The purpose of this study is to create a model that depicts the beliefs and other factors that influence secondary biology teachers when they incorporate newly acquired subject matter knowledge into the planning and instruction of an existing curriculum. Genetics content from Biology Update 1 and Secondary Biology Laboratory Curricula (SBLC) was used to create a case of contextual content. Two secondary biology teachers' perceptions of their planning and teaching were interviewed, and observed during their graduate courses and their classroom teaching of genetics. The model resulting from this study provides insights into the theoretical basis for increasing teachers' content knowledge understandings during and beyond undergraduate preparation. Research findings also indicate that scientists and science teacher educators should develop courses or workshops that assist inservice teachers in keeping their content knowledge relevant. Encouraging teachers to explore their beliefs about science and science teaching would create a better-informed and validated practice. (KHR)