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In some dog breeds, blue eyes can be an indicator of possible vision defects. While such offspring are not doomed to have eye problems, some do develop problems referred to as merle ocular dysgenesis or MOD.
Merle & Dog Genetics
Merle refers to a coat color found in numerous breeds, including Australian shepherds, and to the gene that causes the merle coloration. When sexual reproduction occurs, both parents contribute a set of genes to their offspring. Those genes can be dominant or recessive. The merle gene is dominant. If one gene passes to an offspring, the puppy shows a typical merle pattern without the risks of other problems. However, if both parents pass the merle gene, eye defects as well as deafness can result.
Blue Irises & Merle Genes
The merle gene typically causes a lightening of pigment in the coat as well as in the dog’s irises. Most dogs have brown eyes, so their eyes have a lot of pigment. Dogs who have blue irises have less pigment. Therefore, blue eyes can be a sign of a dog carrying merle genes. Blue eye color, however, does not automatically mean the dog will develop eye problems.
Potential Eye Defects
Merle ocular dysgenesis can cause a number of different eye defects in dogs. The most common MOD condition is microphthalmia, which causes an affected dog to have an usually small eye. In some cases, the eye can be so small as to be barely visible. Smaller eyes also lead to greater vision problems for the affected dog. MOD can also cause off-centered pupils. The degree of off-centeredness determines the severity of the vision problems facing the dog. In other cases, the pupils can have a starburst or eccentric shape, which can increase the dog's light sensitivity. Additionally, MOD can cause persistent papillary membranes, leftover tissues that did not form blood vessels, which can impair the dog's vision. Dogs with two merle genes are also at greater risk of developing cataracts.
Preventing MOD from occurring is not difficult. Breeders should refrain from breeding dogs who display merle-related characteristics to reduce the risks of their offspring inheriting two copies of the merle gene. In other cases, breeders may simply need to identify offspring that exhibit signs of homozygous merles -- two Merle genes present. For example, Great Danes who are white, harlequin or merle may contain two merle genes and if bred together may pass on two copies to their offspring. Those puppies could be examined for eye color, hair color in the inner ear, or merle-related coat colors.