Johannsen: pure lines, genotype and phenotype

1903–1909 Life and health theory

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A distinction between the hereditary constitution and the observable appearance of an organism: genotype and phenotype. It was shown that selection within a 'pure line' (the descendants of a single self-fertilised seed) does not shift the mean trait: Galton's law of heredity worked only on a mixture of lines, because it selected between types already present.

The distinction was introduced in 1909 by the Dane Wilhelm Johannsen, a plant physiologist in Copenhagen. He took beans bought at the market, grew self-fertilising 'pure lines' from single seeds and gave the three words in which all later science speaks: gene (a shortening of de Vries's 'pangene'), genotype and phenotype.

The distinction separated what the biometricians had measured as one thing, and gave genetics a 'hard' heredity that changes by steps and not by selection. Johannsen himself held the gene to be a word 'free from any hypothesis', declined to place it in the nucleus, and never accepted the chromosome theory in full.

Why this date

The axis year follows the first publication of the terms: 'Elemente der exakten Erblichkeitslehre', Jena: Gustav Fischer, 1909 (534 pp. by the BHL scan), where 'gene', 'genotype' and 'phenotype' were first used (Roll-Hansen 2014). The lower bound of the label is the pure-line bean experiment: 'Ueber Erblichkeit in Populationen und in reinen Linien', Jena: Gustav Fischer, 1903. The short English statement is 'The Genotype Conception of Heredity', American Naturalist 45, 129-159 (1911). All bibliographic details are from Roll-Hansen's reference lists (JHB 2009, J. Physiol. 2014). The QID Q51380974 is the Biodiversity Heritage Library entity for the 1909 edition (author Johannsen, Jena, G. Fischer, 1909); Wikidata also holds a duplicate, Q51468262, and a Nature review, Q64283194, which is not the book.

What it grew from

• Three papers of 1900: de Vries, Correns and Tschermak bring back Mendel's laws

  influenced · Confidence: high

  Johannsen knew the rediscovery of 1900 and introduced the word 'gene' precisely for those 'unit factors', 'elements' or 'allelomorphs' in the gametes which, in his words, Mendelian research had demonstrated; the reality of the gene, he wrote, 'is evident in Mendelism'.

  Dispute: Johannsen derived the word 'gene' not from Mendel but from de Vries's 'pangene' (Intracellulare Pangenesis, 1889), and the pure-line experiment of 1903 was set up against Galton's law, not as a Mendelian cross; Mendelism gave Johannsen not a method but the confidence that units in the gametes are real.

  Link sources

  • Nils Roll-Hansen, The holist tradition in twentieth century genetics. Wilhelm Johannsen's genotype concept, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

    Johannsen was well acquainted with recent cytological discoveries and discussions as well as with the 'rediscovery' of Mendel's laws in 1900 (Roll-Hansen 2009, 2014).

  • Robert C. Olby, Mendel, Mendelism and Genetics, MendelWeb (1997) Verified 20 Sep 2026

    The "gene" is nothing but a very applicable little word, easily combined with others, and hence it may be useful as an expression for the "unit factors", "elements" or "allelomorphs" in the gametes, demonstrated by modern Mendelian researches. A "genotype" is the sum total of all the "genes" in a gamete or zygote. . . . As to the nature of the "genes", it is as yet of no value to propose any hypothesis; but that the notion of the "gene" covers a reality is evident in Mendelism.

  • Nils Roll-Hansen, The holist tradition in twentieth century genetics, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

    Since Hippocrates, the dominant conception had been 'transmission of the parent's (or ancestor's) personal qualities', i.e. the phenotype. However, Mendelian hybridization and pure line experiments had shown that this was fundamentally mistaken.

  "Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

• The biometric-Mendelian debate: continuous or discontinuous heredity

  influenced · Confidence: medium

  Johannsen's pure-line experiment answered the central question of the debate, whether continuous variation is inherited: it tested Galton's law on which the biometricians relied, was presented as the culmination of the criticism of orthodox Darwinism by Bateson and de Vries, and was dismissed by Pearson and Weldon.

  Dispute: Roll-Hansen insists that Johannsen was neither a Mendelian nor a biometrician in the English sense: he dedicated the 1903 monograph to Galton as 'creator of the exact theory of heredity' and used biometric methods, and Bateson, when they met in 1904, 'was not enthusiastic', seeing in him too much of a biometrician. The link is therefore drawn from the debate as a whole, not from one side, and with medium confidence.

  Link sources

  • Nils Roll-Hansen, The holist tradition in twentieth century genetics, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

    The experiment had been designed to test Galton's ancestral law of heredity. Galton like other orthodox Darwinians assumed that in normal unitary populations heredity varied continuously, both in time and in amount. This had been contested for instance by Hugo de Vries and William Bateson who claimed that stepwise and infrequent hereditary change (e.g., "mutation") was the main source of evolutionary change.

  • Nils Roll-Hansen, Sources of Wilhelm Johannsen's Genotype Theory, Journal of the History of Biology 42 (3), 457-493 (abstract) (2009) Verified 20 Sep 2026

    Johannsen presented his famous selection experiment as the culmination of a line of criticism of orthodox Darwinism by William Bateson, Hugo de Vries, and others (Johannsen, 1903). They had argued that evolution is based on stepwise rather than continuous change in heredity.

  • Nils Roll-Hansen, The holist tradition in twentieth century genetics, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

    However, when Johannsen sent this paper to Karl Pearson and Frank Raphael Weldon, the contemporary wardens of Galton's biometric heritage, the response was dismissive. Johannsen could not be taken seriously because his knowledge of statistics was deficient. Among the biometricians in England only Udny Yule responded with appreciation

  • Robert C. Olby, Mendel, Mendelism and Genetics, MendelWeb (1997) Verified 20 Sep 2026

    While Bateson derided the Darwinians and biometricians in Britain with his own brand of scornful invective, Johannsen challenged biometricians, Darwinians, and the followers of August Weismann with his penetrating redefinition of heredity, spiced with some scorn.

  "Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

• Bateson gives the science of heredity a name: 'genetics'

  influenced · Confidence: medium

  Johannsen proposed the words 'gene', 'genotype' and 'phenotype' expressly 'to be used in the science of genetics' named by Bateson, and as a replacement for Bateson's own terms 'unit factor' and 'allelomorph'; Olby calls the two men the architects of the new science.

  Dispute: The link is largely terminological and polemical: Roll-Hansen stresses that Bateson received Johannsen coolly at their meeting in 1904, and that 'gene' was introduced not as a development of Bateson's 'allelomorph' but as a word free of its morphological associations. Hence medium confidence.

  Link sources

  • Robert C. Olby, Mendel, Mendelism and Genetics, MendelWeb (1997) Verified 20 Sep 2026

    Therefore I have proposed the words "gene" and "genotype" and some further terms, as "phenotype" and "biotype" to be used in the science of genetics. The "gene" is nothing but a very applicable little word, easily combined with others, and hence it may be useful as an expression for the "unit factors", "elements" or "allelomorphs" in the gametes, demonstrated by modern Mendelian researches.

  • Robert C. Olby, Mendel, Mendelism and Genetics, MendelWeb (1997) Verified 20 Sep 2026

    Thus did Bateson and Johannsen as the architects of the new science of genetics formulate their subject. They sought to distance it from embryology and cytology, and to relate it strictly to the genealogical data of Mendelian experimentation.

  • Nils Roll-Hansen, The holist tradition in twentieth century genetics, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

    His target was the various concepts of hereditary units, whether of particulate nature as in Darwin or Weismann or 'unit characters' without a particulate basis as Bateson's 'allelomorphs'.

  "Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

• Domestic breeds as an argument: artificial selection in the Origin of Species

  influenced · Confidence: medium

  The pure-line experiment was a direct test of Darwin's premise that selection, artificial or natural, changes the hereditary type by accumulating ordinary small variation; Johannsen obtained a negative result and concluded that selection only chooses among types already present.

  Dispute: Roll-Hansen speaks of 'Darwinism in the narrowest sense', that is of the theory of successive selection in general, not specifically of the first chapter of the Origin; the artificial-selection node is chosen because it was the argument from domestic breeds that asserted the accumulative power of selection, and Johannsen ran his experiment as a breeder, on cultivated beans. If the natural-selection branch gets a node of its own, the link belongs there. This is not a refutation: selection as a practice stood, the mechanism was contested.

  Link sources

  • Nils Roll-Hansen, The holist tradition in twentieth century genetics, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

    The theory of 'successive selection' was 'Darwinism in the narrowest sense'. It took as the starting point, on which selection could work, 'the familiar common variability'. In Johannsen's view, the successive selection theory was based on the idea that continuous variation, often called 'individual' or 'fluctuating', was in considerable measure inherited. This was just what he tested in his bean selection experiment, with a negative outcome.

  • Nils Roll-Hansen, The holist tradition in twentieth century genetics, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

    To sum up: Johannsen's criticism of Darwinism objected to the idea that selection as such, whether natural or artificial, was the main cause of hereditary change. In his view, selection could only choose between differences in heredity that already existed.

  • Nils Roll-Hansen, Sources of Wilhelm Johannsen's Genotype Theory, Journal of the History of Biology 42 (3), 457-493 (abstract) (2009) Verified 20 Sep 2026

    He tested and confirmed it experimentally combining the methods of biometry, as developed by Francis Galton, with the individual selection method and pedigree analysis, as developed for instance by Louis Vilmorin.

  "Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

• 'Das Keimplasma': the continuity of the germ plasm and the mouse-tail experiment

  influenced · Confidence: medium

  Johannsen's genotype grew out of a polemic with Weismann's germ plasm: the new terms were introduced to get rid of a 'purely speculative morphological' view of heredity, while the idea itself of a hereditary constitution independent of the body was already present in Weismann and Galton.

  Dispute: The influence here is negative, by repulsion: Johannsen admitted the likeness of the genotype to the 'Keimplasma' but held Weismann up as an example of how not to theorise.

  Link sources

  • Robert C. Olby, Mendel, Mendelism and Genetics, MendelWeb (1997) Verified 20 Sep 2026

    The hallmark of Johannsen's contribution was the introduction of new terms with the deliberate aim of banishing what he regarded as the many erroneous associations which old terms carried. Thus the term germplasm had a Weismannian context, and that was a 'purely speculative morphological view of heredity without any suggestive value.'

  • Nils Roll-Hansen, The holist tradition in twentieth century genetics, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

    What an offspring receives from its parents is the genotype inherent in their gametes. Moreover, each generation produces gametes independently of its personal qualities. This reminds of Galton's 'stirp' and Weismann's 'Keimplasma', but according to Johannsen they had not freed their thinking from morphological preformation.

  • Nils Roll-Hansen, The holist tradition in twentieth century genetics, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

    One reason for this negative attitude to chromosome theorizing was his deep scepticism concerning Weismann's speculations about embryological development.

  "Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

Links under review

• The white-eyed fly: a gene tied to a chromosome, linkage and the first genetic map

  influenced · Confidence: low

  A link between concepts, not a documented influence: the source (Greene) speaks of the genotype-phenotype distinction, not of Johannsen's effect on Morgan.

  Morgan received the word 'gene' ready-made from Johannsen, and the Drosophila work filled it with material content: the abstract Mendelian 'factor' became something lying at a definite place on a definite chromosome.

  Dispute: The quotation asserts a connection of concepts, not an effect of Johannsen upon Morgan: there is no mention of reading or correspondence, and Johannsen himself insisted that the genotype is not reducible to material particles.

  Link sources

  • M. M. Green, 2010: A Century of Drosophila Genetics Through the Prism of the white Gene, Genetics 184 (1), 3-7 (2010) Verified 20 Sep 2026

    So began a short 1910 Science article by T. H. Morgan on the inheritance of the white eye phenotype. Thus began the transformation of Mendel's rather abstract hereditary “factors” to Johanssen's more concrete, if still mysterious, “genes.”

  "Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

Node sources

• Nils Roll-Hansen, The holist tradition in twentieth century genetics. Wilhelm Johannsen's genotype concept, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

  The distinction between 'phenotype' and 'genotype' as well as the term 'gene' was first used in Johannsen's somewhat formidably Germanic 500 page treatise Elemente der exakten Erblichkeitslehre (Introduction to the elements of an exact theory of heredity) published in 1909.

• Nils Roll-Hansen, The holist tradition in twentieth century genetics. Wilhelm Johannsen's genotype concept, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

  He presented the word 'gene' as a suitable short version of 'pangene', which had been used by de Vries.

• Nils Roll-Hansen, The holist tradition in twentieth century genetics, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

  He called them "pure lines," which he defined as descendants from one single parent. Johannsen found that within each of these sub-populations selection had no effect on the average property (e.g., weight of the beans). Thus Galton's ancestral law did not hold. However, if all the pure lines were added together to make one big population, the law gave a correct description of the result of selection. Johannsen concluded that this was due to a selection between stable genotypes.

• Nils Roll-Hansen, The holist tradition in twentieth century genetics, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

  In a paradigm experiment of selecting beans he demonstrated a striking stability of biological type (Johannsen, 1903), which he later called genotype (Johannsen, 1909). This discovery of 'hard heredity' became the basis of genetics as a special line of experimental investigations, leading to the particulate gene concept of classical chromosome theory, and eventually to the discovery of DNA as a key to chemical explanations of heredity.

• Nils Roll-Hansen, The holist tradition in twentieth century genetics, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

  Das Wort Gen ist völlig frei von jeder Hypothese; es drückt nur die sichergestellte Tatsache aus, dass jedenfalls viele Eigenschaften des Organismus durch in den Gameten vorkommende besondere, trennbare und so mit selbstständige 'Zustände', 'Grundlagen', 'Anlagen' – kurz, was wir eben Gene nennen wollen – bedingt sind. (Johannsen, 1909: 124)

• Nils Roll-Hansen, Sources of Wilhelm Johannsen's Genotype Theory, Journal of the History of Biology 42 (3), 457-493 (abstract) (2009) Verified 20 Sep 2026

  The term "genotype" was introduced in W. Johannsen's 1909 (Elemente der Exakten Erblichkeitslehre. Jena: Gustav Fischer) treatise, but the idea of a stable underlying biological "type" distinct from observable properties was the core idea of his classical bean selection experiment published 6 years earlier (W. Johannsen, 1903. Ueber Erblichkeit in Populationen und reinen Linien. Eine Beitrag zur Beleuchtung schwebender Selektionsfragen, Jena: Gustav Fischer, pp. 58–59).

• Nils Roll-Hansen, The holist tradition in twentieth century genetics, Journal of Physiology 592 (11), 2431-2438 (2014) Verified 20 Sep 2026

  Johannsen came to recognize the epoch-making importance of the work done by the Drosophila group, but he continued to insist on the incompleteness of the chromosome theory.

"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

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