Deafness in dogs is not such a rare occurrence and the condition occurs in some breeds more often than others. Boston Terriers are one of the breeds that suffers from congenital deafness, or deafness present at birth. Exactly what causes congenital deafness is not yet entirely known, though most experts agree that it has something to do with pigment genes; these are genes responsible for giving color to the hair cells and the eye cells of an animal. Certain color genes are associated with blindness. More and more research is being done on the topic of deafness in Boston Terriers (and in all other breeds prone to this condition); a test has been developed, called the BAER test, which ascertains whether or not a dog is deaf, whether the deafness is unilateral (one ear) or bilateral (both ears) and the extent of the deafness.
When dogs are born, their ears are folded over and closed, which prevents them from hearing. After they turn 4 weeks old, the ears begin to open and the dog starts to hear. During pre-natal development, pigment cells must be present and healthy for the inner ear to develop properly; the inner ear has essential structures for the process of hearing. If there are no pigment cells, an important inner ear structure does not receive an adequate blood supply and stops functioning. When this structure doesn't function, deafness occurs. The degeneration of the inner ear blood supply happens around the time between the fourth and eighth weeks of life of the puppy. Up until at least 4 weeks old even puppies with a hereditary predisposition to deafness will hear normally. BAER testing should not occur before 8 weeks to ensure that the proper amount of time is given to the degeneration of the blood supply.
Bilateral deafness in dogs is not too hard to spot; a dog that can't hear out of either ear may have a bark that sounds different from the typical bark of the breed (or the dog's bark may change if the dog slowly becomes deaf), may not have any response to sound and may sleep for long periods of time. Dogs that are deaf only in one ear very often appear to be completely normal. They may have only a mild response to sounds and/or it may seem that they experience difficulties when trying to pinpoint the direction from which a sound is coming; a dog with unilateral deafness may cock their head to one side very frequently. The only way to tell for sure if a dog is deaf, though, is by using the BAER test, which stands for Brainstem Auditory Evoked Response.
While it's known that deafness is inherited in Boston Terriers, the method of inheritance is still unclear. There is also evidence to support that deafness is related to certain marking patterns, especially due to the fact that pigment cells are directly involved in the proper functioning of the inner ear. Much more research needs to be done to understand deafness in Boston Terriers, and in dogs in general.