Final answer:
The definition that best describes polygenic traits is D. traits that are controlled by multiple genes. Polygenic traits like human height and skin color involve multiple genes and alleles, leading to a broad range of phenotypes.
Step-by-step explanation:
Polygenic traits are characteristics that are influenced by more than one gene and each gene may possess multiple alleles. Unlike traits controlled by a single gene pair, polygenic traits often result in a wide range of phenotypes. This is because each dominant allele contributes to the expression of the trait, creating a spectrum of possible traits. Examples of polygenic traits in humans include height, skin color, and weight, all of which show a broad variation and can be influenced by environmental factors as well as genetic ones.
For instance, human height is a polygenic trait influenced by at least three genes with six alleles in total. The more dominant alleles for height a person possesses, the taller they are likely to be. Similarly, skin color is influenced by multiple genes, making it a polygenic trait as well.