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The definition of pleiotropy is best captured by the concept that a single gene can affect multiple traits. In genetics, pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. A classic example of this is the gene responsible for sickle cell anemia, which not only affects the shape of red blood cells but also has implications for oxygen transport and can influence susceptibility to certain infections. This phenomenon shows how a single genetic change can have wide-reaching effects on an organism's overall phenotype.

The other options refer to different genetic concepts. The expression of multiple genes influencing many traits relates more closely to polygenic inheritance, where traits are controlled by multiple genes, while the transition of genes between individuals pertains to horizontal gene transfer, commonly discussed in microbial genetics. Lastly, a genetic mutation with a singular effect describes a more straightforward mutation impacting one specific trait, which does not align with the broader implications inherent in pleiotropy.