Final answer:
A reaction norm is the phenotypic possibilities of a single genotype across different environmental conditions, reflecting the interaction between an organism's genes and the environment that influences the traits expressed.
Step-by-step explanation:
The concept of a reaction norm is related to the potential range of phenotypes that a single genotype can produce across various environmental conditions. This biological principle is foundational in genetics and evolutionary biology, and it highlights the interplay between an organism's genetic makeup and the influence of its environment.
Phenotypic plasticity, on the contrary, refers to the measure of the ability to express different phenotypes depending on environmental conditions and is not the opposite of a reaction norm, but rather a concept contained within it.
To illustrate a reaction norm, consider the variety of fur colors exhibited by a population of rabbits that all share a particular genotype for fur color. Depending on various environmental influences like temperature or habitat, these rabbits may develop a range of fur colors from that one genotype, illustrating the reaction norm in action.