About Neuromuscular Diseases

How Are Genetic Diseases Inherited?

Inheritance Patterns

The body's cells don't work properly when a protein is altered or produced in insufficient quantity (or sometimes missing completely). Some neuromuscular diseases can be caused by a spontaneous mutation that is not found in the genes of either parent — a defect that can be passed to the next generation.

Genes are like blueprints; they contain coded messages that determine a person's characteristics or traits. They are arranged along 23 rod-like pairs of chromosomes, with one half of each pair being inherited from each parent. Each half of a chromosome pair is similar to the other, except for one pair, which determines the sex of the individual. Inheritance can happen in three ways:

• Autosomal dominant inheritance occurs when a child receives a normal gene from one parent and a defective gene from the other parent. Autosomal means the genetic mutation can occur on any of the 22 non-sex chromosomes in each of the body's cells. Dominant means only one parent needs to pass along the abnormal gene in order to produce the disorder. In families where one parent carries a defective gene, each child has a 50 percent chance of inheriting the gene and therefore the disorder. Males and females are equally at risk and the severity of the disorder can differ from person to person.
• Autosomal recessive inheritance means that both parents must carry and pass on the faulty gene. The parents each have one defective gene but are not affected by the disorder. Children in these families have a 25 percent chance of inheriting both copies of the defective gene and a 50 percent chance of inheriting one gene and therefore becoming a carrier, able to pass along the defect to their children. Children of either sex can be affected by this pattern of inheritance.
• X-linked (or sex-linked) recessive inheritance occurs when a mother carries the affected gene on one of her two X chromosomes and passes it to her son (males always inherit an X chromosome from their mother and a Y chromosome from their father, while daughters inherit an X chromosome from each parent). Sons of carrier mothers have a 50 percent chance of inheriting the disorder. Daughters also have a 50 percent chance of inheriting the defective gene but usually are not affected, since the healthy X chromosome they receive from their father can offset the faulty one received from their mother. Affected fathers cannot pass an X-linked disorder to their sons but their daughters will be carriers of that disorder. Carrier females occasionally can exhibit milder symptoms of MD.

Content credit: National Institute of Neurological Disorders and Stroke (NINDS)