The Nobel Prize in Physiology or Medicine 1946.
"for the discovery of the production of mutations by means of X-ray irradiation"
Hermann Joseph Muller (1890 - 1967) won the Noble Prize for showing that X-rays could induce mutations in Drosophila melanogaster. He was able to isolate and map specific mutations caused by X-rays showing that these were stable genetic changes.
For a brief description of the technique, see Hermann Muller Invented Balancer Chromosome.
The significance of Muller's work is described in the presentation speech on the Nobel rize website.
It was known, already at the turn of the century, that apparently sudden changes may appear spontaneously in the hereditary mass, which result in changes in the characteristics of the organism. We now know that these changes may be of different types, and among them occur also disturbances in individual genes. These are very rare, however. Even in such a convenient experimental object as the banana fly, introduced by Morgan, where the generations succeed each other rapidly, and thousands of flies can be examined, it is only seldom that mutations are observed. Muller grappled with the task of trying to change the frequency of mutations. He first created procedures, technically extremely elegant, by which the mutation frequency could be measured exactly. When this task - which took several years - had been completed, the effect of different agents on the frequency of mutations was investigated, and the discovery for which the Nobel Prize is now awarded was made, viz. that irradiation with X-rays evokes large numbers of mutations. Experiments could be arranged, for instance, so that nearly 100 per cent of the offspring of irradiated flies showed mutations. Thus a possibility had been created for the first time of influencing the hereditary mass itself artificially.
This discovery aroused a great sensation already when it was first published in 1927 and rapidly led to a great deal of work of different kinds and in the most varied directions. The mechanism of the effect of rays was studied by many research workers, with Muller at their head. Greatly simplified X-ray irradiation, as also ionizing irradiation, could be likened in general to a shower of infinitely small (even compared with the individual cell) but highly explosive grenades, which explode at different spots within the irradiated organism. The explosion itself (or the fragments it throws up) tears the structure of the cell to pieces or disturbs its arrangement. If such an explosion happens to take place in or close to a gene, its structure, and therewith also its effect on the organism, may be changed.
Muller's discovery of the induction of mutations by means of rays has been of tremendous importance for genetics and biology in general.