This study investigates Swedish teachers' use of language when teaching Mendelian genetics in compulsory school. The primary objective of the study is to explore how teachers use the related concepts "gene," "allele," and "anlag" (a Swedish variant of the German word "anlage") and how these are related to trait, central relations that have been identified as problematic in genetics education. Four teachers were recorded while teaching genetics. The empirical data consist of 45 recorded lessons. The teachers' verbal communications were analyzed using a linguistic framework called "thematic pattern analysis." The results showed a predominant use of the word "anlag" over "gene," whereas there was a total lack of use of the word "allele." The word "anlag" was used to mean both gene and trait. "Anlag" was also used as a synonym for allele. The teachers' language conveyed four categories of meanings regarding the gene-to-trait relationship: genes "control traits," "are identified by traits," "have traits," or "are traits." This study thus highlights several linguistic challenges in the teaching of Mendelian genetics that might affect students' learning. Moreover, the study exemplifies how a linguistics methodology can be used to investigate science teaching.