Not all white cats are prone to deafness, but it is true that there is a connection between a cat's coat color and whether they have hearing loss or not. There is a gene that is responsible for a cat being born deaf, that also relates to the cat having a white coat, and it also relates to a cat's eye color. The deafness can be in one ear or both.
White cats and deafness
Congenital deafness, meaning a cat that is born deaf, is seen almost exclusively in white cats. There is a dominant gene, called W (for White), that is an interesting example of a gene being pleiotropic, meaning a single gene that has multiple effects. Gene W is responsible for a cat's coat color. It is dominant, so any cat that has this gene will have a white coat.
This gene also strongly links to blue eye color. Not every cat with Gene W will have blue eyes, and not every white cat will be deaf, but a cat that has the gene will be strongly likely to have blue eyes, a white coat, and be deaf. Statistically speaking, around 1 to 1.5 percent of all cats are deaf and white, and have one or two blue eyes. If a white cat has two blue eyes, it is 3 to 5 times more likely to be deaf than a cat with two non-blue eyes. A cat with a single blue eye is about twice as likely to be deaf as a cat that has no blue eyes at all.
Gene W works to create all-white cats by not allowing pigment to form in the skin. Regardless of whether the cats have any other gene for another coat color, they will always be white. The gene is associated with deafness because it can sometimes cause problems in the cochlea, which is the part of the inner ear that aids in hearing.
Heterochromia in cats
A cat being born with two different-colored eyes has what is called heterochromia. It's hard to find percentages of cats with heterochromia, but it is a fairly rare occurrence. Heterochromia is most commonly seen among cats with white coats, but it can happen in cats of other colors as well.
Different colored eyes are usually caused by another effect of Gene W — it prevents the pigment that gives an eye its color from moving to one or both eyes. Heterochromia doesn't have an affect on a cat's ability to see. And, a non-white cat that has heterochromia isn't more likely to be deaf. This auditory link seems to be related only to white coat color, and not eye color in general.
Blue eyes and deafness
It is a common mis-belief that cats that are white and have blue eyes are more prone to blindness, but that isn't true. They are, however, more prone to congenital deafness than other cats. According to the Cornell Feline Health Center, only 17 to 22 percent of white cats with non-blue eyes are born with hearing loss. 40 percent of cats with one blue eye are deaf, and up to 85 percent of all white cats with two blue eyes have deafness. In cats with heterochromia with a single blue eye, they will be more likely to have an auditory defect on the side of their blue eye.
Louisiana State University has information about older studies on cat breeds with deafness. They list 18 cat breeds that carry the dominant gene W for white coat and that are also at risk for congenital deafness, although there is no data on what this percentage of risk is. These breeds include white Norwegian forest cats, Persians, ragdolls, and white Maine Coons.
There is a dominant gene, called gene W, that will always express itself as a white coat in any cat tat carries the gene. This gene W is what is called pleiotropic, meaning it is one gene that has multiple effects. Its other effects are to make it more likely that aa white cat will have blue eyes, and also that these white cats with blue eyes will also have deafness. But it's important to note that not all white cats will be deaf and not all blue eyed cats will be deaf. Cats that are white that have other eye colors and cats with other coat colors that have blue eyes do not seem to be affected by deafness.