Most people have two functioning copies of the SMN1 gene. People with one non-working copy and one working copy of the gene are called “carriers.” Carriers generally do not show signs and symptoms of spinal muscular atrophy (SMA) but could be at risk to have a child affected with the condition.
SMA is an autosomal recessive genetic condition. This means that for a child to have SMA, they must inherit two non-working copies of the SMN1 gene—typically one from each parent. If only one parent is a carrier, the child is usually not at risk for SMA, though the child does have a 50% chance of also being a carrier themselves.
In rare cases, spontaneous genetic changes in the SMN1 gene can occur during egg or sperm production. In this situation, only one parent will be a carrier. In addition, a small percentage of carriers have genetic changes that cannot be identified through current testing technology. In this case, it will appear that the disease has been caused by a single carrier.