Researchers found that only 17 to 22 percent of white cats with non-blue eyes are born deaf. The percentage rises to 40 percent if the cat has one blue eye, while upwards of 65 to 85 percent of all-white cats with both eyes blue are deaf. Some of these cats are deaf in only one ear. Interestingly, if a white cat with one blue eye is deaf in only one ear, that ear will invariably be on the same side of the head as the blue eye.
In cats, inherited congenital (present from birth) deafness is seen almost exclusively in white coated individuals. The deafness is caused by degeneration of the auditory apparatus of the inner ear and may affect one ear (unilateral) or both ears (bilateral).
Breeding studies have defined the relationship between deafness in white cats and blue eye color. The gene responsible is an autosomal dominant gene termed W (for White). This gene appears to be pleiotropic – i.e., it has more than one effect, being responsible for the white coat color and also blue eyes and deafness. However, while the gene has complete penetrance for white coat color (all cats that carry the gene will have a white coat), it has incomplete penetrance for blue eye color and for deafness (but these two are strongly linked). Thus deafness is strongly linked to the white coat color and blue eye color, but not all white cats or white cats with blue eyes are necessarily deaf. The variable penetrance of deafness and eye color may be caused by interplay with other genes and/or environmental factors.