Also known as dichroic eyes, heterochromia iridis (hetero = different, chromia = colours, iridis = iris), is a condition in which cats have odd-coloured eyes. One will be blue, the other may be green, copper or brown. The condition may be inherited, congenital or acquired.
- Inherited heterochromia, it is mostly found in epistatic white cats (epistatic means the cat is genetically another colour, such as black or grey, but it is hidden by the white masking gene) or in bi-colour cats, due to the white spotting gene.
- Congenital heterochromia may be due to mosaicism in which two distinct populations of cells arise from one fertilised egg, or chimerism in which two zygotes merge early on in the development stage. This possibly explains heterochromia in solid coloured cats who don’t carry the white spotting gene.
- Acquired heterochromia may be caused by uveitis, certain medications, iris tumours and trauma.
Back to epistatic white cats
People commonly associate white animals with albinism, and in many cases this is true, but with cats, true albinism is quite rare and when it does occur, the eyes will be very pale blue or pink. It should be noted that a form of albinism occurs in cats which is responsible for the pointed pattern most commonly associated with Siamese cats but is also found in other breeds such as the Tonkinese, Burmese and Himalayan cats, it can also occur in mixed-breed (domestics).
Most white cats with odd or coloured eyes are white due to the white masking gene (W) a dominant gene, which is why it is represented with an uppercase W. Recessive are shown in lower case w, which symbolises non-white. This gene masks the effect of all other colour genes. It is this gene which is also responsible for deafness in some white cats with blue eyes. Remember that cats have two sets of genes, one from their mother and one from their father. Three combinations can occur:
It is also possible for a cat to be white due to the white spotting gene, which presents in varying grades from 1-10. The higher the grade, the more white, and in some cases only a tiny patch containing only a few coloured hairs may be present.
Coat and eye colour determination
Melanin is a complex polymer found in the hair, skin and eyes, it is responsible for determining your cat’s coat and eye colour. Its role is to protect the skin and eyes from UV radiation. The amount of melanin in the iris determines the colour of the eye. Cats with blue irises have the least amount of melanin and brown eyed cats have the most. All kittens are born with blue eyes, but for those whose eyes change colour, melanocytes begin to
produce melanin in the eye from 6-7 weeks of age when light hitting the eyes triggers this event. As melanin concentration builds up, the eyes begin to change colour. Cats who are epistatic white or cats with the white spotting gene, melanin is prevented from forming in the iris. Epistatic white cats with one blue eye may also be deaf on that side too as this particular gene can cause degeneration of the inner ear.
Types of heterochromia
Heterochromia iridis comes in three forms, complete, central and sectoral.
Complete heterochromia: One eye is blue, the other eye is a different colour (green, yellow, copper).
Central heterochromia: Two colours are present in one eye, usually there is a ring or halo surrounding the pupil with another colour on the outer portion of the iris. This type of heterochromia is rarely seen in cats.
Sectoral heterochromia: Two colours are present in one eye, with one portion being blue, and the other a darker colour.
David Bowie’s odd eyes
Interestingly, David Bowie was known for his odd coloured eyes, and many believe he had heterochromia, however, his different eyes are due to a condition known as anisocoria. This condition occurs when one of the pupils remains permanently dilated (large). David Bowie’s anisocoria is said to be the result of a fight he had with a friend as a teenager. The fixed pupil gave him the appearance of one blue eye and one darker coloured eye.
Image courtesy Martin Cathrae, Flickr
Image courtesy Chris Yarzab, Flickr
Before I finish this article, I would like to mention dark spots which can sometimes appear in your cat’s eyes, which are known as iris freckles, iris hyperpigmentation or iris melanosis. They are similar to moles that people get. Brown spots should always be checked out by a veterinarian as in some cases they can become cancerous over time or more commonly, lead to glaucoma. If you notice any changes to your cat’s eye, seek medical advice.