Have you ever seen a dog whose eyes were each a different color? This condition is called heterochromia, and it’s due to variations in the amount of melanin pigment in the iris, which is the colored part of the eye. Heterochromia can be congenital (inherited) or acquired.
Inherited heterochromia is caused by specific genes that are passed to the puppy from one or both parents. All puppies have blue or bluish-gray eyes when they are born. The eye color changes as they mature, with the final color of the eyes appearing at about 16 weeks. Puppies with inherited heterochromia will display their differently colored eyes by that age. Puppies with different colored eyes usually have normal eyesight and no problems.
Acquired heterochromia, on the other hand, is worrisome. If an adult dog’s eye starts changing color, he needs to be seen by a veterinarian as soon as possible. Acquired heterochromia can be caused by trauma to the eye or by inflammation in the eye. Cancer, some autoimmune conditions, and bleeding disorders can cause ocular inflammation that, in turn, causes the eye or eyes to change color.
Types of Heterochromia
Inherited heterochromia manifests in three basic forms. Complete heterochromia, which is called “heterochromia iridis,” is when the two irises are different colors. Dogs with complete heterochromia are sometimes called “bi-eyed.”
The second form, “sectoral heterochromia” (called “parti-eyed”), refers to two or more colors in the same iris. This variation in color may include flecks, marbling, and /or geometric splits, which is when discrete segments of the iris are different colors.
When the center of the iris is a different shade than its remainder, it’s called “central heterochromia.” Often, the center color doesn’t form a perfect circle, but exhibits streaks or spikes of color that flare irregularly into the outer color.
The Inheritance of Heterochromia
Hereditary heterochromia is caused by specific genes that are often the same genes that determine coat color. Scientists have identified 15 genes that play roles in canine coat color phenotypes. Two genes known to produce heterochromia are named for the coat patterns they produce: merle and piebald.
The merle coat color consists of variegated patches of dark color over a lighter shade of that color. Merle comes in two colors: liver (red merle) and black (blue merle). The coat occurs due to an incomplete (or partially expressed) dominant gene, which means that only one parent needs the merle gene to pass it along to the next generation. With the merle gene, inherited heterochromia is predictive: Dogs who have the merle gene are more likely to have heterochromia than dogs without this gene.
Though it produces stunning coat colors, the merle gene is also associated with a host of ear and eye disorders. Ethical breeders avoid mating two dogs with merle characteristics, as the result of such a pairing is a 25% chance of producing “double-merle” dogs, which are prone to disorders that can include blindness and deafness. Unscrupulous breeders, or those who are ignorant or careless about the dangers of the double-merle genes, may produce blind and/of deaf dogs and pass them off to puppy buyers who are unaware of the additional challenges of managing and training a blind and/or deaf dog. Other unethical breeders may “dump” such puppies in shelters or with rescues.
The piebald gene, aka white spotting gene, causes a more random deletion of melanin that results in a dog with a completely white coat, or white patches and/or white spots on the dog’s coat. This gene can also result in heterochromia and deafness. The suppression of melanocytes by the piebald pigment gene can lead to degeneration of the auditory nerves that enable hearing in very young puppies. This inheritance is more complex than the merle gene, and much more research needs to be done to understand it.
Owing to a preponderance of the genes that cause heterochromia, certain dog breeds are far more prone to having eyes of differing colors than others. In addition to Australian Shepherds and Siberian Huskies, heterochromia occurs in Great Danes, Dachshunds, Dalmatians, Chihuahuas, Shih tzus, Border Collies, and quite a few lesser-known breeds. Some sources suggest that perhaps 10% of Siberian Huskies are either bi-eyed or parti-eyed.
What Matters Most
Breeding for good health is of paramount importance to ethical dog breeders; they want puppy customers to enjoy many wonderful years with a four-legged family member who can enjoy the world with all his or her senses, including eyesight and hearing. So, if you’re purposely looking for a dog with two different colored eyes – and many people do! – just be sure you find an ethical breeder. But make eye color (and coat color!) secondary to the pup’s personality and suitability for your home and what you eventually want to do with the dog. If you’re purchasing a purebred dog, go to the best breeders. Get to know the puppies for making a choice. And, if you get the perfect-for-you bi-eyed beauty, revel in your pup’s peepers!