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Q. My veterinarian said white cats are often deaf. Is that true, and why?

A. About half of all white cats are deaf, and those numbers increase in cats with blue eyes, with deafness often found on the same side as the blue eye — left blue eye, deaf in left ear; two blue eyes, completely deaf. The reason, as you may have guessed, is heredity: The roll of the genetic dice that produces a white, blue-eyed and typically long-haired cat is more likely than not to produce a deaf one as well.

There’s a similar genetic muddle at work in dogs, where merle, piebald and white markings are often linked to both deafness and blindness, again, typically on the same side as blue eyes and white ears. Merle-to-merle matings in breeds such as Collies, Australian Shepherds and Shelties are so controversial that some breeders condemn them, since these breedings can double-up on this dangerous gene, with the resulting puppies deformed to the point of having abnormally small eyes.

While reputable breeders avoid planned matings that will produce such animals, casual pet breeders or “oops” hookups of pets will produce cats and dogs with these markings and problems now and then. But despite their handcap, these animals deserve homes just as much as any other pet and can be wonderful companions. Because their deafness makes them highly vulnerable, though, deaf cats should never be allowed to roam freely, and deaf dogs should stay on leash when out of the house and fenced yard. Altering is also strongly advised.