Subcellular processes like meiosis are frequently problematic for learners because they are complex and, except for the extent that they can be observed under a light microscope, occur outside of our direct experience. More detailed characterization of what underlies various degrees of student understanding of a process is required to more fully inform the design of useful instruction. This qualitative study used microanalytic techniques to characterize expert/novice reasoning on meiosis, with the primary focus on the role played by domain-specific diagrams. Three groups, labelled the expert group, experienced novices, and inexperienced novices, were created based on their expertise in genetics and were each composed of five subjects. Subjects were asked to think aloud as they solved genetics problems. Follow up questions were used to have the participants elaborate on the biological concepts mentioned and diagrams drawn. Results indicate that experts create chromosome diagrams that are more fine-tuned to the relevant biology and use them to: (1) support internal thought processes; (2) record information; (3) check preceding reasoning; and (4) illustrate components of their problem solutions. Based on the research, the paper recommends that instruction concerning expert pictorial skills and reasoning should be incorporated into meiosis instruction. Students should be assessed for both verbal and pictorial knowledge/reasoning. The appendixes include a review of meiosis and chromosome diagrams. (Contains 15 references.) (PR)