Why Do Some Dogs Have One Blue Eye?

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Generally, dogs have brown eyes; however, once in a while, a dog will mix it up and have blue eyes or even one blue and one brown eye -- a condition known as heterochromia. Like you, your dog's eye color is a function of genetics. In dogs, the eye color gene is OCA2, which differentiates blue from brown. Coat color also influences a dog's eye color.

Pigmentation and Eye Color

Eye color is determined by the amount of pigment located in front part of the eye's iris. Though eye colors vary, the iris contains only brown pigment. If a dog's irises contain a lot of pigment, his eyes will look dark or brown; if they have little pigment, his eyes will be lighter in color, such as blue. Young animals often have blue or gray eyes soon after birth because their eye pigmentation hasn't fully developed. The OCA2 gene comes into play because it determines how much pigmentation is in the eye.

One Brown, One Blue

Another term for heterochromia is wall eye. Sometimes a dog may have different amounts of pigmentation in his eyes -- the way his OCA2 genes played out may result in more pigmentation in one eye than the other. Coat color makes a difference, too. Merle dogs, such as border collies, are prone to wall eye because the merle gene dilutes parts of a dog's pigment, including that in his eyes and nose. The more diluted the merle coat is, the more likely the dog is to have blue eyes.

White Coats

When a dog has areas on his coat that are white, those are places where his cells don't produce pigment. If he has large patches of white on his face, chances are his eyes and nose are affected as well -- meaning his eyes may be blue and his nose pink, an unusual affect for a dog. Unlike rodents, dogs don't take on pink eyes when they lack pigment; they are more similar to human albinos, showing blue eyes when pigment isn't present. Dog Genetics reports there are no confirmed cases of full albinism in the dog world, though there is an intermediate form of albinism that causes blue eyes in dogs.

Split Eyes

Once in a while, you may see a dog with blue and brown within the same eye. This condition is called split-eye. It is caused by the random pigment loss associated with the merle gene.

Unusual, Not Valuable or Harmful

Blue-eyed dogs are certainly striking, and a wall-eyed or split-eyed dog certainly gathers his share of attention. Despite their eye-catching looks, dogs with blue eyes, split eye or wall eye are no more valuable or less valuable than the more plentiful brown-eyed specimens. As well, there is nothing medically wrong with a heterochromic dog; his vision is normal.

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