Maize

Many important genetic discoveries were made in maize by a group of scientists brought together at Cornell University in the 1920s and 30s by R. A. Emerson, who is often referred to as the spiritual father of maize genetics. The Emerson group, which included the future Nobel laureates Barbara McClintock and George Beadle, laid the foundation of maize genetics. They assembled information on maize mutants and ultimately produced the first genetic map of maize, based on linkage studies, in 1935. McClintock's first major contribution occurred early in her career (1929), when she perfected the techniques used to visualize maize chromosomes under the microscope. This allowed individual chromosomes to be identified by size, form, and features such as the highly staining regions, called "knobs."

This milestone allowed McClintock and other members of the Emerson group to make major advances in cytogenetics, which combines genetic crossing data and cytological landmarks to locate genes on chromosomes. Cytological landmarks include trisomics, reciprocal translocations, and deficiencies. Another of McClintock's breakthroughs, achieved with the collaboration of her colleague Harriet Creighton, was to establish the cytological proof of crossing over, which refers to the exchange of chromosomal segments during meiosis. Of course, McClintock's most famous discovery was that genetic elements within the genome can move (transpose) from one locus on the chromosome to another. These "jumping genes" (transposable genetic elements of transposons) were later discovered in bacteria, flies, and humans and eventually resulted in McClintock receiving a Nobel Prize in 1983.

In recent years, transposable elements have been exploited as tools for understanding the function of many maize genes. If a transposon inserts into a gene, it will disrupt the function of that gene. The disruption of gene function may result in a mutant phenotype affecting tissues or developmental stages of the plant that give some indication of the function of that gene. For instance, a transposon that inserts into a gene required for chlorophyll production would result in an albino seedling. Because the DNA sequences of many transposable elements in maize are known, they provide convenient molecular tags with which to clone and further characterize the gene into which they have inserted. Corn transposons have also been adapted to mutagenize and "tag" genes in the model plant Arabidopsis thaliana.