Wilhelm Johannsen

JOHANNSEN, Wilhelm (1857–1927), a well-known professor of plant physiology at the University of Copenhagen. Johannsen's early works ("Transactions of the Copenhagen Carlsberg Laboratory") dealt with plant anatomy and physiology; of these, his studies on the structure, development, and physiology of cereal grains are the best known. Johannsen also proposed a method for shortening the dormant period in plants by treating them with ether, which gained practical application in horticulture. However, Johannsen acquired worldwide renown for his work on variability and heredity. In 1903, his work «Über Erblichkeit in Populationen und reinen Linien» (Jena) was published. In this work, Johannsen sets forth the results of his studies on the variability of bean size; these studies, which became classical, laid the foundation for the modern doctrine of variability and the study of the inheritance of quantitative traits. Johannsen studied the variability of bean size in a mixed heterogeneous population of beans and then, through selection and self-pollination, bred a series of hereditarily homogeneous lines from single initial specimens from this population, which he named "pure lines," and subjected them to analysis as well. The following was revealed thereby. In a mixed population, the range of variability is greater than in individual hereditarily homogeneous lines. In a population, selection proves to be effective: if the largest specimens are taken as producers, the average size of the offspring can be shifted in the plus direction, and conversely, by taking small specimens as producers, the average size of the offspring can be lowered. Within an individual "pure line," however, although variability remains, it is no longer subject to selection: the average size of offspring obtained from the largest and smallest beans turns out to be identical. From these experiments, performed on a large amount of material subjected to precise biometric processing, the following propositions emerge. 1. Selection acts only in mixed hereditarily heterogeneous populations; if, however, through self-pollination or repeated close inbreeding and selection, a hereditarily homogeneous material is obtained, the variability still remaining in it is no longer subject to selection, and the trait proves to be hereditarily stable; a natural population is usually a mixture of such "pure lines" or "biotypes." 2. Variability can be hereditary and non-hereditary (fluctuating). The first of these conclusions is now sometimes called "Johannsen's law." These works of Johannsen were of immense theoretical and practical significance, since his conclusions brought much clarity to the conception of variability and the significance of selection, and the method of "pure lines" proved extraordinarily fruitful in the practical breeding of cultivated plants. Johannsen's final major work is his book «Elemente der exakten Erblichkeitslehre» (Jena, 1909; 3rd ed., 1926). This book provides an exposition of the phenomena of variability and heredity and the methods of genetic work. Especially important is the concept of "genotype" and "phenotype" formulated by Johannsen. Johannsen's doctrine of genotype and phenotype, as well as his doctrine of pure lines, are fundamental tenets of modern genetics. To him also belongs the credit for formulating basic genetic terminology. Johannsen owns the review work «Experimentelle Grundlage der Deszendenzlehre» (Kultur der Gegenwart, Band III, Berlin - Leipzig, 1915).