Final answer:
The phenomenon where a genotype's phenotype is expressed only under specific environmental conditions is known as genotype by environment interaction. Gene expression varies with environmental factors, leading to different phenotypes despite identical genotypes, as observed in Mendel's experiments.
Step-by-step explanation:
The condition in which a phenotype associated with a genotype is expressed only in a particular subset of environments is known as genotype by environment interaction. This occurs because the expression of genes can vary depending on environmental conditions. Despite cells having the same genotype, they may display different phenotypes due to such environmental influences. For example, certain traits may only become evident when exposed to specific environmental triggers, such as temperature or nutrition availability.
An illustration of this concept is seen in Mendel's pea plant experiments, where genetic traits like pod color could be influenced by the environment, leading to variations in phenotype despite a shared genotype. Natural selection can act on these phenotypic differences, affecting which alleles are passed on to subsequent generations.