Learning about heredity is important across the K-12 continuum. However, these ideas may be challenging for students. We examined third-grade students' ideas about heredity in the context of a new, six-week, model-based science unit that uses corn as a model organism to support students' ideas about heredity. We analyzed data collected during implementation of the unit, including student artifacts and interviews. We compared these data to those from a pilot version of the curriculum--implemented in the prior year--that was focused on the same disciplinary concepts but was not designed around scientific modeling. Our findings illustrate levels of understanding in students' ideas about three target concepts underlying heredity: life cycles, trait inheritance, and trait variation. We also found that students experiencing the model-based version of the unit exhibited higher levels of understanding for two of the three target concepts than those experiencing the non-model-based curriculum. Analysis of student interviews also showed that students experiencing the model-based curriculum were better able to use key elements of life cycle, such as pollination and reproduction to support their explanations about inheritance. We discuss implications of this work for design and enactment of model-based curricula in elementary grades that can support students' learning about heredity.