Abstract: Genetic inheritance is a fundamental concept in biology, as it is the mechanism for the transmission of traits. Therefore, understanding genetic inheritance is pivotal for science education and biological reasoning. Prior work on children’s understanding of genetic inheritance suggests that children initially have a “mother bias” such that, if parents have different traits, children think that offspring will have the same trait as the mother rather than the father. However, this pattern might be due to prior work only allowing children to select one possible offspring. In this presentation, I will present two novel tasks to assess children's and adults' beliefs about inheritance. Using these tasks, I show that even 4-year-old children have a more sophisticated understanding of inheritance than previously believed, and that this understanding is constrained by different biases than those identified in prior work. I will also show how these biases are present for different traits and animals, are resistant to instruction, and persevere into adulthood. These new insights inform our understanding of children's biological reasoning and can be used to design new genetics lessons.