This study explores how individuals make use of scientific content knowledge for socioscientific argumentation. More specifically, this mixed-methods study investigates how learners apply genetics content knowledge as they justify claims relative to genetic engineering. Interviews are conducted with 45 participants, representing three distinct groups: high school students with variable genetics knowledge, college nonscience majors with little genetics knowledge, and college science majors with advanced genetics knowledge. During the interviews, participants advance positions concerning three scenarios dealing with gene therapy and cloning. Arguments are assessed in terms of the number of justifications offered as well as justification quality, based on a five-point rubric. Multivariate analysis of variance results indicate that college science majors outperformed the other groups in terms of justification quality and frequency. Argumentation does not differ among nonscience majors or high school students. Follow-up qualitative analyses of interview responses suggest that all three groups tend to focus on similar, sociomoral themes as they negotiate socially complex, genetic engineering issues, but that the science majors frequently reference specific science content knowledge in the justification of their claims. Results support the Threshold Model of Content Knowledge Transfer, which proposes two knowledge thresholds around which argumentation quality can reasonably be expected to increase. Research and educational implications of these findings are discussed.