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. It only rarely occurs in cats that lack both the dominant white and the white spotting gene.
As all cats are blue-eyed as kittens, the differences in an odd-eyed kitten's eye color might not be noticeable save upon close inspection. Odd-eyed kittens have a different shade of blue in one eye. The color of the odd eye changes over a period of months, for example, from blue to green to yellow or from green to blue to yellow, until it reaches its final, adult color.
Cultural reactions and folklore
Odd-eyed cats are popular within several breeds, including Turkish Van, Turkish Angora, and Japanese Bobtail.
In 1917, the government of Turkey, in conjunction with the Ankara Zoo, began a meticulous breeding program to preserve and protect pure white Turkish Angora cats with blue and amber eyes, a program that continues today, as they are considered a national treasure. The zoo specifically prized the odd-eyed Angoras that had one blue eye and one amber eye, as the Turkish folklore suggests that "the eyes must be as green as the lake and as blue as the sky."
Mustafa Kemal Ataturk, who founded the Republic of Turkey in 1923, declared that his successor would be bitten on the ankle by an odd-eyed white cat. This prediction dramatically increased the interest in odd-eyed white cats in Turkey.
Muhammad's pet Angora, Muezza, was reputed to be an odd-eyed cat.
In Japan, odd-eyed Japanese Bobtails are very popular and can carry a high price tag.
Deafness in odd-eyed cats
There is a common misconception that all odd-eyed cats are born deaf in one ear. This is not true, as about 60%–70% of odd-eyed cats can hear. About 10%–20% of normal-eyed cats are born deaf or become deaf as part of the feline aging process. White cats with one or two blue eyes do, however, have a higher incidence of genetic deafness, with the white gene occasionally causing the degeneration of the cochlea, beginning a few days after birth.
Eyeshine and red-eye effect
In flash photographs, odd-eyed cats typically show a red-eye effect in the blue eye but not in the other eye. This is due to the combined effect of the (normal) presence of a tapetum lucidum in both eyes and the absence of melanin in the blue eye. The tapetum lucidum produces eyeshine in both eyes but in the non-blue eye a layer of melanin over the tapetum lucidum selectively removes some colors of light.