Researchers are becoming aware of the influence of students' presuppositions in directing their construction of science concepts. When these are entrenched and drastically nonscientific, they predispose the children to alternative explanatory frameworks that are inhibiting, especially in a knowledge domain such as genetics. In this study, we identified such nonscientific presuppositions among 17- to 18-year-old secondary school students from the Igbo community of Southeastern Nigeria. We designed a research-based instructional model to address these presuppositions. The relationship between the levels of nonscientific presuppositions held by students and their achievement in genetics was assessed. Finally, the effect of the instructional model on students' relinquishing these nonscientific presuppositions and on their achievement was determined relative to a comparison group. It was found that this group of students had nonscientific presuppositions that they used in explaining genetic phenomena, and that the present instructional model aided the students in relinquishing these nonscientific presuppositions to a great extent. We conclude that a conceptual change model that addresses explicitly nonscientific presuppositions will lead to an increased understanding of science concepts.