Dr. Charlotte Sumner describes how SMA is inherited.
The other aspect that's very important about this disease, spinal muscular atrophy is that it's an inherited disorder. It's inherited in what we call an autosomal recessive fashion and when a disease is autosomal recessive, it means that both copies of the gene that we have, have to be misspelled. We inherit one of those misspelled copies of the gene what we call mutated gene from and the other one from dad. And a child who develops SMA has two misspelled copies of that gene and will develop the disease.
For two parents who are both carriers for SMA their risk each time of having a child with SMA is 25%, so one in four and that risk continues for each child that they might have.