Research shows that students face challenges as they learn about genetic inheritance. The challenges could emanate from the fact that genetic inheritance involves unseen processes at different organizational levels. We explored students' understanding of heredity and related concepts such as cells and reproduction using a Web-based Science Inquiry Environment (WISE) curriculum unit that was developed to help middle school students learn about genetic inheritance. Our findings suggest that students made significant gains from pretest to posttest. However, despite overall gains, some students struggled to explain the importance of mitotic and meiotic divisions in transferring genetic information.