The acquisition of scientific knowledge is fraught with difficulties and challenges for the learner. The very nature of some scientific domains contributes to the learning difficulties students' experience. Phenomena in these domains are composed of multiple organization levels featuring complicated interactions within and across these levels. Molecular genetics is a compelling example of such a domain. Current instructional practices in genetics leave students ill prepared to benefit from technological advances in genetics or to participate in public discussions about genetic issues. We argue that instruction in genetics needs to be refocused such that it promotes generative understandings of the genetic mechanisms that are at the core of modern genetics. This paper presents the early stages of design-based research involving the development and study of a curriculum unit in genetics for high school students. We employed several design strategies to make the structure and mechanisms of genetic phenomena salient in the activities of the unit. Our findings suggest that the curriculum does, to a large extent, engender a deeper understanding of genetic phenomena. Specifically, we observed gains in students' understanding of the genetic information and the mechanisms that link genes to traits. (Contains 6 tables and 4 figures.)