Induction of instability at selected loci in maize.

McClintock (Nobel Prize 1983) discovered transposable elements or jumping genes. She found that certain parts of chromosome had switched position. This refuted the then-popular theory that genes were fixed in their position on a chromosome. McClintock found that genes could not only move but they could also be turned on or off due to certain environmental conditions or during different stages of cell development. Digital facsimile from PubMedCentral at this link.