About 5% to 10% of Lou Gehrig’s disease cases are directly inherited, meaning the vast majority of people diagnosed have no family history of the condition at all. But the relationship between genetics and ALS is more nuanced than a simple yes-or-no answer. Even in cases that appear to strike randomly, genes can play a contributing role.
Familial vs. Sporadic ALS
ALS falls into two broad categories. Familial ALS, which accounts for roughly 5% to 10% of all cases, runs in families and is caused by inherited gene mutations. A large meta-analysis across 165 studies found that about 8% of ALS cases overall are familial. Sporadic ALS, the remaining 90% to 95%, occurs in people with no known family history of the disease.
That said, the line between familial and sporadic isn’t as clean as it looks. Some people classified as “sporadic” carry the same gene mutations found in familial cases. The most common ALS-linked mutation, a repeated DNA sequence in a gene called C9orf72, shows up in about 37% of familial ALS cases but also in roughly 6% to 7% of sporadic cases. So a person with no family history of ALS can still carry a genetic driver of the disease.
The Genes Most Often Involved
Researchers have identified more than 40 genes linked to ALS. Four of them account for up to 70% of familial cases in European populations:
- C9orf72: The most common genetic cause, found in 25% to 40% of familial cases. The mutation involves an abnormally repeated stretch of DNA that disrupts normal cell function.
- SOD1: The second most common, responsible for 10% to 20% of familial cases. This was the first ALS gene discovered, back in 1993, and it’s the target of the first gene-specific ALS therapy.
- TARDBP: Linked to about 4% of familial cases. Recent research has expanded the conditions associated with this gene to include frontotemporal dementia and parkinsonism.
- FUS: Responsible for about 5% of familial cases, more commonly seen in younger-onset ALS.
More recently identified risk genes include VCP, TBK1, and NEK1, along with a gene called SPTLC1 that has been linked to a rare childhood-onset form of ALS.
How ALS Is Inherited
Most familial ALS follows an autosomal dominant pattern. That means inheriting just one copy of the mutated gene, from either parent, is enough to put you at risk. If a parent carries a dominant ALS mutation, each child has a 50% chance of inheriting it.
Rarely, ALS can follow a recessive pattern, where a person needs two copies of the mutation (one from each parent) to be affected. This is more common in certain juvenile-onset forms. Some SOD1 mutations behave this way in specific populations. In northern Scandinavia, for example, one particular SOD1 variant is carried by about 1% of the population but only triggers ALS when a person inherits two copies, creating a dose effect where more of the abnormal protein is needed to damage motor neurons. X-linked inheritance, where the mutation sits on the X chromosome, has also been documented but is very uncommon.
Carrying the Gene Doesn’t Mean Getting the Disease
One of the most important things to understand about ALS genetics is penetrance, which is the probability that someone carrying a mutation will actually develop the disease. For ALS genes, penetrance is surprisingly low. Population-level estimates put lifetime penetrance at about 54% for SOD1 mutations, 38% for TARDBP, 33% for C9orf72, and just 19% for FUS.
In practical terms, this means that roughly half or more of the people who carry these mutations will never develop ALS. Why some carriers get sick and others don’t remains one of the central unanswered questions. Other genes, lifestyle factors, and environmental exposures all likely play a role in tipping the balance. Researchers describe ALS as a complex disease arising from variable interactions between genetic susceptibility and environmental triggers, though pinning down which triggers matter most has proven difficult.
Genetics in “Non-Hereditary” Cases
Even when ALS appears sporadic, genetics aren’t entirely off the table. Large genome-wide studies have found that common genetic variants, each contributing a small amount of risk individually, can add up. This polygenic risk operates differently from the single-gene mutations that drive familial ALS. Instead of one mutation causing the disease, dozens or hundreds of small genetic differences collectively raise or lower a person’s susceptibility.
Research using genetic risk scoring has also found that ALS shares some genetic overlap with other traits and conditions, including correlations with smoking and physical activity levels. This doesn’t mean those activities cause ALS directly, but it suggests shared biological pathways. The interplay between many small genetic risk factors and environmental exposures likely explains at least some sporadic cases where no single gene mutation can be identified.
Genetic Testing for ALS
Current evidence-based guidelines recommend that all people diagnosed with ALS be offered genetic testing, regardless of family history. The likelihood of finding a causative mutation is higher if you have one or more first- or second-degree relatives with ALS or frontotemporal dementia, if symptoms started at a younger age, or if ALS occurs alongside dementia.
Testing matters for several reasons. It can identify people eligible for gene-targeted therapies, which are now entering clinical use for SOD1 mutations. It also has implications for family members. If a specific mutation is identified, relatives can choose whether to undergo predictive testing themselves, though the reduced penetrance of ALS genes makes interpreting results complicated. A positive result means elevated risk, not certainty.
For family members considering testing, genetic counseling is strongly recommended. The combination of incomplete penetrance, variable age of onset, and the emotional weight of an ALS diagnosis makes this a decision best navigated with professional support.