In 1956, Margaret Bastock published the first demonstration that a single gene could change a behaviour pattern. A Ph.D. student with Niko Tinbergen, Bastock's work was partly inspired by discussions between Tinbergen and the American evolutionary biologist, Ernst Mayr. In this essay, the genesis of Bastock's work is outlined, including reference to archival correspondence between Mayr and Tinbergen, and she is given the credit for developing the study of how a mutation can affect a behaviour pattern. Her paper is described and put into contemporary context, including an analysis of its impact in the 1960s and beyond. Finally, the implications of this study for modern investigations into the genetic bases of behaviour, from behavioural ecology to neuroscience, are discussed. © 2007 The Association for the Study of Animal Behaviour.