Alfred De Bat · Public domain

Hermann Joseph Muller showed that X-rays could induce genetic mutations, the first demonstration that radiation could alter heredity directly rather than simply damage tissue — a discovery that won him the 1946 Nobel Prize in Physiology or Medicine. He spent much of his later career warning publicly about the genetic risks of radioactive fallout from nuclear testing, helping push the dangers of low-dose radiation into public and scientific debate at a time when few researchers were willing to.