The blue colouration you see in a cat’s eyes is due to special pigment cells in the iris known as melanocytes. There are two layers of the iris which contain melanocytes, the stroma is the outermost layer, containing loosely packed melanocytes, the innermost layer, located closer to the rear of the eye, is the epithelium which contains densely packed melanocytes.
The number of melanocytes in the stroma is responsible for the eye colour, high numbers of melanocytes produce brown eyes/copper eyes, fewer melanocytes produce green eyes and very few melanocytes produce blue eyes. A complete absence of melanocytes results in red eyes. The blue eye colour can range from a very pale blue to a deep royal blue.
All kittens are born with blue eyes, which if they are going to change colour, will have done so by around 8 weeks of age.
There are three common reasons a cat will have blue eyes:
1) Dominant White (epistatic white cats) – The gene responsible for blue-eyed white cats is symbolised by the letter W, it is dominant and therefore only one copy is required for the offspring to inherit the trait. This gene suppresses the expression of the coat colour. Which means, for example, you can have a genetically black cat (or red), but if he carries the W gene, the colour will be masked. Blue-eyed, white cats are quite often deaf as the W gene also affects the development of the specialised cells within the ears.
2) Albinism – There are five known alleles for albinism in cats. Full colour (C), Burmese (cb), Siamese (cs), blue-eyed albino (ca), pink-eyed albino (c). Siamese cats (cs) and blue-eyed albino cats (ca) are the only two from the albino group who have blue eyes.
The gene responsible for the Siamese cat and other colourpoint breeds (cs) is recessive, which means the offspring must inherit a copy from each of the parents. In Siamese/colorpointed cats, the gene causes partial albinism, (known as Himalayan albinism) and is also found in other animals such as the rabbit or mouse. The gene causes the coat colour to be heat sensitive, so the cooler extremities such as the nose, ears and tail show colouration and the rest of the body are cream to white.
Blue-eyed albinos are different to epistatic white cats, in the latter, the pigment is masked. There is pigment in the irises with both types but it is greatly reduced compared to other eye colours. Blue-eyed albino cats are so because of an absence of colour, not a masking of colour, as in point no. 1.
3) White spotting gene (piebald) – White-spotted cats are very common, the amount of white can range from almost complete to just a few hairs of white (known as lockets). If the white spotting occurs around the eyes, they will be blue in colour.
4) The Ojos Azules is an extremely rare breed of cat with a coloured coat and blue eyes. Unlike the blue-eyed Siamese who can be prone to strabismus (crossed eyes) and the epistasis white cat who may also be deaf, the gene doesn’t appear to affect the eyes or ears. However, it does cause other congenital problems in the homozygous form (ie; the cat carries two copies of the gene, one from each parent) including cranial defects and stillbirth. To avoid this problem, the Ojos Azules can only be mated to non-blue-eyed cats.