Barbara McClintock and the discovery of jumping genes.

The history of modern genetics begins with the experiments of Gregor Mendel (1822-1884). Mendel found that when hereditary traits were followed through successive generations of hybridisation, the numbers of offspring that resembled parental types were in simple numerical ratios relative to one another – 1:1, or 3:1, or 9:3:3:1, and so on. The most straightforward explanation of these numbers was that the traits were associated with discrete, indivisible entities, later to be called genes. Mendel’s observations languished in obscurity for 34 years until their rediscovery in 1900. Following this, rapidly accumulating data enabled genes to be mapped. Genes were found to be organised into distinct groups that were arranged in a linear order. This, along with other information, suggested that genes kept company with the thread-like structures called chromosomes that existed inside the cell’s nucleus. Over a period of about 50 years biologists dealt with genes and built up a successful predictive science of genetics. All the same, no one knew for sure what genes were made of (a situation that has interesting parallels to the development of the atomic theory). An understanding of the nature of the gene had to await the identification of DNA as the genetic material.