Final answer:
A trait that is polygenic but has a small number of discrete classes or categories is called a threshold trait. This type of trait is influenced by several genes and individuals fall into distinct phenotypic classes when a certain genetic 'threshold' is reached.
Step-by-step explanation:
When a trait is polygenic, but distinguished from continuous and meristic traits by having a small number of discrete classes, it is said to be a threshold trait. Polygenic traits are characteristics that are influenced by more than one gene and can show a broad range of phenotypes. Eye color, skin color, and height are classic examples of polygenic traits because these characteristics are not determined by a single gene but by the combination of multiple genes.
Unlike traits with a continuous variation, such as height with gradations from short to tall, threshold traits have distinct categories. While they are still polygenic, individuals with these traits fall into one of a few discrete categories or classes, representing different phenotypic outcomes based on the cumulative effect of multiple genes reaching a certain 'threshold' to express a particular trait.