One of the most important concepts in clinical genetics is that having a pathogenic variant does not always mean developing the associated condition. Two related concepts explain why: penetrance and expressivity.

Penetrance

Penetrance refers to the proportion of people with a given genotype who show any signs of the associated condition.

If a variant has complete penetrance, everyone who carries it will eventually develop the condition (assuming they live long enough). Huntington disease is a classic example: virtually everyone who inherits the causative expansion will develop the condition if they reach the relevant age.

If a variant has incomplete penetrance, only some people who carry it will develop the condition. BRCA1 pathogenic variants are a well-known example. The lifetime risk of breast cancer for someone with a BRCA1 pathogenic variant is substantially elevated — but not 100%. Some carriers never develop breast cancer.

Penetrance figures are population-level estimates. They describe what happens across many people with a given variant, not what will happen to any specific individual.

Expressivity

Expressivity refers to the range of signs and symptoms that appear in people who do develop the condition.

Even when penetrance is complete — meaning everyone with the variant develops the condition — the severity and specific features can vary considerably. This is variable expressivity.

Neurofibromatosis type 1 is a frequently cited example. Everyone with a pathogenic NF1 variant develops some features of the condition, but the range is enormous: some people have only a few café-au-lait spots, while others develop numerous neurofibromas, learning difficulties, or other complications.

Why Both Concepts Matter for Genetic Counseling

Penetrance and expressivity explain why:

• A positive genetic test result is not a diagnosis or a certainty
• Two family members with the same variant can have very different health histories
• Risk estimates from genetic testing are probabilities, not predictions
• Surveillance and prevention strategies are still valuable even for variants with incomplete penetrance

At Genetic Insights, we help patients understand what a variant classification actually means for their individual risk — not just whether a variant is present.

Key Takeaway: Penetrance describes whether a condition develops at all; expressivity describes how severe it is. Both are usually incomplete, which is why genetic risk is a probability rather than a certainty.

Sources: NHGRI Penetrance; GeneReviews