McClintock's jumping genes

Barbara McClintock had made her name in maize cytogenetics, the study of genes through the chromosomes that carry them. In 1931 she and her graduate student Harriet Creighton showed that genetic crossing-over involves a physical exchange of chromosome segments. In 1944 she became the third woman elected to the National Academy of Sciences, and in 1945 the first woman president of the Genetics Society of America.

At the Carnegie Institution's Department of Genetics at Cold Spring Harbor, New York, she had land to grow corn and no teaching duties. She studied a site on maize chromosome 9 where the chromosome kept breaking, which she named Dissociation, or Ds. Among plants grown in the summer of 1944 she found that Ds broke the chromosome only when a second locus, Activator, or Ac, was present, even on a different chromosome. Early in 1948 she found that both could change position. Where they landed they could turn neighboring genes off, causing unstable mutations, and when they moved away the gene could work again. The color of individual corn kernels was one trait they switched.

She set out the evidence in the Proceedings of the National Academy of Sciences in 1950 and spoke on it at the Cold Spring Harbor Symposium in the summer of 1951. By then Royal Alexander Brink and Robert Nilan at the University of Wisconsin had confirmed transposition in maize. Accounts of the reception differ. Evelyn Witkin remembered dead silence, and McClintock spoke of puzzlement, even hostility; Joshua Lederberg argued in 1983 that she would not have been invited if the organizers had not seen some importance in the work. Her view of these elements as controllers of genes clashed with the picture of genes as fixed in a line on the chromosome, and after meeting skepticism she largely stopped publishing her data in journals.

In the early 1970s molecular biologists found transposition in bacteria and viruses, and later in yeast; it turned out to move genes for antibiotic resistance between bacteria. In 1983, at 81, McClintock received the Nobel Prize in Physiology or Medicine for her discovery of mobile genetic elements, the first woman to win that prize unshared. By a 2012 estimate in PNAS, transposons make up more than 65 percent of the human genome and about 85 percent of the maize genome. She died on 2 September 1992.