How Is Alström Syndrome Inherited? Inheritance Explained

Alström Syndrome is inherited in an "autosomal recessive" pattern. That phrase sounds technical, but it describes a simple idea — both parents have one altered copy of the same gene, and a child needs to inherit altered copies from both parents to be affected. This article walks through what that means with diagrams, examples, and the answers families most often ask after a diagnosis.

Each person has two copies of every gene — one inherited from their mother, one from their father. Most genes work fine even if just one copy is functional, like having two backup batteries. Recessive means a person needs both copies to be altered to develop the condition. Autosomal means the gene is on one of the 22 numbered chromosomes (not the X or Y chromosome that determine sex), so the inheritance pattern is the same in boys and girls.

For Alström, the relevant gene is ALMS1. A child develops Alström when both ALMS1 copies have disease-causing changes — one from each parent.¹

A person with one altered copy and one normal copy of ALMS1 is called a carrier. Carriers are healthy. The single normal copy makes enough functional ALMS1 protein for cells to work properly. Carriers don't have Alström features themselves and aren't expected to develop them.

But carriers can pass the altered copy to their children. When two carriers have a child together, four possible combinations result, each with equal probability:

• One altered copy from mother + one altered copy from father = affected child (25%)
• One altered copy from mother + one normal copy from father = carrier child (25%)
• One normal copy from mother + one altered copy from father = carrier child (25%)
• One normal copy from mother + one normal copy from father = unaffected, non-carrier child (25%)

Combined, that's a 25% chance the baby is affected, 50% chance the baby is a healthy carrier, and 25% chance the baby is unaffected and not a carrier.

Imagine two parents. Each carries one normal copy of ALMS1 (let's call it "A") and one altered copy (let's call it "a").

``` Mother: A a Father: A a

Possible combinations for each child:

Father's gift → A a Mother's gift ↓ A AA Aa a Aa aa

```

• AA — two normal copies, unaffected, not a carrier (25%)
• Aa or aA — one normal, one altered: healthy carrier (50% combined)
• aa — two altered copies, has Alström Syndrome (25%)

This is the same probability for every pregnancy. The outcomes don't "balance out" — having one affected child doesn't reduce the risk for the next pregnancy.²

The parents of a child with Alström almost always carry one altered and one normal ALMS1 copy. They've lived their whole lives without Alström symptoms because their normal copy makes enough functional protein.

That's why it can be such a surprise when a child is diagnosed. The condition often appears with no prior family history because both parents were healthy carriers.

When parents are related (for example, first or second cousins), they share more genetic material than unrelated parents do. Both could carry the same rare altered copy of ALMS1 inherited from a shared ancestor. This raises the risk of having an affected child.

In some communities where consanguineous marriage is common, autosomal recessive conditions like Alström are seen more frequently. Genetic counseling for these families is especially valuable.

If one parent has one altered ALMS1 copy and the other parent has two normal copies:

• Every child has a 50% chance of being a healthy carrier
• No child can have Alström Syndrome (they would need two altered copies)

This is why testing the partner of a known carrier is important when planning a family.

If one parent has Alström Syndrome (two altered copies) and the other parent has two normal copies:

• Every child will inherit one altered copy from the affected parent and one normal copy from the unaffected parent
• Every child will be a healthy carrier
• No child will have Alström unless the unaffected partner is also a carrier

This is the typical situation for adults with Alström who are family planning.

The same odds apply to every pregnancy in the family:

• Each sibling has a 25% chance of being affected
• Each sibling has a 50% chance of being a healthy carrier
• Each sibling has a 25% chance of having two normal copies

A sibling who hasn't shown features of Alström by mid-childhood is very unlikely to be affected — Alström features typically emerge early in life. Carrier testing for adult siblings is straightforward once the family's ALMS1 variants are known.

The most common feeling parents describe after learning the inheritance pattern is guilt — "we did this to our child." That's understandable, and it's also not accurate.

• Carriers have no symptoms and no way of knowing they carry an altered ALMS1 copy without being tested
• Population-wide carrier screening for ALMS1 is not standard
• The chance that two random carriers happen to have a child together is statistical, not anyone's choice

The condition exists. Two healthy people brought a beautiful child into the world. Now the work is supporting that child and the family — not assigning blame.

• Alström Syndrome Genetics: ALMS1 Gene Explained
• What Is the ALMS1 Gene?
• Are Both Parents Carriers? Understanding ALMS1 Carrier Status
• Risk to Siblings of a Child With Alström Syndrome
• Genetic Counseling for Alström Syndrome Families

April 30, 2026.