Expert-novice problem-solving research is extended in this study to include classical genetics. Eleven undergraduates (novices) and nine graduate students and instructors (experts) were videotaped as they solved moderately complex genetics problems. Detailed analysis of these “think aloud” protocols resulted in 32 common tendencies that could be used to differentiate between successful and unsuccessful problem solvers. Experts perceive a problem as a task requiring analysis and reasoning and they tend to use a knowledge-development (forward-working) approach. They make frequent checks on the correctness of their work, use accurate and detailed bookkeeping procedures, and have a broader range of heuristics to apply to the problem. It is clear that studying problem solving using the expert/novice design requires that the problems be difficult enough to require more than more recall and yet simple enough to allow novices a chance for solution. Applying elementary probability concepts seemed to be the most difficult aspect of many of the genetics problems, even for the experts.