Problem-based learning (PBL) is a student centered approach whereby students deal with ill-structured problems while working in small groups. In this study, a path model was utilized to model the relationships among reasoning ability, learning approach, prior knowledge, motivational variables, and achievement in genetics in PBL classes. 126 eighth grade students participated in the study. Results showed that reasoning ability, learning approach, task value, and prior knowledge had direct effects on achievement in genetics. Moreover, reasoning ability and task value were found to have indirect effects which were mediated by learning approach. Interrelationships found among the variables in the path model were discussed considering the characteristics of the PBL environments and suggestions were made for further research.