Modern genetics is a relatively new domain but it is increasingly important for students to have a firm grasp on the content since genetically modified organisms, genetic screenings, and stem cell therapies are becoming more commonplace. In a previous study, we used the Learning Progression-based Assessment of Modern Genetics to assess high school students’ knowledge of genetics concepts after an intensive ~23 calendar week long genetics instructional period. Given that this type of instruction is unique and may represent a “best case scenario,” we are now investigating how students’ knowledge of genetics changes after instruction (i.e. learning loss effect). Using multi-level growth modeling, we find that overall student scores were significantly decreased a year after instruction ended compared to their scores immediately after instruction ended. Interestingly, there was no significant difference in scores a year after instruction ended compared to scores immediately after instruction for 10 of 12 genetics concepts. Two concepts showed a significant reduction in student scores: details of meiosis, and how genetics concepts are related to each other. Our analysis demonstrates students tend to retain mechanistic explanations in genetics while forgetting memorized details.