Heterochromia

This Siberian husky has heterochromia.

I received this suggestion recently and I thought it would make a good topic. Heterochromia is, very simply, when an individual has two different eye colors. One may be blue and the other brown, or many of a number of other combinations. It may even be partial, where only a part of the eye is differently colored. Though uncommon in people, it is quite common in dogs, cats, and certain other animals.

This merle dog has heterochromia

In dogs, of course, there are a number of causes of heterochromia, either partial or complete. Merle is, of course, a very common source of this phenotype. The merle gene causes eumelanin pigment (i.e. black, and the dilutes thereof) throughout the body to be randomly diluted, while other areas will remain fully colored. This not only involved the coat, but also the skin and eyes as well. Thus, it is very common for merle dogs to have pink spots on their noses, pads, lips, or around their eyes. Also, it is very common for merle dogs to have blue spots in their eyes. In both cases, a complete loss of pigment from a select area is not unheard of, such as a completely blue eye or eyes or a completely pink nose. This is especially true of double merles, but is still very possible in dogs with a single copy of the gene.

This extreme piebald dog has a blue eye

Another gene in dogs that can cause blue eyes is the white spotting gene. It has numerous forms, as several different alleles control the amount of color on the body. Anything from Irish white to extreme white piebald can result in dogs with either one or two blue eyes. Again, like merle the white spotting gene causes a loss of pigment on the body. This may only affect the coat, or it may go on to affect the skin and eyes as well. This is why breeds with substantial white markings tend to have puppies with one or more blue eyes born far more often than breeds who are always solidly colored. It is usually considered quite startling to find a dog that is solid, especially solid black, with blue eyes, and that is due to the relative rarity of this occurance

This husky shows classic "bi-eyes"

The third major reason why a dog will have blue eyes is due to a specific inherited gene that controls eye color. This is what causes "husky blue eyes," where dogs will have a blue eye or eyes regardless of their color or markings. Both partial and complete heterochromia are seen, sometimes even on the same dog. Or at least depending on how you're defining the color different. Due to the this gene mainly being found in huskies and other spitzes, it is very common for a dog to be suspected to be a husky mix simply based on its eye color. However, since other breeds can have dogs crop up with one or two blue eyes, and sometimes just a speck of blue in an otherwite brown-eyed dog, it is likely that the gene is present in the domestic dog population as a whole. Either that or there is new mutation occurring to cause the surprise blue eyes.

I will be expanding on this soon with a follow-up post on heterochromia in humans and other animals. In addition, I will look at heterochromia and how it relates to disease and also certain specific types of mutation.


Images are from Wikimedia Commons under Creative Commons licenses: one, two, three, four.