Multiple models of the gene are used to explore genetic phenomena in scientific practices and in the classroom. In genetics curricula, the classical and molecular models are presented in disconnected domains. Research demonstrates that, without explicit connections, students have difficulty developing an understanding of the gene that spans multiple domains, as is necessary to solve authentic genetics problems. This dissertation describes the development and testing of pedagogical tools for teaching multiple gene models with the goal of coherent, integrated, and meaningful understanding. A design-based research study was conducted in collaboration with secondary school teachers in Thailand. Prior to using the tools, most teachers applied the two models separately to different domains, either relying on their default (preferred) model, or making a partial connection between models. A few teachers meaningfully integrated the models, demonstrating complex conceptual linkages and tailoring their use of models to the context. Participating in the development and implementation of the tools helped teachers deepen their own understanding of multiple gene models. Furthermore, results demonstrate that the tools helped high school students achieve an integrated knowledge of both models and appropriately apply the models in problem-solving contexts. The study identifies three principles for designing learning materials: engaging with multiple linked representations, applying multiple models in context, and connecting ideas about gene, protein, and phenotype around a circle of representations. Findings from this study could also improve our understanding of the role representations play in bridging different models, and of knowledge structure in other topics involving multiple models. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]