Expressivity refers to
Expressivity describes variability in severity, intensity, or extent to which genotype expressed among individuals who show trait. For identical mutation, some persons exhibit mild or subtle manifestation while others show severe phenotype due to variation in genetic background modifiers, environmental influences, or random developmental noise affecting threshold. Unlike penetrance which measures all-or-none presence, expressivity is quantitative degree. Examples include polydactyly, neurofibromatosis type 1, and Waardenburg syndrome with broad phenotypic spectrum. Clinicians measure expressiv
Ref: Hartl & Ruvolo, Genetics, 9th ed., Chapter 5: Variable Expressivity and Modifier Genes
