 Several authors suggest that the debates on in-principle reducibility are far removed from explanatory practice in molecular genetics. Wimsatt (1974) highlights theories of inter-level relations and how they facilitate mechanistic explanations (see the entry on   mechanisms in science).   Pointing towards a sematic view of theory, which holds that theories are families of related models, Waters (1990, 1994) maintains that molecular genetics provides explanations for phenomena described by Mendelian theory. The basis for this is that both fields rely on the principle that only phenotypic differences in a population, rather than the presence of traits is explained by genes (or rather by genetic differences). While these authors still speak of theoretical reduction, theory has changed its status. Rather than one theory containing laws being reduced by another theory through logical deduction, theory is conceived in terms of explanatory models which mediate the conceptual and practical relations between Mendelian and molecular genetics.