The Secret Behind Cats With Two Different Eye Colors
📝 Summary
Odd-eyed (heterochromia iridis) cats have two differently colored irises and usually appear in white cats; it is a trait, not a disease. Japan and Thailand historically saw the look as lucky. The dominant white (W) gene suppresses pigment cells, so the iris lacks melanocytes and looks blue, and the same missing cells fail to form the ear's sound sensors - which is why many white blue-eyed cats are deaf. About 60-80 percent of white blue-eyed cats have some hearing loss, and an odd-eyed cat's blue side carries a 30-40 percent deafness chance. Such cats also dislike bright sun and should not be startled when deaf.
1 What odd eyes are
Heterochromia iridis means one iris changes color; despite the “-ia” it is not an illness, just a description of two-toned eyes, commonly called odd eyes. In old Japan, cats with one gold and one blue eye were seen as lucky, and in the Siamese region of Thailand the look was called “diamond eyes” and treasured as rare.
In people, heterochromia can be a condition from genetics, disease, or injury, but in cats it is usually just a trait.
2 Which cats get them, and the genetics
Odd eyes show up most in white or mostly-white cats, and more often in Turkish Van, Angora, and Japanese Bobtail lines; breeders sometimes select for the look. The driver is the dominant white (W) gene, which overrides all other coat-color genes. Because the W gene carries no pigment cells (melanocytes), the iris lacks them and appears blue – blue eyes are simply a lack of pigment, not a blue pigment.
The same missing melanocytes were supposed to form the ear’s sound-sensing cells, so their absence can also cause hearing loss. Coat color, eye color, and hearing all trace back to melanocytes, so how strongly the W gene affects them decides both the blue eye and the deafness.
3 Hearing risk and care
About 60-80 percent of white blue-eyed cats have some hearing impairment, versus 10-20 percent of white cats with other eye colors; in an odd-eyed cat, the blue-eyed side carries about a 30-40 percent chance of deafness in that ear. Some white blue-eyed cats hear normally as kittens but lose hearing by age five or six. Researchers think more genes are involved, so the picture is not fully settled.
White blue-eyed cats also handle direct sun poorly due to missing pigment. A deaf cat is fine in a familiar home – just do not startle it while it sleeps; walk heavily enough that it feels the vibration so it knows you are near.
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