Understanding genetics, an important topic in the study of biology, is critical for scientific literacy in agriculture, health, and forensic investigations. However, it has been observed that teachers find genetics difficult to teach and pupils do not perform well in it. Teachers can use expository teaching or active learning methods, but the secondary school curriculum in Malawi advocates the latter. The aim of this study was to find out how genetics is taught and how teachers justify their choice of method. This study used a multi-case design involving six teachers and observed their Mendelian genetics lessons and asked them to justify their choice of teaching method. The findings show that teachers mostly use expository methods utilizing lecture and question and answer teaching techniques. Contextual factors such as large classes, limited teaching time, and lack of resources were cited as reasons for their choice of expository techniques. In addition, they said that the topic was abstract and learners could not construct their own understanding. Furthermore, they indicated that they did not know what activities to arrange for the learners. The paper discusses the difficulties experienced in implementing active learning methods in teaching genetics and recommends possible avenues for teacher education and further research in Malawi.