Define polygenic and pleiotropic traits and explain their significance for researchers’ efforts to link human genotypes to phenotypes.

Polygenic traits are those traits that are controlled by more than one gene (poly- means many). The examples for polygenic traits are eyes colour, hair colour, human height, etc. 
Pleiotropic traits are those traits that are controlled by one gene. For instance, if one gene controls two different traits, then those traits are pleiotropic traits. The example is gene for phenylketonuria and sickle-cell anemia.
Knowing and understanding how different traits are controlled (by one or by many genes) is important for linking human genotypes to phenotypes.