Hereditary deafness is a major concern found in white cats, and especially so if one or both eyes are blue.
Research has found that only 17-22% of white cats with non-blue eyes are born deaf. The percentages rise to nearly half (40%) if the cat has one blue eye. An upwards of 65-85% of all white cats with two blue eyes are deaf. Some of these cats are deaf in a just one ear. Interestingly enough, if a white cat has one blue eye, the ear that is deaf tends to fall on the same side as the blue eye.
Cats with only one deaf ear out of the two tend to appear normal and their issue may never be known to their humans. Even if born completely deaf, cats can live perfectly fine lives as long as you take heed to not put them in situations where they must rely on audible cues (i.e. the outdoors). There is no treatment for hereditary deafness.