First a person may be affected by a genetic defect which has just occurred in a family for the first time, in one egg or one sperm of his or her parent. This is called a new mutation. Secondly, a parent may have been so slightly affected by the disease that it was unnoticed and may have been undiagnosable even by an experienced physician. Thirdly, one important mode of inheritance, autosomal recessive, very seldom affects more than one generation. Medical experts do not all agree on the best way to classify MD's.
Classifications tend to change as knowledge increases.
However, there is, generally, agreement about the types of MD which have been recognised for many years and which are seen relatively often. For information about these and other types of nerve and muscle disorders, see A Synopsis of Disorders or contact the Muscular Dystrophy Association in your State. The primary cause is the genetic defect. In the Duchenne and Becker types, the defect is in the gene which enables the muscle fibres to make dystrophin. The muscle fibres in Duchenne and Becker dystrophies are deficient in dystrophin, more severely so in the Duchenne type than in the Becker type.