 Genetics is devoted to the study and manipulation of heredity and variation in living organisms. Genetics is so pervasive in twenty-first century science—in reproductive screening technologies like preimplantation genetic diagnosis, in assessments of what species are endangered, in public health programs that track antibiotic-resistant bacteria, to name a few—that it is easy to forget what these disparate practices all have in common with one another: a focus on the patterns and mechanisms of trait transmission from one generation to the next in order to understand and potentially control that process. This contemporary focus can be traced back to the first years of the twentieth century, when genetics took shape as a unique field of study. 