Transposable Genetic Elements

Barbara McClintock originally theorized that unusual patterns of phenotypic variance (in corn kernels) could be explained by gene transposition. However, this explanation did not coincide with traditional Mendelian inheritance that genetic information was fixed within the genome, and her views were not widely accepted within the scientific community until the 1960s, when evidence of transposition began to accumulate.

McClintock was a cytogeneticist working on maize. She noted that, in maize, there was a pigment-bearing layer of the kernel, called the aleurone layer, that changed color from kernel to kernel and generation to generation. She also noted a baffling result that occurred when a homozygous plant for purple aleurone (CC) was crossed with a colorless aleurone homozygote (cc). About one-half of the kernels of offspring corn were solid purple, and one-half were purple with varying sizes of colorless spots, suggesting breakage (and loss) of the C locus. However, one of 4,000 analyzed Figure 1. Schematic diagrams of various classes of transposable elements. kernels was colorless with purple spots, indicating a "gain-of-function" of the c locus. McClintock identified a locus (called Ds, which stands for dissociation) that induced nearby chromosome breakage only in the presence of another gene, which she called Ac (for activator). She concluded that the inactivation of pigment production is caused by insertion of the Ds gene into the C locus (disrupting pigment products and yielding the colorless background), and the eventual movement of the Ds out of the gene, restoring pigment production and yielding the purple spots. All of this could occur within a single kernel.

Further support for the mobility of the Ds and Ac genes was the inability to determine their chromosomal locations, which differed among plants. High mutational reversion rates have been subsequently identified in other organisms, with mobile elements now offering a plausible explanation.