There is physics behind the answer to the question, which may displease some readers and I’ll understand why! By six or seven weeks of age a kitten’s eyes begin to change colour from blue to the colour that their genes dictate that their eyes should be. In human babies the change takes place at 6-8 months of age. I have discussed this before and you may know why kittens’ eyes are blue. It’s because they do not contain pigmentation in the iris and the colour blue is a result of the refraction (scattering) of sunlight through the eye. The physics behind this process is identical to the physics which creates a blue sky.
Why the sky is blue
White light from the sun is made up of all the colours of the rainbow. The gases in the atmosphere of the earth scatters this white light. It breaks up the white light by separating blue and violet from the rest of the light. These colours are scattered in every direction which is why we see blue. The other colours of the spectrum pass straight through the atmosphere to our eyes. The reason why blue is scattered is because the wavelength of blue light is the right size to hit and bounce off the molecules of gas in the atmosphere.
Pigment producing cells – melanocytes
At about seven weeks, the pigment producing cells in the iris, called melanocytes, start creating melanin, which is the pigment that creates the colour in the iris of the eye. As you know, there is a huge range of colours. If an adult cat’s eyes are to be blue the pigmentation is not created or not fully created. The blue color is entirely due to white light refraction throughout the cat’s life. The same applies to humans. People with blue eyes feel the effects of bright sun more than others because their irises are missing pigmentation which blocks the sun’s rays. They should wear sunglasses.
Sometimes the melanocytes do not produce pigment because they are switched off (my terminology) by the presence of a more dominant gene which might be the dominant white gene or the piebald gene (white spotting gene).
Dominant white and white spotting genes
The dominant white gene creates a white cat. The hair strands are devoid of pigment. The eyes are also devoid of pigment which is why they are blue in white cats. Sometimes white cats have odd-eye colour meaning that one eye is coloured (usually a radiant yellow) and the other is not i.e. it is blue. FYI – you may know to that white cats are not infrequently deaf because the gene referred to affects the development of the auditory system in the cat. I’m told that the hearing deficit is due to “degenerative changes in the succule and cochlea” (Robinson’s Genetics) which in turn is due to a lack of migration and viability of the cells from the neural crest during development. Deafness might be bilateral or unilateral (in one or both ears).
The white spotting gene has a similar effect on eye colour which is why you might see a bi-coloured cat with blue eyes or odd-eyes. You won’t see odd-eyes in a tabby cat for instance because this type of cat does not carry the white spotting or piebald gene except perhaps for the Ojos Azules cat.
Eye colour, as mentioned, varies tremendously. The colour may show shades from emerald green to grey-green to gold, from yellow to deep copper or from pale grey to sapphire blue depending upon the cat breed or the individual cat.
Sometimes eye colour is controlled by a single gene (monogenic). The blue or bluish colour (sapphire blue) “produced by the genes for dominant white, Siamese, and blue-eyed albino and the unique aqua eye created in the Tonkinese heterozygote” are examples (Robinson’s Genetics for Cat Breeders and Veterinarians Fourth Edition)
