Which of the following best defines a pleiotropic effect?

A pleiotropic effect is best defined as one gene affecting multiple traits. This phenomenon occurs when a single gene influences two or more seemingly unrelated phenotypic traits. Such effects are significant in the field of genetics because they illustrate how genes can have widespread implications on an organism's biology beyond a single characteristic.

This concept is crucial in understanding genetic disorders that may arise from mutations in a single gene, leading to a range of symptoms and traits manifesting in an individual. For instance, the gene responsible for Marfan syndrome is pleiotropic, as it affects the connective tissues, leading to characteristics like long limbs and cardiovascular issues.

The other options focus on different genetic relationships: multiple genes affecting a single trait describes polygenic inheritance, a scenario where various genes contribute to one phenotypic characteristic. A trait that is strictly dominant refers to how certain traits can be expressed over others in a heterozygous condition, while traits that are inherited independently pertain to Mendel’s law of independent assortment, which explains how different traits are passed to offspring without affecting each other's inheritance. Each of these concepts is distinct from pleiotropy, which centers specifically on the influence of a single gene on multiple traits.