Pleotropy is defined as which of the following?

Pleiotropy occurs when one single gene influences multiple phenotypic traits. This means that a mutation or variation in that gene can have effects on different, often unrelated, characteristics of an organism. An example of pleiotropy is seen in certain genetic disorders, where a single gene mutation can result in a variety of symptoms affecting different body systems. This illustrates the complexity of genetic regulation and how one gene can play multiple roles in an organism's development and physiology.

The other options do not accurately reflect the definition of pleiotropy. Multiple genes influencing a single phenotype refers to polygenic inheritance, environmental factors affecting gene expression relates to epigenetics, and variations of a single phenotype across populations pertain to population genetics, which are distinct concepts from pleiotropy.