I don’t think that I have seen an all-grey cat with odd-eyes before. This picture is on the Reddit.com website. And the eye colours are interesting. The brown eye is very dark and the light green eye is, in contrast, quite light. I think this cat is fairly rare. I don’t think the owner realised it. For that reason, I’ve told her!
All-white cats quite often have one eye which is blue and the other which is gold or yellow. And eyes of different colour are also quite often present on bicolour cats which have white coats with patches of colour. This is because heterochromia iridium (the technical name for odd-eyes) is linked to the dominant white gene in all-white cats and the piebald gene which creates the piebald coat of bicolour cats.
I always thought that these genes, themselves, affected the pigmentation in the iris of the eyes of these cats. Specifically, I believed that the genes removed the melanin pigmentation from the iris of one eye rendering it blue. The blue coloration then being due to refraction of white light through the eye. However, Dr John Bradshaw in his book Cat Sense tells us that the gene which controls eye colour is on the same chromosome as the piebald gene and the dominant white gene and the linkage between the coats of these cats and their odd-eyes is because of the close proximity of the genes on the same chromosome. I don’t know what the right answer is which is why I’m giving you both!
The reason why I am publishing this photograph, posted by the Reddit user u/thecryptbeekeeper, is because it’s pretty rare.
Heterochromia iridium can be inherited genetically or it can be caused by a developmental defect i.e. congenital, and thirdly it can be caused by an illness, injury or medication which means that it was acquired. I don’t know which category this grey cat falls in.
Sarah Hartwell, a cat genetics expert says that heterochromia is mostly found in cats with white coats or white patches as I’ve mentioned. She also says that occasionally it can be seen in coloured cats. As odd-eyes are quite rare anyway there are obviously rarer in fully coloured cats which is why I told the owner of this cat that she has a fairly rare cat even if he is a random-bred cat.
SOME MORE ON ODD-EYES: