Most people have two functioning copies of the SMN1 gene. People with one faulty copy and one functioning copy are called “carriers.”
Even though carriers don’t have SMA themselves, they can still pass the faulty gene on to their children.
Approximately 1 in 50 people is a genetic carrier for SMA. Most have no idea they are carriers until they have a child born with SMA.
How SMA is Inherited
SMA is a recessive disease, which means that, generally, a child will only have SMA if both parents pass on the SMN1 mutation.
When two parents are carriers, there is:
- A 25% chance that their child will be unaffected
- A 50% chance that their child will be a carrier
- A 25% chance that their child will have SMA
If only one parent is a carrier, the child is usually not at risk for SMA (though they do have a 50% risk of being a carrier). However, in very rare cases, mutations in the SMN1 gene can occur during egg or sperm production. In this situation, only one parent will be a carrier of the SMN1 mutation. In addition, a small percentage of carriers have a mutation that cannot be identified through current testing. In this case, it will appear as though the disease has been caused by a single carrier.
A DNA test is the only way to detect carrier status. The DNA test is simple procedure, based on a small blood draw. In the general population, this test can detect about 95% of carriers. However, in African-American populations, detection is closer to 70%. This is because an undetectable mutation is more frequent in African-American populations than in other races.
Deciding whether or not to undergo genetic testing is highly personal, and we strongly recommend discussing it with a physician or genetic counselor. Usually, the testing is only recommended—and only covered by insurance—for those known to be at risk for genetic diseases. Carrier screening via saliva is also available as an alternative to a blood draw.
For couples who are carriers, reproductive decisions can be sensitive. A number of options are available, such as adoption, prenatal testing, and pre-implantation genetic diagnosis (PGD). PGD screens embryos for genetic disorders and selects the unaffected embryos for implantation.
Cure SMA believes that your family has the right to choose whatever option is best for your own values. We help families understand their options and provide resources to support their decision-making process. We do not advocate any specific course of action, nor do we pressure families to choose one way or the other.
We encourage each family to discuss their situation with a physician, genetic counselor, and—if helpful—a therapist or a spiritual advisor.