Controlling Elements and the Gene

In a recent brief review (McClintock, 1956), a description was given of types of elements carried in the maize chromosomes that serve to control gene action and to induce, at the site of the gene, heritable modifications affecting this action. These elements were initially discovered because they do not remain at one position in the chromosome complement. They can appear at new locations and disappear from previously determined locations. The presence of one such element at or near the locus of a known gene may affect the action of this gene. In so doing, it need not alter the action potentials of the genic substances at the locus. Therefore, these elements were called controlling elements. It was also shown that controlling elements fall into groups, the members of each operating as an integrated system in the control of gene action.