The mysteries of heredity began to succumb to science in the late 19th and early 20th centuries. Key steps were the adoption of the fruit fly as an experimental subject and the discovery by Hermann Muller that nuclear radiation causes mutations. These led to maps of the fly's chromosomes as a chain of genes - essentially today's model. Muller's career was nearly ruined by his socialist views and he exiled himself to Germany and then Russia. He left Russia in a hurry in 1937and after a few years in Edinburgh he returned to the USA, where he struggled to find a suitable academic post. Nonetheless, he warned of the genetic hazards of radiation, particularly weapons fallout, when this was officially unwelcome. He also promoted eugenics - racial improvement - right up to his death. This book is not just a brief biography of Muller but a history of how the ideas he inherited, supported and originated subsequently evolved and flowered or foundered.