Which term describes the degree to which a phenotype is expressed among individuals with the same genotype?

• A. Expressivity
• B. Expression
• C. Homozygosity
• D. Heterozygosity

Answer: (A)

Expressivity captures how strongly a genotype is manifested as a phenotype across individuals who carry the same mutation. It explains why two people with the identical genetic change can show different severities or appearances of the trait, from mild to more pronounced features. This is distinct from penetrance, which is about whether the phenotype appears at all in individuals with the genotype. The other terms don’t describe this variation in severity: expression is a broader term often used for gene activity or phenotype in general, while homozygosity and heterozygosity refer to having two identical alleles or two different alleles, respectively, not to how the trait is expressed.