In which scenario would you observe variable expressivity?

Variable expressivity refers to the phenomenon where individuals with the same genetic makeup display different traits or phenotypes. In the chosen scenario involving identical twins, even though they share the same genetic material, they may exhibit differing traits due to a variety of factors such as environmental influences, other interacting genes, or random developmental variations. This scenario exemplifies variable expressivity perfectly, as it highlights how the same genetic information can lead to a range of observable characteristics in different individuals.

The other scenarios presented do not demonstrate variable expressivity. For instance, individuals with the same alleles showcasing the same characteristics would imply uniform expression, contradicting the concept of variable expressivity. Similarly, an allele that is always expressed fully indicates a lack of variation in expression, and alleles that are completely dominant suggest clear and consistent phenotypic outcomes without the variability implied by expressivity.