In which scenario is polygenic inheritance most likely to be observed?

Polygenic inheritance is characterized by the involvement of multiple genes influencing a single trait. This scenario leads to a continuous range of phenotypes, rather than distinct categories, such as those seen in traits controlled by a single gene. For example, human height and skin color are the result of the cumulative effect of several genes, each contributing to the final phenotype.

In contrast, the other scenarios primarily focus on the interaction between alleles of a single gene rather than the influence of multiple genes on one trait. Complete dominance occurs when one allele completely masks the effect of another, codominance involves both alleles being fully expressed, and the competition of two alleles typically relates to single-gene inheritance rather than the polygenic trait spectrum. Therefore, the situation where multiple genes affect a single trait is where polygenic inheritance is most likely to be observed.