Autosomal dominant is a pattern of inheritance where a single copy of an altered gene is enough to cause a trait or disorder. You have two copies of almost every gene, one from each parent. In autosomal dominant conditions, just one altered copy overrides the normal copy, and the trait shows up regardless of what the other copy says. The “autosomal” part means the gene sits on one of the 22 non-sex chromosomes, so males and females are affected equally.
How One Gene Copy Takes Over
Your cells carry two versions (called alleles) of each gene. In autosomal dominant inheritance, the altered version is “dominant,” meaning it produces its effect even when the other version is perfectly normal. This follows a principle first described by Gregor Mendel: if an organism inherits at least one dominant variant, it will display that trait.
At the molecular level, this happens in a few different ways. Sometimes the normal copy simply can’t produce enough protein on its own to keep things running properly. When 50% of the expected protein isn’t enough, the body shows symptoms. This is called haploinsufficiency. Other times, the mutant gene produces a faulty protein that actively interferes with the normal protein, dragging overall function below what even 50% would provide. Faulty proteins can clump together with normal ones, for example, or jam up a structure that needs two identical pieces to work. In either case, one bad copy is enough to cause problems.
Inheritance Risk for Children
If one parent carries a single copy of a dominant altered gene (and the other parent has two normal copies), each child has a 50% chance of inheriting it and a 50% chance of not inheriting it. Those odds apply independently to every pregnancy, the same way a coin flip doesn’t change based on previous flips.
If both parents carry the same dominant mutation, the odds shift. Each child then has a 75% chance of inheriting at least one copy and a 25% chance of inheriting none. In practice, though, it’s rare for both parents to carry the same dominant mutation unless the condition is very common in a population.
Not Everyone With the Gene Is Affected the Same Way
One of the most confusing things about autosomal dominant conditions is that two people carrying the exact same mutation can look very different. A parent might have mild symptoms while their child is severely affected, or a person might carry the mutation and show no signs at all. This variability comes down to two related concepts.
The first is incomplete penetrance: some people with the mutation never develop symptoms. The genotype is there, but the expected clinical picture doesn’t appear. This is why apparently unaffected parents can pass a condition to a child who is clearly affected.
The second is variable expressivity: among those who do show symptoms, the severity and type of symptoms differ. Marfan syndrome is a good example. Mutations in the same gene can cause severe cardiovascular and skeletal problems in one person, while another family member is simply tall and thin with long fingers. Similarly, mutations in a gene linked to hearing can cause deafness in one person and only mild hearing loss in another.
Both phenomena are shaped by other genes in the background, environmental factors, epigenetics, and lifestyle. This is why genetic counseling often emphasizes that carrying a mutation doesn’t automatically predict exactly what someone’s experience will be.
Conditions That Can Appear Without Family History
Many people assume autosomal dominant conditions always run in families, but that’s not always the case. New (de novo) mutations can arise spontaneously in a sperm or egg cell, meaning a child is the first person in the family to be affected. In a large study of over 7,500 individuals with severe developmental disorders, researchers found that about 42% carried new mutations in their coding DNA rather than inheriting them from a parent. Each person averaged roughly two new mutations in their protein-coding genes.
Parental age influences how often these spontaneous mutations occur. The study found a strong paternal age effect, with older fathers contributing more new mutations, alongside a weaker but real maternal age effect. This is one reason certain autosomal dominant conditions appear more frequently in children of older parents, even when neither parent is affected.
Common Autosomal Dominant Conditions
Hundreds of conditions follow this inheritance pattern. A few of the most widely recognized include:
- Huntington’s disease: a progressive neurological condition that typically appears in mid-adulthood, causing movement, cognitive, and psychiatric changes.
- Marfan syndrome: a connective tissue disorder affecting the heart, blood vessels, bones, and eyes. Severity ranges widely, from life-threatening aortic problems to subtle skeletal features.
- Achondroplasia: the most common form of short-limbed dwarfism. About 80% of cases arise from new mutations rather than being inherited from an affected parent.
- Familial hypercholesterolemia: causes dangerously high LDL cholesterol from birth, significantly increasing the risk of early heart disease.
- Hereditary transthyretin amyloidosis: a condition where misfolded proteins build up in the heart and nerves, leading to progressive organ damage.
Genetic Testing and Family Screening
Genetic testing for autosomal dominant conditions serves several purposes: confirming a diagnosis in someone with symptoms, predicting risk in relatives who have no symptoms yet, and prenatal testing during pregnancy. The process typically starts with a family member who is already affected. Testing that person first identifies the specific mutation in the family, which makes it much easier to interpret results for relatives. A “negative” test in a relative is only truly reassuring if you know exactly which mutation to look for.
Predictive testing, where an at-risk but symptom-free person gets tested, is particularly important for conditions like Huntington’s disease, where knowing your status may influence major life decisions. For other conditions, early detection allows monitoring or preventive treatment before symptoms progress.
Gene-Editing Treatments on the Horizon
Autosomal dominant conditions have historically been difficult to treat because you can’t simply add a working gene. The problem isn’t a missing protein; it’s a harmful one being actively produced. That’s made gene-silencing and gene-editing approaches especially promising.
For hereditary transthyretin amyloidosis, a CRISPR-based therapy delivered through a single infusion reduced levels of the disease-causing protein by roughly 90% in clinical trial participants, with the reduction sustained throughout the trial’s duration. Results were published in the New England Journal of Medicine in late 2024, and a global phase III trial is underway.
A similar approach is being tested for hereditary angioedema, where CRISPR is used to reduce an inflammatory protein that triggers painful swelling attacks. Participants receiving the higher dose saw an 86% reduction in the target protein and significantly fewer attacks. A phase III trial began dosing patients in January 2025.
For familial hypercholesterolemia, a different gene-editing technique called base editing produced a 55% reduction in LDL cholesterol in one participant, durable over two years of follow-up. These therapies represent a shift from managing symptoms to addressing the genetic root cause, something that was purely theoretical for dominant conditions just a decade ago.