Argumentation is known to improve understanding of primary literature and aid in achieving the goals of undergraduate biology education. However, argumentation has not traditionally been taught in the undergraduate classroom setting, leading to deficits in this skill among students. To support students to develop strong argumentation skills, and understand where students are struggling when constructing written arguments, it is essential to evaluate how students construct their arguments. Hence, my project aimed to develop a framework to answer the following questions: (1) What is the structure of students’ arguments: how much of their argument is addressing a claim, evidence, and mechanism? (2) How often do students include relevant and correct information in their arguments? (3) What are the common ways that students describe evidence in their written arguments? (4) Does argument quality improve after engaging in peer-review? Using a Toulmin framework, we assessed 240 arguments, written by students who had taken an undergraduate genetics course at the University of California, San Diego, over two years. Current findings reveal that students’ arguments were largely describing evidence, with a low inclusion of biological mechanisms to explain the data. Additionally, the majority of students described evidence as general trends instead of conducting data analysis. Assessing correctness and relevance of statements that make up an argument was not sufficient to reveal the struggles students have with writing scientific arguments. Also, our data suggested that a peer-review process might benefit students with more careful instruction.