SMA is a disorder that results when a child is born with two faulty (mutated)SMN1 genes-one inherited from the child's mother and one inherited from the father.
Like all genes, the SMN1 gene contains the genetic blueprint for a particular protein, which has a specific function in cells. The role of SMN1 is to produce a protein called survival of motor neuron. This protein is critical to the survival and healthy functioning of specialized nerve cells, called motor neurons, that the brain uses to control voluntary muscle movement.
SMA is an autosomal recessive genetic disorder, which means that the child must have inherited not just one, but two copies of the mutated SMN1 gene to have SMA. Having just one faulty SMN1 gene is not sufficient to cause the disease, as a single normal gene can perform all the required functions of two normal genes. While a person with one faulty SMN1 gene does not have SMA, he (or she) is, however, a carrier of SMA.
If a pregnancy occurs between two SMA carriers, the following can occur:
Their child inherits two faulty SMN1 genes (one from each parent) and has SMA. There is a 25% (one in four) chance of this occurring.
Their child inherits just one faulty SMN1 gene (and one normal gene) and, like each of his parents, does not have SMA but is an SMA carrier. There is a 50% chance of this occurring.
Their child inherits two normal copies of the SMN1 gene and does not have SMA and is not a carrier. There is a 25% chance of this occurring.
A blood test prior to conception or early in pregnancy (called the "SMA carrier test") can tell couples with a high degree of certainty whether they are both carriers of the mutated SMN1 gene. If not, they can rest assured that their baby is highly unlikely to have SMA. If so, the couple still has many options.
Athena Diagnostics thanks Families of SMA for permission to post Genetics of SMA