This paper explores conceptual variation in the depiction of gene function in upper secondary school textbooks. Historically, concepts in genetics have developed in various scientific frameworks, which has led to a level of incommensurability as concepts have changed over time within their respective frameworks. Since students may have difficulties in understanding concepts where there is implicit variation in descriptions of the same phenomena, we have developed a concept mapping instrument and applied it to study the gene function concepts in biology and chemistry textbooks that are widely used in Sweden, and others used in a selection of English speaking countries. The data were then further examined using content analysis. In the present paper we describe the conceptual variation of gene function as it is presented in the textbooks, and analyze the ways in which students' understanding may be influenced. We conclude that it may be difficult for students to gain a modern, process-oriented understanding of gene function if textbooks are used as foundations for the planning and execution of lessons.