The brain controls muscles via signals that travel through nerves. A disease that affects the nerves is called a polyneuropathy: poly- (many), neuro- (nerves), -pathy (a disease). Due to a quirk in the way an embryo develops, one of the longest nerves in the body supplies the muscles of the voice box (larynx). The vocal folds vibrate as air moves over them allowing a dog to bark. When the dog breathes in, muscles in the larynx pull the vocal folds aside so that air can move easily into their lungs. These nerves also help to close the larynx when the dog swallows so they do not choke on their food.
If nerves are unable to convey messages properly, the muscles become weak or paralyzed. The longest nerves are often affected first; hence laryngeal paralysis is the first symptom. The vocal folds cannot be pulled out of the way as the dog breaths in. They vibrate noisily and can obstruct the flow of air into the lungs particularly when the dog is exercised or hot. The dog may also choke on their food or water or regurgitate, which can result in pneumonia.
The next longest nerves in the body go to the back legs, thus they are often affected next. The dogs have difficulty getting up and wobble as they walk. Eventually the front legs will also be affected. The symptoms do not occur until after weaning age, and thus the disease is called juvenile laryngeal paralysis/polyneuropathy or JLPP for short.
JLPP is inherited as a recessive trait. In a recessive disease, both parents of an affected pup are carriers but do not show any symptoms of the disease. All animals have two copies of each gene, one that is inherited from the mother and one inherited from the father. A dog that has one normal gene and one gene that causes the disease is a carrier of the trait. They show no symptoms because the one good gene is enough for their brain to develop normally, but they will pass that bad gene on to about half of their offspring. If a carrier dog is bred to another carrier, then some of the pups (25% on average) will get a bad gene from each parent. Without one good gene to carry the day, the brains cannot function normally and the unlucky pup has JLPP.
The DNA test can identify carriers of the mutation so that breeders can make wise breeding decisions. While the initial response is often to want to eliminate all carriers of a mutation from the breeding pool, this is not the recommended approach. Eliminating all the carriers will narrow the gene pool and limit the breeding choices available. This reduces desirable genetic diversity in the breed and may inadvertently increase the incidence of other hereditary problems that may be lurking the lines that are free of the JLPP mutation. There are often highly desirable traits in the lines with a mutation that made them popular to begin with. Eliminating those lines can eliminate those desirable traits, throwing out the baby with the bathwater. The best approach is to test breeding stock and ensure that two carriers are never bred together so that no affected pups are produced. A carrier that has desirable traits can be bred to a dog who is tested clear of the mutation since no affected pups can result from such a breeding. When selecting future breeding stock, the gene status can be considered in deciding which pups to keep but does not have to be the sole factor. Over time, the prevalence of the mutation in the breed will decline while a genetic bottle-neck is avoided. The key is to ensure that carriers are never bred to each other so that no affected pups are produced.
Video of JLPP Affected Puppy