Johannsen was born in Copenhagen. While very young, he was apprenticed to a pharmacist and worked in Denmark and Germany beginning in 1872 until passing his pharmacist's exam in 1879. In 1881, he became assistant in the chemistry department at the Carlsberg Laboratory under the chemist Johan Kjeldahl. Johannsen studied the metabolism of dormancy and germination in seeds, tubers and buds. He showed that dormancy could be broken by various anesthetic compounds, such as diethyl ether and chloroform. In 1892, he was appointed lecturer at Royal Veterinary and Agricultural University and later became professor of botany and plant physiology. He taught plant physiology. His best-known research concerned so-called pure lines of the self-fertile common bean. He was able to show that even in populations homozygous for all traits, i.e. without genetic variation, seed size followed a normal distribution. This was attributable to resource provision to the mother plant and to the position of seeds in pods and of pods on the plant. This led him to coin the terms phenotype and genotype. Johannsen's findings led him to oppose contemporary Darwinists, most notably Francis Galton and Karl Pearson, who held the occurrence of normal distributed trait variation in populations as proof of gradual genetic variation on which selection could act. Only with the modern synthesis, was it established that variation needed to be heritable to act as the raw material for selection. He created the terms phenotype and genotype, first using them in his paper Om arvelighed i samfund og i rene linier and in his book Arvelighedslærens Elementer. This book was rewritten, enlarged and translated into German as Elemente der exakten Erblichkeitslehre. It was in this book Johannsen introduced the term gene. This term was coined in opposition to the then common pangene that stemmed from Darwin'stheory ofpangenesis. The book became one of the founding texts of genetics. Also in 1905, Johannsen was appointed professor of plant physiology at the University of Copenhagen, becoming vice-chancellor in 1917. In December 1910, Johannsen was invited to give an address before the American Society of Naturalists. This talk was printed in the American Naturalist. In 1911, he was invited to give a series of four lectures at Columbia University. Johannsen was a corresponding member of the Academy of Natural Sciences of Philadelphia. Johannsen died from work poisoning in 1927.
