Part One interweaves the present-day story of the Rosens, a young family on an odyssey to find a cure for their four-year-old daughter's rare genetic disease, with stories of the exciting discoveries of the early pioneers in genetics — Gregor Mendel, Thomas Hunt Morgan, Francis Crick, and James Watson. This episode also tracks the dark period in human history when a little genetic knowledge was used to justify terrifying human experiments that culminated in the Holocaust.