleles: N = Normal/Unaffected, Z = Multiple congenital ocular anomalies; Silver
Breeds appropriate for testing: Appaloosa, Arabian, Ardenne, Finnhorse, Gypsy Cob, Gypsy Vanner, Icelandic Horse/Pony, Kentucky Mountain Horse, Miniature Horse, Missouri Foxtrotter, Morgan Horse, Mountain Pleasure Horse, Norwegian Nordland, Paint Horse, Quarter Horse, Rocky Mountain Horse, Saddlebred, Shetland Pony, Swedish Warmblood, Welsh Pony, related breeds
Explanation of Results:
- Horses with N/N genoytpe will not have multiple congenital ocular anomalies (MCOA) or the silver dilution, and cannot transmit this MCOA variant to their offspring.
- Horses with Z/N genotype will likely develop a less severe form of multiple congenital ocular anomalies (MCOA), and will be silver dilute. They may transmit this MCOA variant to 50% of their offspring. Matings with N/N horses will result in a 50% chance of producing a foal that will likely develop a less severe form of MCOA.
- Horses with a Z/Z genotype will likely develop a more severe form of multiple congenital ocular anomalies (MCOA) and will be silver dilute. These horses will pass down this MCOA variant to all of their offspring. Matings with any genotype will results in a foal that will likely develop some form of MCOA.
Chestnut horses with one or two copies of the MCOA variant will not be silver dilute but are still likely to develop ocular abnormalities/MCOA.
Sample Collection
Horse DNA tests are carried out using cells from the roots of a hair sample (roughly 20-30 hairs).
1. Grab about 10 hairs at the base.
2. Wrap the hairs around your finger and give it a quick pull.
3. Check the ends to make sure the pulled hairs have roots.
4. Repeat the process until you have collected about 20-30 hairs with intact roots.
5. You can choose different places on the mane or tail. NOTE: For foals, we recommend pulling all hairs from the tail only.
6. Tape the hairs to the submission form and fold the form along the dotted line to protect the sample. Do not use ziploc bags as they can cause condensation that allows mold to grow on the hair.
7. Place the folded form containing the sample in a paper envelope and mail it to the laboratory.
Multiple Congenital Ocular Abnormalities syndrome (MCOA) is an eye disorder associated with presence of the silver dilution mutation in PMEL17 gene. MCOA is characterized by defects that occur in the anterior and posterior segment of the eye. The severity of the syndrome is dosage related; it varies depending on the number of copies of the silver mutation present. Horses with 1 copy of silver (N/Z) have less severe signs (Cyst phenotype), typically presenting as translucent cysts (or fluid filled vesicles) that can be 1 centimeter in diameter. Horses with 2 copies of silver (Z/Z) have more severe signs (MCOA phenotype) with cysts and additional abnormalities such as enlargement of the cornea, abnormally formed iris and/or retina, among others, which impair vision and can cause blindness.
MCOA shows incomplete penetrance, which means that there is variation in presentation of MCOA among horses with N/Z and Z/Z genotypes. Research suggests that there may be a progressive change in vision of horses with the silver mutation. To avoid producing offspring with the severe MCOA associated with 2 copies of silver mutation, breeders should not mate two horses with silver mutation (N/Z or Z/Z, regardless of base color) to each other. Although the silver mutation has no effect on the coat color phenotype of chestnut/sorrel horses, it can cause ocular abnormalities if present. Chestnut horses from silver lineages should be tested to inform breeding decisions. Regardless of base color, it is advisable that horses with the silver mutation be examined by a veterinary ophthalmologist to determine the extent and severity of ocular condition.
Testing for the silver dilution/MCOA is important in assisting clinicians, owners, and breeders in identifying affected horses. Breeder can use results from the test as a tool for selection of mating pairs to avoid producing homozygous (Z/Z) individuals.