It is widely argued that a more general ‘genetic literacy’ of the public is required so that appropriate use is made of the new human genetic technologies, such as DNA testing, and there is effective public participation in debates about the regulation and development of new technologies. Despite a century of popularisation and school education in Mendelian genetics, public understanding remains very limited. An hypothesis is put forward that suggests that the limited public understanding results from the existence of a lay understanding of inheritance that conflicts with the scientific account of Mendelian genetics and impedes its uptake. It is further proposed that the lay understanding of inheritance is derived by children from a developmentally earlier social understanding of kinship relations. Given that the lay knowledge of inheritance is grounded in social concepts of kinship that are sustained by every day social activities and relationships, it may be particularly resistant to change. A test of the hypothesis is described and evidence relating to the development of concepts of inheritance and family in children is discussed.