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).
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.
In 1881 British ophthalmologist
Richard Woltereck studied aquatic animals around Germany in the early twentieth century, and he extended the concept of Reaktionsnorm (norm of reaction) to the study of genetics. He also provided some of the first experimental evidence for the early twentieth-century embryological theory of heredity called cytoplasmic inheritance.
View image of Boveri here.
Multiple theories about what determines sex were tested at the turn of the twentieth century. By experimenting on