Cat Colours

Heterochromia (heterochromia iridis) is the condition of having different-coloured irises. Some people call this having “Odd Eyes”. Having sectoral heterochromia myself, I have always wondered why this condition seems to be more common in cats than in humans. After having done some research, this is what I found:

The simple answer for why heterochromia is more common in cats than in humans is inbreeding. When it comes to inherited heterochromia, that I will touch upon in a bit, this gene is recessive (it is only expressed when both the mother and father pass on the gene) and is thus almost always masked by another more dominant gene. In animals, this is the same, but it is way more common for cats from the same litter to produce offspring together and thus express the gene for heterochromia. Incest is obviously highly frowned upon by humans. Heterochromia in humans is mostly due to mutations, which is also explained below.

There are three different types of heterochromia: complete (one iris is completely brown and the other one completely blue), sectoral (a part of the iris has a different colour, e.g. half of my eye is brown and the other half is blue) and central heterochromia (there is discoloration in a ring around the pupil).

In addition to this, heterochromia can be either inherited, congenital or acquired. Inherited heterochromia is mostly found in epistatic white cats (meaning that the cat is genetically another colour, but this is not expressed). These epistatic cats have the socalled masking white gene: this gene is dominant over other colours and thus “overpowers” the other colours that the cat should have otherwise got. If the trait is congenital, this means that the colouration of the irises is caused by a gene mutation. When we say heterochromia is acquired, it is caused by medications, iris tumours and/or trauma. Many believed David Bowie had Odd Eyes, because one of his eyes was darker than the other. This is often mistaken for acquired heterochromia but is, in reality, a condition called anisocoria. This means that one pupil is permanently dilated, and in Bowie’s case, this happened due to a fight he had as a teenager.

Another fun ‘eye fact’ is that all cats are born with blue eyes. Melanin determines what eye colour an individual gets. The more melanin, the darker your eyes: browneyed cats have the most melanin and blue-eyed cats have the least. When born, there is hardly any melanin present in the eyes, however, at around 6-7 weeks, melanocytes (cells that produce melanin) begin to produce melanin, thus potentially changing the colour of the eyes. This process is triggered by light and is more or less the same in humans, pretty much all babies are born with blue eyes.

And lastly, a common misconception is that heterochromia in cats is an indication that the cat is deaf: this is an especially well-known myth when it comes to white cats. Heterochromia does not imply the cat is deaf, however, it can occur in cats with heterochromia. The deafness is then found in the ear that is on the same side as the blue eye and is caused by the same white masking gene that was mentioned before. This same gene (that can cause heterochromia) also causes deafness in some cats. However, just from looking at a white cat with heterochromia, you cannot tell if that is the product of a white masking gene or another reason.

Tortoiseshell cats are per definition females. You might have never known this, but what you probably will know is that females have two X chromosomes and men only have one. The genes that result in a tortoiseshell cat are located on the X chromosome. In order to get this black, brown and orange fur coat, cats will need two X chromosomes, and this is why: when two dominant genes are put together from two X chromosomes, that separately produce black/brown and orange fur, they are called “co-dominant” - meaning that they will both be expressed. The result will be a tortoiseshell cat. Each individual cell will decide randomly whether it is going to produce black/brown or orange fur and that is how you get the patchy fur coat that we have in mind when thinking of these cats.

Furthermore, this same genetic concept explains why orange cats are usually male (however, not exclusively). The gene for orange fur is also carried on the X chromosome. As males only have one of these X chromosomes, the presence of just one orange gene (that they get from their mother) is enough for them to become orange. However, in females, you will need to have two of these genes, so both the X chromosomes need to carry this, for this colour to be expressed. Any other colour will override the orange colour, so if the dad or mum does not pass on this gene, there will be no orange female offspring. Cats add a splash of colour to all of our lives and hopefully you will now understand a little better where they get theirs from!

By Giulia Pipolo