Any of a group of inherited progressive muscle disorders caused by a defect in one or more genes that control muscle function. MD is characterized by a gradual, irreversible wasting of skeletal muscle. It is a sex-linked trait most often passed on to sons by their mothers. The most common form, Duchenne MD, begins to weaken the legs of boys by age 3 and inevitably gets worse with each passing year. There is no cure for this disorder, and it usually results in death before the age of 30.