Final answer:
The multiple-gene or multiple-factor hypothesis states that many observable characteristics are controlled by multiple genes, acting in unison. These genes work together to determine a single heritable trait. Polygenic inheritance is a term used when multiple genes contribute to a phenotype.
Step-by-step explanation:
The multiple-gene or multiple-factor hypothesis in biology refers to the idea that many observable characteristics are influenced by multiple genes, each with two or more alleles. These genes work together to control a single heritable trait. For example, eye color in humans is determined by at least eight genes. Each gene may have different variations, or alleles, that contribute to the overall eye color phenotype. This phenomenon is also known as polygenic inheritance.