The total amount of melanin (pigment) in the iris of the eye determines the range of eye colours in cats and in humans. The same principles apply to all animals with a similar eye anatomy.
The pigment ‘melanin’ is produced by cells called melanocytes. The amount of pigment produced and the way that melanocytes produce the pigment and how it is delivered is controlled by genetics. Genetics are complicated. I don’t intend to go over the genetics of eye colour in real detail although I touch in genetics in this article. I can leave thr details to Sarah Hartwell (messybeast.com). She’s the expert.
Suffice to say that eye colour in cats shows a huge variation from blue through to brown. These two extremes in eye colour are caused by no pigmentation for blue eyes and a maximum amount of pigmentation for brown eyes. The reason why cat eyes are blue is because of the refraction of daylight through the eye. Blueness in eyes is not caused by a blue pigment but by white light refraction. But deep brown eyes are caused by the full presence of melanin in the iris. In between these two extremes there is green, yellow, gold and copper. I discuss eye colour in more detail on another page which you can access by clicking on this link.
The different degrees of melanin or pigmentation in the iris results in the differences in colour when combined with a variation in the degree of refraction of light through the eye and the effect of genes such as the dilution gene. My assessment comes from the fact that melanin is a dark brown colour. It is a single colour. Therefore to obtain the variations in colour from blue through to brown the brown pigmentation needs to be modified by different degrees of refraction and the effect of genes on melanin production. “Refraction” is quite a complicated physical property which you can look up but in essence it occurs when white light passes through one medium to another which causes the light to be split. This happens in the sky when blue light is seen because the white light has been refracted by minute particles in the air. Blue light is always refracted off.
In domestic cats, selective breeding results in the pedigree, purebred cats having striking eye colours. The range of eye colour in cats and people is due to a polygenetic action. “Polygenetic” means more than one gene. Therefore it is a complicated science. My book on cat genetics (Robinson’s Genetics) tells me that “there appears to be no reliable evidence of monogenetic control of eye colour” (i.e. single gene control).
The cat genes for dominant white, Siamese, and blue-eyed albino and the unique aqua eye created in the Tonkinese cat influence eye colour. Also, the brown and dilution genes (i.e. the genes that create a diluted coat color) can have an effect on eye colour. Although the effect is masked by the range of variability seen in eye colour.
You probably know this but all kittens are born with blue eyes because for the first few months of life eye pigment is being built up to create their adult eye colour. As young kittens don’t have any pigment in their eyes they are blue for the reasons stated above i.e. refraction of white light.
The deep sapphire blue eyes in pointed cats such as the Siamese are “a result of the pigment producing effects of the albinism allele combined with a low inherent amount of pigmentation”.
Odd-eye color is due to the dominant white gene or piebald gene affecting the depositing of melanin in one of the eyes. It prevents the melanin being deposited in the iris which turns the eye blue while the other eye has pigment which is why it is usually yellow.
To conclude and to recap, the cause of the range of eye colours in domestic cats is the cat’s genetics. A cat’s genes are inherited. The genes dictate the production of melanin and the amount of melanin in the iris of the eye dictates the eye’s colour together with the refraction of light.
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