This paper reports on the relationship between the subject matter knowledge of two underqualified teachers and their topic-specific pedagogical content knowledge (TSPCK) as they taught genetics at two community secondary schools in Malawi. The study was qualitative and used the multiple case study approach. The sample was purposefully chosen. Data were collected through video-recorded lesson observations and tape-recorded interviews. Guided by literature on teaching genetics and on TSPCK, thematic analysis was done on the data to identify emerging themes and categories that best provided meaning. The categories used in analysing the data were the five components of TSPCK: learners' prior knowledge; curricular saliency; what makes the topic easy or difficult to understand; representations including analogies; and conceptual teaching strategies. For both teachers, their knowledge of subject matter influenced three of the five components of TSPCK: knowledge of subject representations; teaching strategies; and issues that make the topic difficult/easy to understand. The other two components, that is, knowledge of learner's prior knowledge on the topic and curricular saliency, depended more on the way the teachers perceived the purpose of teaching genetics for their learners--either for success in the high-stakes national examinations or for understanding and applying this knowledge in their everyday life experiences. Although these findings cannot be generalised owing to the small sample size, they do highlight the need for adequate subject matter knowledge and awareness of the purpose of teaching genetics for teachers to have strong TSPCK. Therefore, tertiary institutions involved in secondary school teacher preparation in Malawi and the Ministry of Education, Science and Technology should design relevant interventions to improve the underqualified teachers' subject matter knowledge on genetics and sensitise them on the purpose of teaching genetics, so that their TSPCK of genetics can be strengthened which, in turn, would improve the teaching of the topic in community secondary schools for the country to have a genetics-literate public.