Though argumentation is a key component of scientific knowledge construction practices, it rarely occurs in classrooms worldwide. Moreover, related literature in the context of genetics and evolution learning primarily addresses them separately. In contrast, this paper is part of broader research on genetics and evolution instruction taught together through scientific practices. We focused on students' arguments and data use while working on a task that required developing explanatory links between sickle cell disease and malaria. Two research questions were framed: What is the nature of students' arguments while explaining these links? What data do they mobilise and how do they use them to support their arguments? Our qualitative approach followed discourse analysis methods. To assess the quality of arguments, theoretical and empirical criteria were used, including the justifications and causal language in the arguments as well as theoretical considerations of genetics and evolution knowledge. The analysis of the second question focused on data use and their sources. The results pointed to students' difficulties in building quality arguments using clear causal language, and to use of genetics and evolution notions in their conclusions. We suggest implications regarding how to address processes occurring on different time scales in combined genetics and evolution instruction.