While learning and teaching difficulties in genetics have been abundantly explored and described, there has been less focus on the development and field-testing of strategies to address them. To inform the design of such a strategy a review study, focus group interviews with teachers, a case study of a traditional series of genetics lessons, student interviews, and content analysis of school genetics teaching were carried out. Specific difficulties reported in the literature were comparable to those perceived by Dutch teachers and found in the case study and the student interviews. The problems associated with the abstract and complex nature of genetics were studied in more detail. The separation of inheritance, reproduction and meiosis in the curriculum accounts for the abstract nature of genetics, while the different levels of biological organisation contribute to its complex nature. Finally, four design criteria are defined for a learning and teaching strategy to address these problems: linking the levels of organism, cell and molecule; explicitly connecting meiosis and inheritance; distinguishing the somatic and germ cell line in the context of the life cycle; and an active exploration of the relations between the levels of organisation by the students. (Contains 1 table.)