Wilhelm Ludvig Johannsen helped found the science of genetics with his creation and experimental support for the concepts of the gene, genotype, and phenotype.

Natural selection, Natural selection theory one of the mechanisms for evolutionary change, was first formulated by Charles Darwin Darwin, Charles in 1859 in On the Origin of Species by Means of Natural Selection: Or, The Preservation of Favoured Races in the Struggle for Life. On the Origin of Species by Means of Natural Selection (Darwin) Darwin described a process in which certain individual organisms are better able to survive and reproduce offspring relative to other individual organisms because of inherited differences. Over time, the inherited properties of a population change gradually and continuously, and sometimes the changes lead to a new population of organisms (speciation). Evolution;theory Darwin also tried to account for the origins of inherited differences with his concept of “provisional hypothesis of pangenesis,” featured in a chapter in The Variation of Animals and Plants Under Domestication (1868). He integrated a variety of ideas into his theory and believed that environment sometimes could directly influence the inherited characteristics passed to offspring (soft inheritance). Genetics;terminology
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Elemente der exakten Erblichkeitslehre (Johannsen)
[kw]Johannsen Coins the Terms “Gene,” “Genotype,” and “Phenotype” (1909)
[kw]”Gene,” “Genotype,” and “Phenotype,” Johannsen Coins the Terms (1909)[Gene, Genotype, and Phenotype, Johannsen Coins the Terms (1909)]
[kw]”Genotype,” and “Phenotype,” Johannsen Coins the Terms “Gene,” (1909)[Genotype, and Phenotype, Johannsen Coins the Terms Gene, (1909)]
[kw]”Phenotype,” Johannsen Coins the Terms “Gene,” “Genotype,” and (1909)[Phenotype, Johannsen Coins the Terms Gene, Genotype, and (1909)]
Genetics;terminology
Genes
Genotypes
Phenotypes
Elemente der exakten Erblichkeitslehre (Johannsen)
[g]Denmark;1909: Johannsen Coins the Terms “Gene,” “Genotype,” and “Phenotype”[02300]
[c]Science and technology;1909: Johannsen Coins the Terms “Gene,” “Genotype,” and “Phenotype”[02300]
[c]Biology;1909: Johannsen Coins the Terms “Gene,” “Genotype,” and “Phenotype”[02300]
[c]Genetics;1909: Johannsen Coins the Terms “Gene,” “Genotype,” and “Phenotype”[02300]
Johannsen, Wilhelm Ludvig
Jennings, Herbert Spencer
Pearl, Raymond
Morgan, Thomas Hunt

Not even some of Darwin’s strongest supporters, such as the biologists Thomas Henry Huxley Huxley, Thomas Henry and August Weismann Weismann, August or the biostatistician Francis Galton, Galton, Francis accepted all the parts of his theory. Weismann was a strong supporter of natural selection but rejected soft inheritance. He believed in the continuity of the “germ plasm,” which was separated from the body (soma). Galton also argued that the inherited properties, or “stirps,” were passed from generation to generation with little change. Both Galton and Huxley questioned Darwin’s emphasis on small, heritable differences and continuous change. They argued instead for selection of “sports,” or mutations, and believed that evolution proceeds through rapid, discontinuous jumps. Galton also developed the “law of filial regression,” Law of filial regression in which offspring tend to exhibit the average of the race or type to which they belong rather than the average of their parents for quantitative characteristics such as height. Thus two very short parents would tend to have taller children.

The rediscovery of Gregor Mendel’s Mendel, Gregor theory of heredity in 1900 only exacerbated the disputes. Karl Pearson Pearson, Karl and Walter Weldon, Weldon, Walter founders of the biometrical school, considered themselves to be followers of Galton but rejected Galton’s belief in discontinuous evolution. They sided with Darwin in favor of natural selection of small, heritable variations and rejected Mendel’s theory. Early Mendel supporters opposed the biometricians and argued that evolution proceeds by discontinuous leaps. The Mendelians believed that differences in continuous traits are too small to generate sufficient selection pressure to cause evolutionary change.

Wilhelm Ludvig Johannsen played a significant role in helping to bridge the gap between the opposing camps. Johannsen was trained as a pharmacist and attended lectures in botany at the University of Copenhagen but never obtained a degree. In 1881, he joined the chemistry department of the newly established Carlsberg laboratory. He resigned his post in 1887 but continued his research there and discovered a method of reviving dormant winter buds. The success of this discovery helped him gain a position as lecturer in botany and plant physiology at the Copenhagen Agricultural College in 1892.

Johannsen was affected profoundly by Galton’s work on heredity and was influenced also by Darwin’s writings on natural selection. He decided to try to distinguish between Galton’s ideas on selection, which had been experimentally supported by the plant physiologist Hugo de Vries, Vries, Hugo de and Darwin’s theory of natural selection, as defended by Pearson and Weldon. De Vries considered selection to be ineffective, whereas Pearson and Weldon believed selection to be a continuous force for population change.

Unlike his predecessors, Johannsen chose for his experiments a species that reproduces by self-fertilization, so that “pure lines” consisted of all individuals descended from a single self-fertilized individual. Genetics;pure-line studies[Genetics;pure line studies]
Pure-line genetics studies[Pure line genetics studies] In the autumn of 1900, Johannsen randomly selected 5,000 princess bean (Phaseolus vulgaris) seeds. The following spring, he planted 100 of the seeds that were of average weight relative to the total of 5,000. He also planted 25 each of the smallest and the largest seeds by weight. After the fall harvest, he discovered that, whether described in terms of the original groups (25 small, 100 average, 25 large) or as a combined total of 150, he got normal distributions. When he then reclassified the groups of large and small beans according to the average weight of their own offspring, however, he discovered that selection had resulted in larger and smaller average weights, respectively, when compared with the average for the original total population. Later, in 1902, he applied selection to the offspring of a single plant and found no change in average values. This indicated that selection on pure lines had no effect.

Wilhelm Ludvig Johannsen (left) with English geneticist William Bateson in 1924.

(Courtesy, California Institute of Technology)

Johannsen presented the findings from his three years of experiments at the meeting of the Royal Danish Scientific Society on February 6, 1903, and published an article in both Danish and German in the same year. Johannsen described his views on the evolutionary process in his book titled Om arvelighed og variabilitet (on heredity and variation). An extended version of that book, retitled Arvelighedslaerens elementer (the elements of heredity), Arvelighedslaerens elementer (Johannsen) was published in 1905. In 1906, Johannsen presented a paper at the Third International Conference on Genetics in which he extended his conclusions to hybridized lines on the basis of one incomplete experiment.

In 1908, Herbert Spencer Jennings at Harvard and Raymond Pearl at the Maine Agricultural Experiment Station conducted pure-line studies on the microorganism Paramecium and in fowls, respectively. Although the researchers’ conclusions were partly erroneous, both sets of experiments were considered to provide support for Johannsen’s pure-line theory.

Johannsen then wrote an even larger version of his textbook in German and published it in 1909 under the title Elemente der exakten Erblichkeitslehre (elements of an exact theory of heredity). He provided many statistical analyses of quantitative traits and discussed his experimental work. He noted how general populations represent mixtures of many pure lines, each with a different inherited constitution. Although individuals might vary for a particular continuous trait within a pure line, the inherited characteristics do not vary. He coined the term “phenotype,” defining it as the statistical average of the environmentally influenced variable appearances of individuals.

Johannsen also recognized a need to characterize the inherited properties of an organism. The nature of the inherited material was controversial. Physicalists such as Johannsen wanted to interpret biological processes in terms of forces. They rejected any efforts to tie Mendelian segregation patterns to cytologists’ discoveries about chromosomes and cellular division. Embryologists favored a corpuscular inherited factor. De Vries called the inherited units either factors or pangens, after Darwin. Johannsen borrowed from de Vries and coined the term “gene” as a unit of calculation or accounting. He wanted a term unconnected to any of the contemporary theories about its nature, and, as Frederick B. Churchill has noted, he used the term “gene” as if it referred to a chemical or physiological process rather than a thing.

Johannsen then discussed the relationship of phenotypic and genotypic differences in terms of populations. The term “genotype” in this context refers to the inherited differences among pure lines. Johannsen did not distinguish between transmission of traits and their development, nor did he apply his genotype-phenotype distinction to individuals.

Elemente der exakten Erblichkeitslehre was the first and most influential textbook of genetics on the European continent. In it, Johannsen introduced the new science and defined its key concepts. Although there were a few critics of his pure-line work, most geneticists in 1910 accepted his theory and rejected any significant role for natural selection in the evolutionary process.

From 1908 to 1911, however, while Johannsen, Jennings, and Pearl were criticizing Darwinian selection, three sets of experiments were performed that helped to bring the work of Darwin and that of Mendel into harmony. First, the geneticist William E. Castle Castle, William E. experimentally demonstrated the action of selection. Second, Edward Murray East East, Edward Murray and Herman Nilsson-Ehle Nilsson-Ehle, Herman showed how Mendelism accounted for continuous variation and apparent blending in the offspring of parental traits. (East also noted that Johannsen’s genotype-phenotype distinction was essentially a later version of Weismann’s somatoplasm-germplasm dichotomy.) Finally, experimentation by Thomas Hunt Morgan and his students with the fruit fly Drosophila showed how the Mendelian factors or genes might be tiny variations. Morgan’s group also showed how genes are carried on the chromosomes (color-staining bodies in the nucleus of cells). Morgan’s chromosome theory may have hastened the acceptance of the contemporary definitions of genotype and phenotype—that is, the distinction between the expressed and outward appearance of an individual (phenotype) and the genetic information stored in the germ plasm (genotype).

In the third edition of Elemente der exakten Erblichkeitslehre, published in 1926, Johannsen used the modern definitions of genotype and phenotype and incorporated cytological discoveries and the work of Morgan’s group. He still rejected a corpuscular gene, however. He was not alone in this; disputes about the composition of the genetic material continued until Oswald Avery, Cohn MacLeod, and Maclyn McCarty showed in 1944 that deoxyribonucleic acid (DNA) is the genetic material for almost all organisms, and James D. Watson and Francis Crick demonstrated in the 1950’s how hereditary information is encoded in the DNA. DNA

The science Johannsen helped found, genetics, has thrived. Since his discoveries, an ever-increasing number of behavioral, anatomical, and physiological traits have been shown to have inherited components. Knowing how specific traits (including genetic diseases such as cystic fibrosis) are transmitted permits genetic counselors to advise couples about the risk of passing particular conditions on to any children they conceive together.

By the early years of the twenty-first century, research on the molecular biology of the gene and chromosome (the genotype) had proliferated. Such research included the manipulation of the genetic material of food plants and animals in order to increase production or resistance to disease; identification of potent mutagens, particularly those affecting DNA, in the sperm or egg; and mapping of specific genes’ locations on particular chromosomes. In addition, studies concerning how traits are phenotypically expressed by organisms may eventually provide avenues through which scientists can mediate that expression—for example, by curing genetic diseases or by preventing viruses (such as the human immunodeficiency virus, or HIV) from exploiting individuals’ genetic machinery to reproduce themselves. Genetics;terminology
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Elemente der exakten Erblichkeitslehre (Johannsen)

• Bowler, Peter J. The Mendelian Revolution: The Emergence of Hereditarian Concepts in Modern Science and Society. Baltimore: The Johns Hopkins University Press, 1989. Provides details about the work and life of Johannsen and his intellectual milieu. Good bibliography.
• Carlson, Elof Axel. Mendel’s Legacy: The Origin of Classical Genetics. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory Press, 2004. Based heavily on early twentieth century sources, this book traces the roots of genetics in breeding analysis and studies of cytology, evolution, and reproductive biology. Highly illustrated.
• Churchill, Frederick B. “William Johannsen and the Genotype Concept.” Journal of the History of Biology 7 (Spring, 1974): 5-30. An interesting analysis of Johannsen’s central ideas and the consistency in his work during two time periods: from the pure-line studies until publication of Elemente der exakten Erblichkeitslehre and after publication of the text’s third edition.
• Dunn, L. C. A Short History of Genetics: The Development of Some of the Main Lines of Thought, 1864-1939. 1965. Reprint. Ames: Iowa State University Press, 1991. A clear, very readable description of the important personalities (including Johannsen) and discoveries that contributed to the origins of the field of genetics. Bibliography includes most of Johannsen’s major works.
• Mayr, Ernst. The Growth of Biological Thought: Diversity, Evolution, and Inheritance. Cambridge, Mass.: Harvard University Press, 1982. A superb history of conceptual developments (including gene, genotype, and phenotype) and the people responsible for them by one of the great evolutionary biologists of the twentieth century. Good bibliography.
• Provine, William B., comp. The Origins of Theoretical Population Genetics. 2d ed. Chicago: University of Chicago Press, 2001. Excellent coverage of the “war” between the biometricians and the Mendelians. Details Johannsen’s role in that dispute.
• Roll-Hansen, Nils. “Drosophila Genetics: A Reductionist Research Program.” Journal of the History of Biology 11 (Spring, 1978): 159-210. Describes the philosophical and scientific biases of Morgan, his students, and his colleagues, including Johannsen.

Bateson Publishes Mendel’s Principles of Heredity

McClung Contributes to the Discovery of the Sex Chromosome

Sutton Proposes That Chromosomes Carry Hereditary Traits

Punnett’s Mendelism Includes Diagrams Showing Heredity

Bateson and Punnett Observe Gene Linkage

Hardy and Weinberg Present a Model of Population Genetics

Morgan Develops the Gene-Chromosome Theory

Sturtevant Produces the First Chromosome Map