Microscopic image of the chromosome structure in the salivary glands of sciara taken by Barbara McClintock in 1934. Throughout her long and distinguished career, McClintock's work focused on the genetics of maize and the relationship between plant reproduction and subsequent mutation. Beginning in the late 1920s, she studied how genes in chromosomes could "move" during the breeding of maize plants. She did groundbreaking research on this phenomenon, where she determined the physical correlate of genetic crossing-over. Later, during the 1940s and 1950s, McClintock showed how certain genes were responsible for turning on or off physical characteristics, such as the color of leaves or individual corn kernels. She developed theories to explain the suppression or expression of genetic information from one generation of maize plants to the next that defied the common wisdom of molecular biology prevalent during the 1950s. In 1983, at the age of 81, she received the Nobel Prize in Physiology or Medicine for her work on "mobile genetic elements," the ability of genes to change position on the chromosome.