Final answer:
The topic sentence of the paragraph is "A genetically modified organism is one whose genes have been altered," which introduces the concept of genetically modified organisms and their purpose.
Step-by-step explanation:
The topic sentence of the paragraph provided is: "A genetically modified organism is one whose genes have been altered." This sentence defines a genetically modified organism (GMO) and sets the stage for the rest of the paragraph which explains how genes from one species are inserted into another to introduce desired traits such as pest resistance or prolonged freshness. The paragraph goes on to provide examples of GMOs, namely genetically engineered corn and tomatoes to illustrate these modifications.