Genetics has become increasingly important in everyday life, a fact that is reflected in state and national science education standards. This study examines the relationship between 474 fifth graders' understandings of the core ideas of "inheritance of traits" and "variation of traits". A confirmatory factor analysis, supplemented with qualitative analyses of students' responses, revealed a relationship between these concepts and contributed evidence about the nature of students' challenges in learning these ideas. While most students demonstrated some normative ideas, many struggled to explain "variation of traits" as resulting from equal inheritance of genetic information from two parents. Instead, they focused on generational patterns of visible traits or on the influence of nongenetic factors. Deepening understanding of young students' heredity-related thinking can improve preparation of older students for learning advanced genetics-related ideas.