What makes some dogs have two different coloured eyes (Heterochromia iridis) ?

Most dogs have brown eyes, or at least eyes that fall within the brown colour spectrum, such as amber or chocolate. Blue eyes and green eyes are the next most common shades in dogs, but some dogs even have white eyes, or two eyes that are distinctly different colours!

The state of having eyes that are not the same colour as each other-such as one blue eye and one brown eye-is called heterochromia iridis, and it is in fact a genetic anomaly, which may come accompanied by other physical anomalies too.

If you know a dog with two differently coloured eyes and wonder what caused it, or are just interested in the genetics of what may cause a dog to have two differently coloured eyes, read on to learn more about heterochromia iridis in dogs, why it occurs, and what it means for the dog in question.

What dictates the eye colour of a dog?

By far the most common eye colour in dogs is brown, or shades from within the brown spectrum, such as amber. The colour of the eye is dictated by the melanin levels of the iris-and when puppies are born they have blue eyes because it takes a while for melanin production to begin, and the eyes only change colour as they age, or for some dogs, stay blue forever.

The blue eye colour is not strictly speaking a colour in itself, but a lack of colour-when the eye does not produce any melanin, or only produces it in very small amounts, the reflection of light off the eye leads to their blue appearance. For dogs whose eyes do change colour to become brown later on, this is caused by melanin production in the eye, and the more melanin the eye produces the darker they will be-so if a dog has green or light amber eyes, their eyes produce less melanin than a dog whose eyes are a deep chocolate brown.

How do dogs get odd coloured eyes?

In dogs with heterochromia iridis, the melanin level produced by the two eyes differs-leading to eyes of two distinctly different colours such as one brown and one blue, or one amber and one green.

Heterochromia iridis is a genetic anomaly, which is what makes odd eyes so rare-for a dog to display the trait, they must inherit a certain combination of genes from both parent dogs, and this is generally a recessive combination that would naturally be bred out of the gene pool with subsequent matings, unless a dog with two odd eyes was mated with another who also had odd eyes to match!

Ergo, it is challenging to deliberately breed a dog with two odd-coloured eyes, although the trait can appear even in a dog whose parents both had two eyes of the same shade. That said, because individual breeds of dog have distinct gene pools with only a limited amount of genetic diversity available from breeding dogs from within the same breed grouping, the trait tends to be more common in some breeds than others.

Siberian huskies and Australian shepherd dogs are two of the breeds that can most commonly be seen with odd coloured eyes, although the trait can theoretically present in dogs of any breed.

The level of melanin produced by the eyes is dictated by genetics, and so this in turn dictates the eye colours of the dog in question-but in rare cases, a recessive genetic anomaly or defect will lead to the two eyes producing different levels of melanin and so appearing different colours.

The trait can be caused by genetic mosaicism or chimerism, but the difference in colours will not affect your dog’s vision-because eyes of all different colours can of course see perfectly well!

However, because of the hereditary nature of heterochromia iridis and the fact that it is after all a genetic defect, sometimes the condition can indicate the presence of a larger genetic anomaly in the dog. A certain and rather unusual coat colour and pattern combination called blue merle, which is again caused by a genetic anomaly, can lead to a range of different physical manifestations of the gene, which may include a higher than normal possibility of odd coloured eyes.

This in itself is not a problem, but when two dogs that carry the blue merle gene are bred, this causes a further mutated gene expression called double merle, which is apt to lead to a range of problems in dogs that possess it.

The double merle gene can lead to problems with both the dog’s eyesight and hearing, as well as giving them a distinctive and attractive blue coat pattern, and potentially, odd-coloured eyes. Breeding of dogs that carry the merle gene should be undertaken with great care in order to avoid producing pups with the double merle gene and associated problems, and in many cases, dogs with the blue merle gene are not used for breeding at all.