With their black and white spotted markings and their familiar image atop fire trucks, Dalmatians are among the most well known of dogs. Less well known is the fact that they can suffer from a genetic defect that causes them to spend their lives in silence or with impaired hearing.

In fact, Dalmatians have the highest prevalence of deafness of any breed. About 30 percent lose their hearing because of a genetic defect.

Every animal has two genes for each of his traits, one inherited from the mother and one from the father. Deafness can be associated with a single gene or multiple genes. In this specific deafness, multiple genes are thought to be involved. This means that dogs can be completely deaf or only deaf in one ear. And, deaf parents can have offspring that are not deaf if all the associated genes are not inherited by the offspring. However, breeding deaf dogs, even those with deafness only in one ear, is not recommended.

Inside a dog's ear, there is an organ that converts sound waves into electrical impulses that travel to the brain and are processed as sound. In this type of genetic defect, the organ responsible for the conversion starts to degenerate in the first few weeks of life.

Some puppies are affected in only one ear and some are affected in both. Since normal puppy ear canals do not open until they are 6 to 14 days of age, a puppy with this genetic defect never hears out of one or both ears.

Testing for deafness can be difficult. If you try to use a loud sound, such as clapping, a puppy that is deaf in only one ear will still respond to the noise. And some puppies may seem to hear because they adapt or respond to other cues that they may see. The most accurate way to measure hearing ability is an electrical test called "BAER" test, which gauges the brain's ability to detect sound. BAER stands for Brainstem Auditory Evoked Response.

Deaf animals may be difficult to arouse from sleep, may be more aggressive than normal littermates and may be very vocal. They are difficult to train, but with patience and proper effort, deaf dogs can make good pets.