This book offers an integrated historical and philosophical examination of the origin of genetics. The author contends that an integrated HPS analysis helps us to have a better understanding of the history of genetics, and sheds light on some general issues in the philosophy of science. This book consists of three parts. It begins with historical problems, revisiting the significance of the work of Mendel, de Vries, and Weldon. Then it turns to integrated HPS problems, developing an exemplar-based analysis of the development and the progress in early genetics. Finally, it discusses philosophical problems: conceptual change, evidence, and theory choice. Part I lays out a new historiography, serving as a basis for the discussions in part II and part III. Part II introduces a new integrated HPS method to analyse and interpret the historiography in Part I and to re-examine the philosophical issues in Part III. Part III develops new philosophical accounts which will in turn make a better sense of the history of scientific practice more generally. This book provides a practical defence of integrated HPS: the best way to defend integrated HPS is to do it.