This is one of 30 lessons from the NSTA Press book Scientific Argumentation in Biology. The lesson engages students in an argumentation cycle in which they determine if an individual could be the abducted son who disappeared 20 years prior. The lesson engages students by posing the question of whether or not the individual could be a member of this family. The lesson begins by discussing background information of Short Tandem Repeats (STRs) of DNA segments as genetic markers. The students are then given diagrams of the STR Family Relationship Analysis Test from four different chromosomes and a diagram of the lab protocol for the setup for the analyses. The test analysis is for one parent, four potential siblings and the individual in question. Through analysis of the diagrams, students must then decide and present evidence as to whether or not the individual in question could be a family member. Students are challenged to collect evidence needed to construct an argument defending their claim of relationship. As they construct this argument, students build and apply knowledge of genetic markers and STRs. This lesson is intended for middle or high school students. The reading level of this activity is more suitable to secondary level or advanced middle school. Teachers are encouraged to refer to the preface, introduction, student assessment samples, and appendices provided in the full book for important background on the practice of argumentation and resources for classroom implementation.