Understanding genetics is one of the most significant learning difficulties faced by students. To improve teaching genetics and the students’ representational competence within the classroom have been proposed using multiple representations. This study presents an analysis of representational levels used by high school students learning Mendelian genetics and determining the construction trajectories of these representations. An assessment instrument was used to analyse the representational levels of 186 students with three different teaching strategies. The levels were used to analyse and classify students’ answers, written texts, and drawings and diagrams. The result shows that learning Mendelian genetics is a gradual process in which students appeal to various levels of representations that can determine different trajectories and demonstrate progress on comprehension. We identify five trajectories, ranging from responses that rely on descriptions of observable traits to responses from students who could integrate processes or use models.