This interrupted case study immerses students in one of the most pivotal discoveries in genetics: the physical location of genes on chromosomes. Set in the early 20^th century, the case follows Thomas Hunt Morgan’s investigation into an unexpected white-eyed mutation in Drosophila melanogaster. Through a sequence of experimental crosses and unpredicted results, students apply and challenge classical Mendelian ratios, examine deviations from predicted outcomes, and ultimately discover the principle of X-linked inheritance. Designed for high school or introductory undergraduate biology courses, the case engages learners through guided interruptions that promote critical thinking, hypothesis generation, and conceptual understanding. It introduces key genetic concepts such as alleles, autosomal versus sex-linked inheritance, dominance, and genetic linkage. As the narrative unfolds, students trace the historical and scientific reasoning that led to the confirmation of the chromosome theory of inheritance, illustrating how empirical evidence shapes scientific understanding.