Final answer:
A trait that is polygenic and has a small number of discrete classes is known as a quantitative trait. Polygenic traits like height and skin color are controlled by multiple genes whose combined effects result in a continuous range of phenotypes.
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 quantitative trait. A quantitative trait is controlled by more than one gene, and each gene can have multiple effects on the phenotype, which often results in a small number of discrete categories. For example, kernel color in wheat or the number of seeds in a pod can show such quantitative variation.
Polygenic traits such as height, skin color, and weight are examples where multiple genes contribute to a complex phenotype. In human genetics, these traits do not adhere to simple Mendelian inheritance because they are influenced by interactions between several genes, which makes their inheritance more complex. Each dominant allele in a polygenic trait contributes to the phenotype in a cumulative or additive way.