A teaching unit on genetics and human inheritance using problem-solving methodology was undertaken with fourth-level Spanish Secondary Education students (15 year olds). The goal was to study certain aspects of the students' learning process (concepts, procedures and attitude) when using this methodology in the school environment. The change experienced by students in the process of problem-solving is discussed: the analysis of the problem, the formulation of hypothesis, the design of a solution, the putting into practice of that strategy and the analysis of results. As genetics is one of the few biology topics best addressed through problem-solving, and problem-solving traditionally has been treated in Spain as that of closed problems, it is important that we have shown that working with genuine problematic situations allows students to successfully solve closed problems without prior training.