Using the think-aloud interview technique, 16 undergraduates and 11 genetics graduate students and biology faculty members were asked to solve from 1 to 3 classical genetics problems which require pedigree analysis. Subjects were classified as either successful or unsuccessful and the performances of these groups were analyzed from videotaped recordings of the interviews. A number of previously reported findings were corroborated. Additional observations are discussed in terms of genetic knowledge, use of production rules, strategy selection, use of critical cues, hypothesis testing, use of logic, understanding of issues of probability, and the thinking process itself. Taken collectively, these findings evidence a remarkable similarity between the successful solution of pedigree problems and the processes of medical diagnosis and scientific investigation. This convergence of research findings suggests a qualitative advance in the understanding of problem solving. Based on this understanding, recommendations for classroom instruction are presented.