Heterochromia is a variation in coloration. Most often this term is used to describe color differences in the iris, but it can also be applied to hair or skin color variations.

Eye color, especially the color of the iris, is determined by the concentration and distribution of melanin. There are only 2 pigments in the irises of mammals — eumelanin and pheomelanin. The overall concentration of these pigments, the ratio between them, the variations in the distribution of the pigment in the stroma layers of the iris, and the effects of light scattering play an important role in determining eye color.

The main cause of heterochromia is a lack of genetic diversity, at least in domestic animals. This is due to a mutation in the genes that are responsible for the distribution of melanin. Usually, they are damaged because of the homogeneity of the chromosomes. For example, heterogeneous eye color occurs in 2 cases in cats:

When there is a dominant white gene in the genome of an animal, it disguises any other color genes and makes the cat completely white. These are, for example, Turkish Van, Turkish Angora, Persian, and Khao Manee.
When there is the white spotting gene in the genome, responsible for the bicolor and black and white color. This gene prevents the melanin pigment from entering one eye during fetal development, resulting in a cat having one blue eye and one green, yellow, or brown eye. For instance, this coloration can be seen in Turkish Van, Sphynx, and Japanese Bobtail cats.

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