Concepts of molecular biology and genetics are difficult for many biology undergraduate students to master yet are crucial for deep understanding of how life works. By asking students to draw their ideas, we attempted to uncover the mental models about genes and gene expression held by biology students (n = 23) and experts (n = 18) using semistructured interviews. A large divide was identified between novice and expert conceptions. While experts typically drew box-and-line representations and thought about genes as regions of DNA that were used to encode products, students typically drew whole chromosomes rather than focusing on gene structure and conflated gene expression with simple phenotypic outcomes. Experts universally described gene expression as a set of molecular processes involving transcription and translation, whereas students often associated gene expression with Punnett squares and phenotypic outcomes. Follow-up survey data containing a ranking question confirmed students' alignment of their mental models with the images uncovered during interviews (n = 156 undergraduate biology students) and indicated that Advanced students demonstrate a shift toward expert-like thinking. An analysis of 14 commonly used biology textbooks did not show any relationship between Punnett squares and discussions of gene expression, so it is doubtful students' ideas originate directly from textbook reading assignments. Our findings add to the literature about mechanistic reasoning abilities of learners and provide new insights into how biology students think about genes and gene expression.