Wilhelm Ludvig Johannsen

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Wilhelm Ludvig Johannsen


Danish Geneticist and Botanist

Wilhelm Johannsen is considered one of the founders of genetics. His research provided evidence supporting the mutation theory of Hugo de Vries (1848-1935), which holds that there are sudden, spontaneous appearances of new characters or traits in existing species. Johannsen's research appeared to counter Charles Darwin's (1809-1882) theory that changes in species occur slowly through the process of natural selection alone. Johannsen came to believe that both natural selection and mutations in the hereditary units of germ cells act as influences on subsequent progeny, thereby changing species through the evolutionary process.

Born the son of a Danish army officer and German mother, Johannsen's schooling in Copenhagen was stopped short of the University level because his father did not have the financial means to provide for his tuition. Johannsen served as a pharmacy apprentice in Denmark and Germany, and passed the pharmacist's examination in 1879. Johannsen's keen mind allowed him to become self-taught and trained in the fields of chemistry and botany while he worked as a pharmacist. Johannsen never attained a formal university degree, but was awarded several honorary degrees, became a professor of botany and plant physiology at the University of Copenhagen, served as rector of the university, and became a member of the Royal Danish Academy of Sciences. Johannsen was considered to be a well-read person of the humanities, aesthetics, philosophy, and several languages.

In 1881 he began working at the Carlsberg Laboratory researching plant physiology problems, particularly the metabolism of plants, buds, tubers, and seeds during the processes of germination, dormancy, and ripening. This research included his discovery of a method for breaking the dormancy period of winter buds (1893). At this point Johannsen shifted the focus of his research from plant physiology to plant heredity.

Greatly influenced by both Darwin and Francis Galton (1822-1911), Johannsen would soon become a leading authority on heredity, studying self-fertilizing plants and creating "pure lines" of plants whose hereditary make-up would all be identical. Johannsen focused his analysis on hereditary characters that expressed continuous variation, such as fertility, size, and responses to environmental stimuli—traits that were most useful to plant and animal breeders. Any variation that Johannsen observed in the "pure line" progeny that was not heritable would be due to environmental factors, while physical and physiological differences that were heritable would be the result of a mutation in the germ cells. Johannsen concluded that the process of mutation could produce a new character in a species that natural selection would then act upon, either maintaining the new character or removing it from the population. Johannsen recognized the importance of the genetic structure of populations to evolutionary biology, and the concepts of heredity that he developed at this time both preceded the discovery of DNA and survived all subsequent technical advances made since then.

Johannsen published his text The Elements of Heredity in 1905, in which he introduced the terms "gene," to refer to a unit of heredity, "genotype" as the totality of an individual's genes, and "phenotype" as the overall appearance and processes of an individual. The phenotype represents the summation of the genotype and the environmental conditions encountered by the individual. It was the first and most influential textbook of genetics in Europe, and contained many mathematical and statistical methodologies useful for analyzing quantitative data from genetics experiments. Johannsen's text and his subsequent role as a science historian and critic served to both establish the modern sciences of genetics and evolutionary biology and to remove outdated concepts and ideas that did not stand up to the rigors of objective scientific inquiry. Johannsen used his position to act as a purging filter for any nineteenth-century myths of biology that were based on mysticism, superstition, and teleology, and thus performed an overdue, lasting service for biology.