In classical genetics, what do we call the percentage of individuals with a specific genotype who actually express the corresponding phenotype (i.e., who show the trait under standard conditions)?

Introduction / Context:
In human genetics and Mendelian inheritance, the terms “penetrance” and “expressivity” are often confused. This question tests your understanding of how often a genotype produces its expected phenotype in real organisms, which is a cornerstone for risk prediction in medical genetics and for interpreting pedigree data.

Given Data / Assumptions:

• A particular genotype is known (for example, a dominant disease allele).
• We observe a set of individuals carrying that genotype.
• We assess whether they show the associated phenotype.

Concept / Approach:
Penetrance refers to the proportion (often expressed as a percentage) of individuals with a given genotype who display the expected phenotype. If 80 out of 100 carriers show the trait, penetrance is 80%. Expressivity, by contrast, describes the degree or severity of phenotype among those who are affected. Distinguishing these concepts is essential when evaluating variable outcomes from the same genotype.

Step-by-Step Solution:

Verification / Alternative check:
In disorders with incomplete penetrance, not all carriers are affected; clinical genetics references consistently define penetrance as this percentage, confirming the term selection.

Why Other Options Are Wrong:

• Expressivity: measures variation in severity among affected individuals, not the proportion affected.
• Inheritance percent: nonstandard terminology.
• Environmental influence: a factor that modulates penetrance, not the definition itself.
• Trait prevalence: frequency of the phenotype in a population regardless of genotype, not conditioned on genotype.

Common Pitfalls:
Confusing “how many show it” (penetrance) with “how strongly they show it” (expressivity). Also, mixing up population prevalence with genotype-conditioned probability.

Final Answer:
Penetrance