Wilhelm Ludvig Johannsen studied plants and helped found the field of genetics, contributing methods and concepts to the study of heredity around the turn of the twentieth century in Denmark. His experiments on heredity and variation in plants influenced the methods and techniques of geneticists, and his distinction between the genotype of an organism—its hereditary disposition—and its phenotype—its observable characteristics—remains at the core of contemporary biology.
Wilhelm Johannsen first proposed the distinction between genotype and phenotype in the study of heredity while working in Denmark in 1909. The distinction is between the hereditary dispositions of organisms (their genotypes) and the ways in which those dispositions manifest themselves in the physical characteristics of those organisms (their phenotypes).
Richard Woltereck first described the concept of Reaktionsnorm (norm of reaction) in his 1909 paper "Weitere experimentelle Untersuchungen über Art-veränderung, speziell über das Wesen quantitativer Artunterschiede bei Daphniden" (Further investigations of type variation, specifically concerning the nature of quantitative differences between varieties of Daphnia).