Penetrance and Variable Expressivity
Definition
Two concepts that explain why people carrying the same genetic variant experience different outcomes:
Penetrance, does the disease appear?
| Type | Meaning |
|---|---|
| Complete penetrance | Everyone carrying the variant shows signs of the disease |
| Reduced (incomplete) penetrance | Some people carrying the variant do not develop features of the disease |
Example: BRCA1/BRCA2 variants increase cancer risk, but some people do not develop cancer even though they carry the mutation.
Variable Expressivity, what does the disease look like?
Among affected people carrying the same variant, the range and severity of symptoms differ.
Example: In Marfan syndrome, among people carrying the same FBN1 gene variant, the picture ranges from mild features (tall stature, slender build) to life-threatening cardiovascular complications.
The Difference
| Answers the question | |
|---|---|
| Penetrance | Does the disease appear? (yes / no) |
| Expressivity | How does the disease appear? (mild / severe, which symptoms) |
Risks and Limitations
This is the main reason genetic counseling is difficult:
Both arise from a combination of unknown genetic, environmental and lifestyle influences. For this reason, it is difficult to predict which family member will become ill and how severely they will be affected, and risk assessment and family planning decisions become more complex.
In other words, a positive genetic test does not mean a certain fate.