Final answer:
Barbara McClintock was the first person to propose the existence of transposable elements. Her research on maize genetics in the 1940s and 1950s led to the discovery of transposons, which are mobile segments of DNA that can move within an organism's genome. Transposons have been found to play a role in regulating gene expression and protein expression.
Step-by-step explanation:
The individual who first proposed the existence of transposable elements was Barbara McClintock. In the 1940s and 1950s, McClintock's research on maize genetics led to the discovery of transposons, which are mobile segments of DNA that can move within the genome of an organism. Transposons can regulate gene expression, protein expression, and virulence. Initially, McClintock's findings were not accepted by the scientific community, but later discoveries confirmed the existence and importance of transposons.