  Rachel Ankeny (2000, 2001) considers several ways in which organisms might plausibly be considered models. She suggests that mapped and sequenced genomes for these various HGP model organisms serve as “descriptive models.” A genome reference sequence is a model because it is, first, “an idealized, abstract entity constructed from the natural organism” and, second, “‘model’ is used to indicate a promissory note about this organism providing a framework for pursuing explanatory questions and ultimately serving as a prototype for understanding more complex organisms” (Ankeny 2000, p. S267). Genome reference sequences are descriptive because they are “constructed largely without motivation by hypotheses to be tested or traditional explanatory questions” but rather as preliminary to such work (Ankeny 2000, p. S267). In this, they are similar to earlier efforts to use wiring diagrams for C. elegans to model neural structure: these diagrams were based on data from several worms but were presented as canonical; the worms were “wild type” and presumed to exhibit species-typical structure; the diagrams served as a tool to investigate abnormalities (Ankeny 2000). Nonhuman genome reference sequences become tools as their corresponding organisms are used as experimental models for understanding basic biological processes common to many species or diseases processes found in humans—for example, by knocking out genes in mice. Here, analogical reasoning is at work; “[m]odels in this sense of the term seem to provide what might be termed strong causal analog models” (Ankeny 2001, p. S255). According to Kenneth Schaffner (1998a), this is quite typical of biological explanation: unlike physicists, biologists frame explanations “around a few exemplar subsystems in specific organisms … used as (interlevel) prototypes to organize information about other similar (overlapping) models” (p. 278).