This paper aims to provide developmental data on two connected naive inheritance concepts and to explore the coherence of children's naive biology knowledge. Two tasks examined children and adolescents' (4, 7, 10, and 14 years) conceptions of phenotypic resemblance across kin (in physical characteristics, disabilities, and personality traits). The first task required participants to predict and explain feature outcomes in both an offspring and a sibling, in a modified version of the phenotypic similarity task. In the second task, participants offered explanations for instances of parent-offspring dissimilarity and grandparent-offspring resemblance (phenotypic difference task). The inclusion of two tasks and a broad age range revealed significant age trends between 4 and 10 years in naive inheritance concepts. However, there was little consistency in children's inheritance explanations within or across tasks. The findings are discussed with reference to debates concerning the development and structure of naive biology concepts. (Contains 6 tables.)