Final answer:
Quantitative traits are phenotypes that are conditioned by multiple loci and form continuously variable traits, influenced by both genotypes and environmental factors.
Step-by-step explanation:
Phenotypes that are conditioned by multiple loci and form continuously variable traits are most appropriately termed quantitative traits. These traits are influenced by both genotypes and environmental factors, and they exhibit a wide range of variability that can be measured, such as height or weight. Unlike Mendelian traits, which are determined by single genes and exhibit discrete categorical differences, quantitative traits are polygenic and show continuous variation.