Genes normally direct the production of one or more proteins necessary for healthy cells. During its lifetime, a muscle cell produces thousands of proteins. With each of the metabolic diseases of muscle, a gene defect leads to an absence or abnormality in one of the proteins necessary for energy production in muscle cells.

Most of the metabolic diseases of muscle are inherited in an autosomal recessive inheritance pattern, which means that a child must inherit the defective gene from both parents in order for the child to have the disease. The parents themselves usually do not have the disease, but each child has a 25 percent chance of being born with the disease. If only one parent has the defective gene, a 50 percent chance exists that a child will inherit the defective gene. In those cases, the child will be a carrier of the flawed gene and can pass it on to children, but will usually not show signs of the disease.

Autosomal refers to the fact that the genetic defect may be located on any of the 46 rodlike structures, called chromosomes, that hold the genes found in each human cell, except the two that determine a person's sex. With an autosomal inheritance pattern, male and female children are equally affected.

In rare instances, some forms of these diseases follow an autosomal dominant pattern or an X-linked recessive pattern. A disease governed by the dominant pattern will develop when only one parent passes on the defective gene. In this case, each child has a 50 percent chance of inheriting the disease.

X-linked refers to a gene that is on the X chromosome which, along with the Y chromosome, determines sex. Males have one X chromosome and one Y chromosome, while females have two X chromosomes. Therefore, inheritance of a gene on the X chromosome is different from that seen for one on an autosomal chromosome. In the X-linked recessive pattern, the disease develops mostly in males. Females who inherit the defective gene can pass the disease on to their sons but rarely show signs of the disease themselves.