Final answer:
When a single gene affects multiple phenotypic traits, it is known as pleiotropy. However, most human traits, such as skin color and eye color, are polygenic, meaning they are controlled by multiple genes.
Step-by-step explanation:
When a single gene influences multiple phenotypic traits, the effect is called pleiotropy. However, it's important to note that most traits are not controlled by a single gene. Instead, they are often the result of polygenic inheritance, where several genes, each with multiple alleles, contribute to a single phenotypic trait.
An example of polygenic inheritance is human skin color, which involves the combined action of at least three genes. In contrast to Mendel's initial hypothesis about individual traits being controlled by single genes, we now understand that characteristics such as height, skin color, and eye color are influenced by the interaction of multiple genes. For instance, at least eight genes are known to contribute to the determination of human eye color.