Final answer:
Phenotypes for traits controlled by many genes are hard to predict due to the complex interactions between those genes and the additive nature of their contributions to the final trait expression.
Step-by-step explanation:
It is harder to determine which phenotype a person will have when a trait is controlled by many genes because of option C: Because multiple genes interact to influence the trait, and their effects are often additive or interactive. In contrast to simple Mendelian inheritance patterns, which involve a single gene that can produce a predictable phenotype from dominant and recessive alleles, polygenic traits are significantly more complex. This complexity means that predicting the phenotype becomes challenging, as many variables and various degrees of genetic interactions are involved.
Most human traits are polygenic rather than Mendelian. For example, skin color and height are influenced by the interaction of several genes, which makes the final phenotype a result of the cumulative effect of all these genes. Furthermore, polygenic traits can be influenced by environmental factors, adding an additional layer of variability to the phenotype.