McClintock , Barbara

(1902–1992) American geneticist

The daughter of a physician, McClintock was born in Hartford, Connecticut, and educated at Cornell's College of Agriculture, where she received her PhD in 1927 for work in botany. She remained at Cornell until 1936 supported by various grants from the National Research Council and the Guggenheim Foundation. But there was no future at Cornell for her as, until 1947, only the department of home economics appointed women professors. Fortunately a new genetics department was being set up in the University of Missouri by Craig Stadler, who knew and admired her work, and she was offered a post as assistant professor there, although it was made clear to her that any further advancement would be unlikely. She left in 1941, and in 1944 was elected to the National Academy of Sciences, becoming only the third woman to be so honored. McClintock then joined the Carnegie Institute's Cold Spring Harbor Laboratory, New York, where she remained until her death.

By the 1920s Morgan and other geneticists, working mainly with the Drosophila fruit fly, had established that gene action was connected with chromosomes and thereby established the new discipline of cytogenetics. Drosophila chromosomes, however, before the discovery of the giant salivary chromosomes by T. Painter in 1931, were too small to reveal much detail. McClintock chose to work with a variety of maize that possessed much more visible chromosomes. Further, the development of new staining techniques allowed McClintock to identify, distinguish, and number the ten maize chromosomes.

Morgan and his group had also demonstrated the existence of ‘linkage groups’ in Drosophila – groups of genes, such as those for white eyes and maleness, linked together because the genes themselves were sited near each other on a chromosome. In a series of papers published between 1929 and 1931, McClintock established similar linkage groups in maize. Because maize chromosomes were more visible under the microscope than those of Drosophila, McClintock was able to identify the chromosomal changes responsible for a change in phenotype and thus confirmed Morgan's work.

McClintock's own Nobel Prize for physiology or medicine, awarded in 1983, was for later work done on the so-called ‘jumping genes’. In the 1940s at Cold Spring Harbor, McClintock planted her maize and began to track a family of mutant genes responsible for changes in pigmentation. She was struck by the fact that mutation rates were variable. After several years' careful breeding, McClintock proposed that in addition to the normal genes responsible for pigmentation there were two other genes involved, which she called ‘controlling elements’.

One controlling element was found fairly close to the pigmentation gene and operated as a switch, activating and turning off the gene. The second element appeared to be located further away on the same chromosome and was a ‘rate gene’, controlling the rate at which the pigment gene was switched on and off. She further discovered that the controlling elements could move along the chromosome to a different site and could even move to different chromosomes where they would control different genes. McClintock gave a full description of the process oftransposition, as it became known, in her 1951 paper,Chromosome Organization and Genic Expression. McClintock's work was largely ignored until 1960 when controlling elements were identified in bacteria by Monod and Jacob.