 (a) Whereas theory reduction assumes that reduction is a relation between theories, models of explanatory reduction permit other features as the relata of a reductive explanation, such as fragments of a theory, generalizations of varying scope, mechanisms, and even individual facts. One approach pursued along these lines is due to Kenneth Waters (1990, 1994, 2000). Instead of construing theory relations in terms of the Nagel-Schaffner model, he identifies a key principle of inference operating in classical genetics and molecular genetics: the “difference making principle” (gene differences cause differences in phenotypes). This principle can be ascribed to  genes  in both classical and molecular genetics. An explanatory reduction is achieved between them because their causal roles correspond. Waters (2008) extends this account by emphasizing how the investigative practices of classical genetics were “retooled” by developments in molecular genetics, which accounts for the success of the corresponding explanations.