Two people can carry the same pathogenic variant in the same gene and have very different health outcomes. Penetrance and expressivity are the concepts that explain why.

Penetrance

Penetrance describes the proportion of people who carry a pathogenic variant and actually develop the associated condition.

If a variant has complete penetrance, essentially everyone who carries it will develop the condition at some point in their lifetime — assuming they live long enough.

If a variant has incomplete penetrance, only a fraction of carriers will develop the condition. The others carry the variant but never show signs of the associated disease.

Example: Pathogenic variants in BRCA1 and BRCA2 are associated with significantly elevated lifetime risk of breast and ovarian cancer, but not every carrier develops cancer. The penetrance is high but not 100%.

Expressivity

Expressivity describes how much variation exists in the severity or features of a condition among people who do develop it.

If a condition has variable expressivity, different carriers may have mild, moderate, or severe forms of the same condition — even within the same family carrying the same variant.

Example: Neurofibromatosis type 1 (NF1) shows highly variable expressivity. Some people have only a few café-au-lait spots; others develop more extensive features including tumors along nerves.

Why Both Concepts Matter

Penetrance and expressivity explain why genetic risk is rarely all-or-nothing:

• A positive result does not guarantee a condition will develop
• A negative result does not guarantee a condition will not develop through other mechanisms
• Family members with the same variant may have very different experiences

Other factors — including other genetic variants, lifestyle, environment, and chance — interact with a pathogenic variant to influence whether and how a condition appears.

Implications for Genetic Counseling

Understanding penetrance and expressivity is essential for interpreting genetic results accurately. At Genetic Insights, we help patients understand not just whether a variant is present, but what the scientific literature says about how often and how severely that variant affects health — and what that means for their specific situation.

Key Takeaway: Penetrance describes how often a variant causes a condition; expressivity describes how variable the severity is. Both explain why the same variant can have very different effects in different people.

Source: NHGRI Genetics Glossary — Penetrance