An odd-eyed cat is a cat with one blue eye and one eye of either green, yellow, or brown. This is a feline form of complete heterochromia, a condition that occurs in some other animals. The condition most commonly affects white-colored cats, but may be found in a cat of any color, provided that it possesses the white spotting gene.
The odd-eyed coloring is caused when either the epistatic (dominant) white gene (which masks any other color genes and turns a cat completely white) or the white spotting gene (which is the gene responsible for bicolor and tuxedo cats) prevents melanin (pigment) granules from reaching one eye during development, resulting in a cat with one blue eye and one green, yellow, or brown eye.