How does one get SMA?

Many people have been asking me recently about how SMA is inherited. We had no one in our extended family that we are aware of affected by SMA. SMA is what’s called an autosomal recessive condition. This means that both myself and my husband carry a mutated SMN 1 gene. Carriers have no symptoms and recently it has even been suggested that carrying SMA may have some evolutionary advantages in terms of an inherent ability to fight infections. It is estimated as many as 1 in 40 people could carry the mutated SMN 1 gene, which equates to just over 2% of the population. It is likely other members of my family will also carry the mutated SMN 1 gene. Both myself and Will don’t know which of our parents we inherited the gene from and didn’t even know we carried the gene ourselves until Helena was diagnosed with SMA and this is usually the case with an SMA diagnosis.


So, all genes come in pair. A carrier has one normal gene and one SMA causing gene. When 2 carriers of SMA come together (such as myself and Will) we each pass on 1 of our pair of genes to our child. So the possible combinations result in a 25% chance every time we become pregnant that we could have a child with SMA if the baby inherits both our SMA causing genes. We also have a 25% chance of having a baby that does not have the SMA causing gene at all and we have a 50% chance of having a baby that, like us, would be a carrier of SMA, but not actually have the condition itself.

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