There are parts of every cell in the human body that recieve instructions from genes. The parts of the cell that get the instructions are, by nature, protien. When the instructions contain a mistake, this is called a deletion. In SMA, the instructions given to nerves that control muscles contain a deletion that causes a protien deficiency. The gene responsible for the instruction to motor neurons is called SMN 1, or survival motor neuron.
Each person has 2 pairs of genes for each instruction given: one gene is inherited from mother and one from father. Some diseases will show up if only one of the inherited genes contains an instruction error. Other diseases, such as SMA, need both mother's and father's inherited genes to contain a mistake before the disease will present itself. The SNM 1(survival motor neuron) gene, which is responsible for SMA, is one of those types of genes. This means that for a child to have SMA, both parents must have contributed an SMN 1 with faulty instructions (the deletion mentioned above). Even then, the chance is 1 in 4 per pregnancy that the child will have SMA. One in 40 adults are carriers of the gene that causes SMA.
