The most common cause of spinal muscular atrophy (SMA) is a mutated or missing gene called survival motor neuron 1 (SMN1). This condition is inherited in an autosomal recessive manner. If both parents are carriers, there is a 25% (one in four) chance their child will have SMA. One in every 40 people carries the gene variation that causes SMA. SMA occurs when the mutated gene is passed from both parents to the child, unbeknownst to the parents who may be carriers.