The overall aim of this thesis is to investigate how teachers talk about genetics in actual classroom situations. An understanding of how language is used in action can give detailed information about how the subject matter is presented to the students as well as insights in linguistic challenges. From the viewpoint of seeing language to be at the very core of teaching and learning, this study investigates teachers’ spoken language in the classroom in topics within genetics that are known to be both crucial and problematic. Four lower secondary school teachers in compulsory school grade 9 (15-16 years old) were observed and recorded through a whole sequence of genetic teaching. The empirical data consisted of 45 recorded lessons. The teachers’ verbal communication was analyzed using thematic pattern analysis, which is based on the framework of systemic functional linguistics (SFL). The focus of the thesis is to determine how teachers talk about the relationships between the concepts of gene, protein and trait, i.e. the functional aspects of genetics. Prior research suggests that this is a central aspect of genetics education, but at the same time it is problematic for students to understand because the concepts belong to different organizational levels. In the first study I investigated how the concepts of gene and trait were related in the context of Mendelian genetics. My results revealed that the teachers’ way of talking resulted in different meanings regarding the relationship between gene and trait: 1) the gene as an active entity causing the trait 2) the gene as a passive entity identified by the trait 3) the gene as having the trait, and 4) the gene as being the trait. Moreover it was found that the old term anlag was regularly used by the teachers as synonym for both gene and trait. In the second study I examined how teachers included proteins in their lessons, and if and how they discussed proteins as a link between different organizational levels. This study showed that teachers commonly did not emphasize the many functions of proteins in our body. The main message of all teachers was that proteins are built. Two of the teachers used proteins as a link between gene and trait, whereas two of them did not. None of the teachers talked explicitly about genes as exclusively coding for proteins, which implies that the gene codes for both proteins and traits. The linguistic analysis of teachers’ talk in action revealed that small nuances in language used by the teachers resulted in different meanings of the spoken language. Thus, my work identifies several linguistic challenges in the teaching of genetics. / <p>This thesis is written within the framework of the Hasselblad Foundation Graduate School, a four-year programme financed by the Hasselblad Foundation.</p>