Many constructions of race are associated with phenotypical traits and geographic ancestry, and scholars like Carl Linnaeus have proposed scientific models for the organization of race since at least the 18th Century. Following the discovery of Mendelian genetics and the mapping of the human genome, questions about the biology of race have often been framed in terms of genetics. A wide range of research methods have been employed to examine patterns of human variation and their relations to ancestry and racial groups, including studies of individual traits, studies of large populations and genetic clusters, and studies of genetic risk factors for disease.