Heterochromia in Animals

Purrrfect: Complete heterochromia in a female cat

This difference in colouration – in humans or animals – is called heterochromia. Usually found in the iris, heterochromia can also affect the hair or skin, though we're going to focus on eyes here, and specifically those of man's beast and second beast friends, dogs and cats.

What have we here? Mismatching eyes in a cat

Heterochromia occurs due to a relative excess or lack of the pigment melanin. Think loosely along the lines of Michael Jackson, except more likely to be inherited, resulting from genetic mosaicism (cells with different genotypes), or due to disease or injury – the possible causes of heterochromia.

Bright eye: Heterochromia of the eye in a pet dog

When someone tells you you've got nice eyes, what they're really saying in a less clinical way is that they like your distribution and concentration of melanin, as this is what determines your eye colour. Irises affected by heterochromia are either hyperpigmented or hypopigmented.

We've been expecting you Mr Bond: Domestic cat with full heterochromia

Heterochromia of the eye also comes in two packages: complete heterochromia, where one iris is a different colour from the other, and partial heterochromia, where part of one iris is a different colour from the rest of it. Partial heterochromia is less common – and less eye catching.

Eye eye: Siberian Husky with complete and partial heterochromia

OK, so now most of the boring science bit is done, on with the animals. Although rare in humans, complete heterochromia is more common in other species, and almost always involves one blue eye – often found in a white patch, where melanin is lacking from the skin and hair.

Introducing Kitty Pazzaz: An archetypal odd-eyed cat

Cats are one of the most commonly affected species, especially breeds like Turkish Van and Turkish Angora. Known colloquially as odd-eyed cats, these flash felines are usually white or mostly white, with one blue peeper and one normal eye of copper, orange, yellow or green.

Different again: Heterochromia in non-white cat with green eyes

The odd-eyed colouring is caused when either the dominant white gene or white spotting gene stop melanin granules from reaching one eye during the cat's development from kitty to moggy. The condition also affects cats of colours other than white, so long as they have that white spotting gene.

Say cheese: Flash photography effect on blue but not green eye

But we're digressing into genetics again. When taking snaps of odd-eyed cats, a red-eye effect can be seen in the blue coloured eye but not the other. Eyeshine is produced in both eyes but in the normal, non-blue peeper a layer of melanin removes certain light colours.

What are you looking at? Copper bi-eye Siberian Husky

As for dogs, complete heterochromia is commonly observed in Siberian Huskies, the eyes of which are normally light blue, dark blue, amber, or brown.

Also starring Scarlet: Complete heterochromia in a Siberian Husky

With complete heterochromia, one eye of the husky may be brown and the other blue, or with partial heterochromia, both may be half brown and half blue.

Whether you find heterochromia in canines and felines unnerving, alluring, or just plain different, you can't deny it's eye-catching.
What's more, who's to say the pets in question aren't peering back at gawpers wondering why their eyes are so boringly homochromatic.

Sources: 1, 2, 3

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