Summary: Epigenetics--from Phenomenon to Field

By the time of the 21st Symposium, McClintock's ideas about "controlling elements" had developed (Mc-Clintock 1957). Two were particularly relevant with regard to epigenetics. In the Spm controlling element system, she had uncovered variants that allowed her to distinguish between trans-acting factors that could "suppress" a gene (reduce or eliminate its phenotypic expression), and those factors that could mutate it. She also noted that some controlling elements could suppress gene action, not only at the locus where they had inserted, but also at loci that were located some distance on either side of them. Others were discovering this "spreading effect"