Pedigree problems are typical genetics tasks in schools. They are well suited to help students learn scientific reasoning, representing realistic genetic problems. However, pedigree problems also pose complex requirements, especially for secondary students. They require a suitable solution strategy and technical knowledge. In this study, we examined the approaches used by N = 89 secondary school students when solving two different pedigree problems. In our qualitative analysis of student responses, we examined how two groups of secondary students with varying degrees of experience in genetics constructed arguments to support their decisions. To do so, we categorized I = 516 propositions from students' responses using theory- and data-driven codes. Comparison between groups revealed that "advanced genetics" students (n = 44) formulated more arguments, referred more frequently to specific family constellations, and considered superficial pedigree features less often. Conversely, "beginning genetics" students did not use a conclusive approach of step-by-step falsification but argued for the mode of inheritance they believed was correct. Advanced genetics students, in contrast to beginners, to some extent used a falsification strategy. Finally, we demonstrate which family members students used in their decisions and discuss a variety of typical but unreliable arguments.