This paper reports on findings from an analysis of trait-formation tasks in the genetic sections of high school biology textbooks. Drawing on studies exploring a range of problems in students' explanations of trait formation, we investigated trait-formation tasks and analysed their potential for knowledge integration and multi-level mechanistic reasoning. Among the 216 trait-formation tasks, 39 tasks (18%) were classified as integrative tasks addressing the relationships between genes, proteins and traits. The remaining 177 tasks (82%) were non-integrative tasks addressing selected aspects of the relationship between genes and proteins alone, respectively the relationship between proteins and traits alone. Among the non-integrative tasks, 64% addressed the molecular level alone and thus did not encourage students to address the higher levels of biological organisation. 13% of the non-integrative tasks (n = 23) and 5% of the integrative tasks (n = 2) addressed the role of the environment in trait formation. The findings are related to three main arguments from the genetics literature, concerning knowledge integration and multi-level mechanistic reasoning. As educational implications, we recommend that biology educators make use of integrative tasks to foster students' understanding of trait formation from an integrated model connecting genes, proteins, environmental factors and traits to overcome gene-centred and gene-deterministic beliefs.