Transmission genetics labs are valuable for undergraduate development of analytical thinking skills. However, these labs typically present the scientific process as artificially linear, with a single "right answer." This resource describes an inquiry-based approach to teaching a transmission genetics lab focused on plant cotyledon color. Student groups are required to develop a hypothesis based on limited information, determine phenotype categories based on observation rather than given information, and use qualitative and chi-squared analysis to evaluate their hypotheses. Optional activities to further enhance the scientific thinking component of the project are also described.