We describe the development, validation, and use of the "Learning Progression-Based Assessment of Modern Genetics" (LPA-MG) in a high school biology context. Items were constructed based on a current learning progression framework for genetics (Shea & Duncan, 2013; Todd & Kenyon, 2015). The 34-item instrument, which was tied to 12 constructs within the genetics learning progression, was administered to 65 high school students at three time points (pre, middle, and post) across a 23-week period of instruction. The LPA-MG, its 12 constructs, and 34 items demonstrated high reliability and construct validity with respect to the Rasch model and indicated by satisfactory fit. Furthermore, it demonstrated utility in providing both quantitative and qualitative information about how students moved along each construct. Using a repeated measures ANOVA design, we found that students made gains across the 23 weeks that were both large and significant at the 95% confidence level. Students also moved along each construct in qualitatively meaningful ways during the 23 weeks of instruction.