Final answer:
A specific trait that requires a combination of two or more genes is called a polygenic trait. These genes may work together in complementary or synergistic ways and often result in continuous variation in the population.
Step-by-step explanation:
When a combination of two or more genes is required to produce a specific trait, it is called a polygenic trait. In this situation, the genes may function in complementary or synergistic ways to produce the observed phenotype. For instance, human skin color is influenced by at least three genes, making it an example of polygenic inheritance. This is in contrast to simpler cases where a single gene with two alleles determines a trait.
In polygenic inheritance, each gene can have multiple variants, or alleles, which contribute to the trait. Such traits often show continuous variation, where individuals exhibit a range of phenotypes with small differences from one another due to the combined effect of multiple genes. Traits like eye color and human height are products of multiple genes acting in concert; for eye color in humans, at least eight genes with different alleles are involved in determining the trait.