Is amyotrophic lateral sclerosis inherited ?
About 90 to 95 percent of ALS cases are sporadic, which means they are not inherited.  An estimated 5 to 10 percent of ALS is familial and caused by mutations in one of several genes. The pattern of inheritance varies depending on the gene involved. Most cases are inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. In most cases, an affected person has one parent with the condition. Some people who inherit a familial genetic mutation known to cause ALS never develop features of the condition. (This situation is known as reduced penetrance.) It is unclear why some people with a mutated gene develop the disease and other people with a mutated gene do not.  Less frequently, ALS is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition. Because an affected person's parents are not affected, autosomal recessive ALS is often mistaken for sporadic ALS even though it is caused by a familial genetic mutation.  Very rarely, ALS is inherited in an X-linked dominant pattern. X-linked conditions occur when the gene associated with the condition is located on the X chromosome, which is one of the two sex chromosomes. In females (who have two X chromosomes), a mutation in one of the two copies of the gene in each cell is sufficient to cause the disorder. In males (who have only one X chromosome), a mutation in the only copy of the gene in each cell causes the disorder. In most cases, males tend to develop the disease earlier and have a decreased life expectancy compared with females. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.