Why Some Dogs And Cats Have Two Different-Colored Eyes
It’s more common than you think.
Animal eyes come in a variety of beautiful, striking colors — blue, green, brown, violet, yellow and, occasionally, a combination of two or more.
This condition, known as heterochromia iridum, has inspired quite a few myths and legends. One myth even claims that dogs with this special trait have the power to see heaven with one eye and earth with the other.
So what is the real reason why dogs and cats have two different-colored eyes? The cause of this trait is not quite as mystical as it seems.
You can chalk this variation in eye pigmentation up to good ol’ genetics, explains veterinary ophthalmologist Dr. Laura Proietto.
“Much like many other details of our body who make us who we are, DNA is the culprit. The color of the eye is determined by multiple genes that program how much pigment is present in the iris itself,” Proietto tells The Dodo. “A blue eye has less pigment than brown and this difference occurs during development in utero.”
This differentiation in melanin, the pigment that gives our skin, hair and eyes their distinctive shade, is evident as soon as a puppy or kitten takes her first look at the world.
It’s more common in certain breeds
The more variation in coat pattern, the more likely a dog is to have blue eyes or different-colored eyes, notes Proietto. The difference in color between eyes (heterochromia iridum) or within the same eye (heterochromia iridis) is more common in breeds that carry the merle gene, which produces coats with a “great variation in pigmentation,” such as dapple, harlequin or white coats.
While rare in breeds like Labradors and poodles, explains Proietto, having eyes of different colors (sometimes referred to as being “bi-eyed”) is fairly common in Australian cattle dogs and shepherds, boxers, collies, Great Danes, dachshunds, Dalmatians, malamutes, Old English sheepdogs, Siberian huskies and Weimaraners.
In cats, the trait can be found in breeds including Siamese, Burmese, Abyssinian and Persian. Complete heterochromia can often be found in white cats, where the white gene or white spotting gene creates a loss in pigment, turning one eye blue and the other green, yellow or brown. A feline with this coloring is commonly referred to as an “odd-eyed cat."
It can make animals more sensitive to light
There are no specific health risks associated with the condition, but you may want to keep your pet out of bright sunlight — or get them a pair of shades. “Heterochromia iridis [color variation centralized to one eye] can make dogs and cats more sensitive to sunlight,” Proietto explains. “Without the protection of pigment to help block the bright light, even when the pupil is constricted, more light reaches the retina at the back of the eye which can be painful like when we see a flash or accidentally look at the sun.”
They are otherwise healthy
When occurring naturally, a dog or cat born with this unique eye color is usually perfectly healthy — however, they should never be bred that way on purpose, as animals can be born deaf or blind. The condition should not affect a dog or cat’s eyesight unless they have other genetic abnormalities in eye structure sometimes associated with heterochromia, such as a coloboma, explains Proietto, which in rare cases can cause blindness.
Of course, for concerned pet owners, it’s better to be safe than sorry. “If your pet has a merle, dapple, harlequin or white hair coat it is important to have their eyes evaluated in case they have other developmental abnormalities that aren't seen as readily as the color of their eyes,” Proietto recommends. “This is often the sole manifestation of color variation, but can indicate that further examination would be in their best interest.”
The only other time pet owners should be truly concerned about eye color? “If the color of the eye changes. That is never considered normal and is an indication that something is going wrong.”