Studies have highlighted the contributions of argumentation to the teaching of Natural Sciences. However, analyses investigating its presence and quality in textbooks are still scarce. This qualitative study examines arguments expressed in the statements of Mendel’s First Law in high school Natural Sciences textbooks approved by the PNLD/2021. For the analysis, Toulmin’s Argument Pattern was adopted as the theoretical and methodological framework, complemented by the instrument proposed by Penha and Carvalho (2015) and adapted by Silva (2021). Among the seven textbook collections analyzed, one does not present any argumentative elements in its approach to Mendel’s First Law, five display only partially satisfactory argumentation, with elements that do not fully fulfill their functions, and only one contains a satisfactory argument, in which each identified element adequately serves its purpose. The results reveal significant gaps in the arguments related to Mendel’s First Law, highlighting the need to improve this approach in textbooks.
