Final answer:
Polygenic inheritance is key to recognizing traits that vary along a continuum in the population, as these traits are influenced by multiple genes and alleles, resulting in a range of phenotypic outcomes.
Step-by-step explanation:
The key to recognising a trait whose expression is determined by the effects of two or more genes, known as polygenic inheritance, is the observation that the trait varies along a continuum in the population. Unlike Mendelian inheritance where traits are dictated by a single pair of alleles, polygenic traits involve multiple genes and alleles, contributing to a wide range of phenotypic outcomes. Traits such as height, weight, skin color, eye color, and hair color are all examples of polygenic traits with variations due to different combinations of alleles.
In polygenic inheritance, each dominant allele 'adds' to the expression of the trait, resulting in a continuous spectrum of phenotypic expression, rather than discrete categories. Therefore, a trait that shows continuous variation, meaning there are gradations from one extreme to the other within a population, is likely influenced by multiple genes and thus polygenic. For instance, human height is controlled by at least three genes with six alleles, and the combination of these alleles contributes to the overall height of an individual.