Most ALS cases are not inherited, but family history does matter

About 90 percent of ALS cases occur in people with no family history of the disease. These are called sporadic ALS. The remaining 10 percent run in families — familial ALS — and follow a pattern of inheritance you can predict based on your relatives' diagnosis.

If you have a parent, sibling, or child with ALS, your risk is higher than the general population's, but it is not certain. The actual risk depends on which gene carries the mutation, whether you inherited it, and factors scientists still do not fully understand. Knowing your family history helps you and your doctor watch for early signs and make informed decisions about genetic testing.

Key Takeaways

  • Familial ALS accounts for roughly 10 percent of all ALS cases and is usually inherited in an autosomal dominant pattern, meaning one mutated gene from one parent is enough to cause the disease.
  • If one of your parents has familial ALS, you have a 50 percent chance of inheriting the mutation, but inheriting the mutation does not may provide you will develop symptoms.
  • The most common familial ALS gene is C9orf72, followed by SOD1 and FUS; genetic testing can identify which mutation runs in your family.
  • People with a family history of ALS should discuss genetic testing and screening options with a neurologist or genetic counselor before symptoms appear.
  • Sporadic ALS can still have a genetic component; some people carry mutations that increase risk without a clear family pattern.

How familial ALS is inherited

Familial ALS is usually inherited in an autosomal dominant pattern. That means you need only one mutated copy of the gene — from either your mother or father — to develop the disease. If one parent carries the mutation, each child has a 50 percent chance of inheriting it.

A small number of familial ALS cases follow an autosomal recessive pattern, where you need two mutated copies (one from each parent) to develop the disease. Recessive ALS is rarer and often appears in families where both parents are carriers but do not have symptoms themselves.

X-linked ALS, inherited through the X chromosome, is extremely rare. It affects males more severely than females because males have only one X chromosome.

The three most common ALS genes

C9orf72 is the most common cause of familial ALS in North America and Europe, accounting for about 40 percent of familial cases. People with C9orf72 mutations often develop symptoms in their 40s or 50s, though the age varies widely. Some people with the mutation never develop ALS in their lifetime, a phenomenon called incomplete penetrance.

SOD1 was the first ALS gene identified and accounts for about 20 percent of familial cases. SOD1 mutations tend to cause faster disease progression than C9orf72. FUS mutations cause about 5 percent of familial ALS and are often associated with earlier symptom onset, sometimes in the 20s or 30s.

More than a dozen other genes have been linked to ALS. A genetic counselor or neurologist can explain which genes run in your family and what that means for your risk.

What inheriting the mutation actually means

Inheriting a mutation does not mean you will definitely develop ALS. This is the most important distinction. Some people carry a familial ALS mutation their entire lives and never show symptoms — they have the mutation but not the disease. Scientists call this incomplete penetrance, and it happens most often with C9orf72.

If you do develop symptoms, the age at which they start can vary dramatically, even within the same family. One parent might develop ALS at 45 and another at 70, or a sibling might never develop it at all despite carrying the same mutation. This unpredictability makes it hard to predict your own timeline.

Environmental factors, other genes, and random biological events all seem to influence whether a mutation leads to disease. Researchers are still working to understand why some people with the mutation stay healthy while others develop ALS.

Genetic testing: what it can and cannot tell you

Genetic testing can identify which mutation runs in your family, but it cannot predict whether you will develop ALS or when. A positive test means you carry the mutation; a negative test means you do not carry the specific mutation being tested, though other genetic factors might still affect your risk.

Testing is most useful when someone in your family already has an ALS diagnosis. A neurologist or genetic counselor can order testing to confirm which gene is involved, then relatives can be tested for that specific mutation. Testing before anyone in the family has symptoms is less common but may be an option worth discussing with a genetic counselor.

Genetic testing requires a blood sample and usually takes several weeks for results. Some insurance plans cover testing when there is a family history of ALS; others do not. Ask your doctor about cost and coverage before the test.

What to do if ALS runs in your family

If you have a parent, sibling, or child with ALS, tell your primary care doctor and ask for a referral to a neurologist. You do not need to wait for symptoms to appear. A neurologist can perform a baseline exam, discuss your risk, and explain what early signs to watch for — muscle weakness, twitching, difficulty speaking, or trouble swallowing.

Ask your neurologist whether genetic testing makes sense for you. If your family member has had genetic testing, you already know which gene to test for, which simplifies the process. If they have not, testing them first can identify the mutation before you decide whether to be tested yourself.

Consider meeting with a genetic counselor, who can explain inheritance patterns specific to your family, discuss the pros and cons of testing, and help you understand what results would mean for your life. Many ALS centers have genetic counselors on staff or can refer you to one.

Sporadic ALS and genetic risk

Even though sporadic ALS does not run in families, it still has a genetic component. Researchers have identified common genetic variations that increase the risk of sporadic ALS, and some people with sporadic ALS carry mutations in the same genes that cause familial ALS — they just do not have a family history.

This means that if you develop sporadic ALS, your children or siblings have a slightly elevated risk compared to the general population, though the risk is much lower than if you had familial ALS. Genetic counselors can discuss this with you if you want to understand your family's risk more clearly.

Frequently Asked Questions

If my parent has ALS, will I definitely get it?

No. If your parent has familial ALS, you have a 50 percent chance of inheriting the mutation, but inheriting the mutation does not may provide you will develop symptoms. Some people carry the mutation their entire lives without ever showing signs of ALS.

Can I get tested for ALS before I have symptoms?

Yes. If ALS runs in your family and someone has had genetic testing, you can be tested for that specific mutation. Testing before symptoms appear does not change your medical care right now, but it can help you and your doctor plan ahead and watch for early signs.

What is the difference between having the mutation and having ALS?

Having the mutation means you carry the genetic change; having ALS means you have developed the disease with symptoms like muscle weakness or difficulty speaking. Some people carry the mutation their whole lives without ever developing ALS.

If I have sporadic ALS, should my family members be tested?

Genetic testing is less straightforward for sporadic ALS because no specific familial mutation has been identified. Talk to your neurologist or genetic counselor about whether testing would be useful for your family. Your siblings and children have a slightly higher risk than the general population, but it remains low.

How much does genetic testing cost?

Cost varies depending on which genes are tested and your insurance coverage. Some plans cover testing when there is a family history of ALS; others do not. Ask your doctor or genetic counselor about the cost before the test and whether financial assistance is available.