What does variable expressivity refer to?

Variable expressivity refers to the phenomenon where individuals with the same genotype may exhibit different phenotypes. This means that even when they carry the same genetic makeup, the physical traits that are expressed can vary significantly due to a variety of factors, including environmental influences, modifier genes, and developmental differences. For example, two individuals may have the same genetic mutation that predisposes them to a condition, but the severity of the condition can differ widely between them.

The context of the other options provides clarity on why they do not represent variable expressivity. The second choice implies that alleles are expressed equally in all individuals, which contradicts the concept of variable expressivity, where differences in phenotype can occur even among genetically identical individuals. The third choice suggests a fixed way of expressing alleles, while variable expressivity, by definition, involves variations in expression. Lastly, the fourth option pertains to the dominance relationship between alleles rather than the variability of phenotypic expression resulting from a single genotype. This clearly distinguishes variable expressivity as a concept that highlights the diversity of traits arising from the same genetic foundation.