Mendel, Johann Gregor
MENDEL, JOHANN GREGOR
(b. Heinzendorf, Austria [later Hynčice, Czech Republic], 22 July 1822; d. Brünn [later Brno, Czech Republic], 6 January 1884),
botany, genetics, meteorology. For the original article on Mendel see DSB, vol. 9.
Mendel scholarship has traditionally been guided by the questions posed by Ronald Fisher in 1936: what did Mendel discover? How did he discover it? And what did he think he had discovered? Answers to these seemingly naïve, but politically and epistemologically charged questions have varied considerably over time. For Mendel’s “rediscoverers” in 1900, the law of segregation was the centerpiece of Mendel’s discovery. Commemorating the fiftieth anniversary of the rediscovery of Mendel’s paper, Cyril Dean Darlington pointed out that it was only now, fifty years later, that geneticists were about to rediscover what Mendel had truly discovered, namely the material nature of the determinants of heredity, which Mendel had called “elements.”
Geneticists paid attention to Mendel’s historical achievements in order to strengthen and defend their discipline, which by 1950 had become stigmatized as a reactionary science in countries under Communist control. In 1962, the American human geneticist Curt Stern visited Brno to speak with Jaroslav Křížženecký, who had been dismissed as professor of animal breeding and genetics in 1950 and had been imprisoned for two years in 1956. Stern and Kříženecký, wanted to organize an international conference at Brno on the occasion of the hundredth anniversary of the publication of Mendel’s Pisum paper. The Mendel museum in Brno, which had been closed down in 1950, was renovated in 1963, and the conference took place in 1966, despite Kříženecký, untimely death in 1964. Both events marked the “rehabilitation” of Mendel and genetics as a scientific discipline in the Communist block.
Since then, historians have moved on from Fisher’s questions to emphasize questions about the intellectual and social context in which Mendel worked: what were biologists’ prevailing ideas about variation and heredity? What practical problems concerning hybridization faced breeders in Moravia? Biographical studies have revealed other important details about Mendel’s life, such as his work in meteorology. Sociologists of science have analyzed the Mendel story as a reflection of changing interests in the scientific community. This article is divided into two sections, the first on Mendel’s life and nineteenth-century background, the second on his hybridization studies and their relationship to evolution and the development of genetics.
Overview of Mendel’s Career . Mendel’s father was a diligent and industrious peasant and his mother was a
gardener’s daughter, a fact biographers have seen as instrumental in stimulating their son Johann’s interest in growing plants. Mendel’s education included six years at the gymnasium (secondary school) in Troppau (Opava) and two years at the Lyceum attached to the university of Olmütz (Olomouc), where he studied philosophy, mathematics, and physics as preconditions for university study.
In 1843 Mendel entered the Augustinian monastery in Brno with the name Gregor, finding there the best possible conditions for pursuing his studies. During his theological studies in Brno, Mendel attended courses in agriculture at the Philosophical Institute, where he became acquainted with the role of hybridization in creating new plant varieties. In 1849 the abbot at Brno, Cyrill Napp, sent him as a substitute teacher to the gymnasium at Znojmo.
From 1851 to 1853, in his studies at the University of Vienna, Mendel paid much attention to physics, mathematics, chemistry, and plant physiology, and acquired the theoretical background and the skill required to perform experiments and undertake independent research. The teaching of the physicist Christian Doppler had a formative impact on Mendel’s experimental methodology. He also took courses at the university from Franz Unger, who was an early proponent of cell theory and interested in paleobotany, biogeography, and hybridization.
In 1854 Mendel was appointed teacher of physics and natural history at the newly established Realgymnasium in Brno. He devoted all his free time to his long-term research program on plant hybridization (principally peas), drawing on the experience of breeders and plant hybridization experiments conducted by botanists. Mendel became a member of a number of regional associations of breeders, agriculturalists, and naturalists.
In 1864 Mendel was elected abbot of the monastery and entrusted with new duties. The previous extent of experimentation ended, but he did not lose his interest in the problem. A recently discovered fragmentary note (Orel, 1996, p. 187) shows that in the mid-1870s he returned to the enigma of Hieracium hybrids (see below) and found that cross-fertilization of these polymorphic species yielded segregating multifactorial traits in accordance with his theory of variable hybrids.
In 1857 Mendel began his meteorological observations in the monastery and was soon recognized as the authority on this subject in Moravia. He published the results of meteorological observations made throughout the province, and promoted weather forecasting. He wrote some magazine articles about promising new ways to transmit weather data to Vienna and to disseminate weather forecasts to farmers. His remarkable theoretical knowledge and talent for observation is shown in his ten-page report on the whirlwind in Brno in October 1870. He used physics to explain its origin through the meeting of two air streams of different dimensions and properties, arising shortly after the occurrence of the storm.
Having been elected abbot, Mendel was soon appreciated for his support for applying hybridization techniques to create new plant varieties. His newly built apiary became a pioneering research site for the improvement of beekeeping. He paid special attention to investigating controlled crossing to create a new race of bees. After his death in 1884 Mendel’s achievements were recognized by plant breeders and beekeepers. His naturalist colleagues commemorated his activities in meteorology and his hybridizing experiments (Matalová, 1984).
Background: Concepts and Questions . Mendel’s study can be viewed narrowly as a sequence of hypotheses tested through experiments, which led to the explanation of the essence of heredity. But to fully appreciate the questions that inspired Mendel and the methods that he used, he must be placed within a rich, multilayered context that historians have begun to evaluate. Scholars now know that Mendel was influenced by a wide variety of problems and ideas stemming both from major botanists and local Moravian agriculturalists.
His main innovation was his transition from questions about inheritance and transformation of species to answers involving transmission and distribution of individual traits. He selected the genus Pisum (garden peas) as the best experimental model plant. He reduced the problem to be investigated to discrete trait pairs (of seeds and plants) that existed in alternative forms and were reliably distinguishable. His aim was to explain the “generally applicable law of formation and development of hybrids” as one way of “finally reaching the solution to a question about the developmental history [Entwicklungs-Geschichte] of organic forms.” (1865, pp 3–4; Kříženecký,, pp. 57–58; see Stern and Sherwood, p. 2). Here, as for most German scientists of his day, the term Entwicklung embraced development at both individual and species levels.
Sander Gliboff (1999) has argued that Unger’s combination of plant morphology and biogeography provided the overarching intellectual framework for Mendel’s hybridization studies. Unger’s goal was to discover patterns and quantified laws of distribution and Entwicklung, including historical Entwicklung (i.e., species transformation). His method counted plant species and followed their changing proportions in the flora over geographic space or over geological time. What Unger wanted to provide was a “physics” of the plant organism. Gliboff finds the parallels to Mendel’s famous pea studies to be striking. The monk looked at a short-term Entwicklung, but employed a similar methodology: he quantified the changing proportions of the different traits, from generation to generation, and formulated laws of change.
Roger Wood and Vítecaron;zslav Orel have brought to light the existence of a unique regional network of sheep breeders and professors of agriculture and science in Moravia, which provided the immediate social and intellectual context for Mendel’s work. In 1814, this group formed a Sheep Breeders’ Society in Brno; they were trying to find ways to control the characteristics of the animals they bred. Producers of fruit trees and vines were also interested in problems of heredity. In 1818, this society was engaged in a search for a law of hybridization. They realized that traits might be hidden, yet somehow transmitted to future generations.
The abundant communication of animal and plant breeders with naturalists in Moravia in the 1830s (before Mendel came to Brno) led them to ask about the definition of species, the force of hybridization, and the creation of heritable new kinds of plants. Professor Johann Karl Nestler indicated how nature produces, “through forces beyond the hand of man,” constant natural species and how, in contrast, humans modify the variations in organisms “with increasing or disappearing inheritance” (Wood and Orel, 2005, p. 262). The discussion of sheep breeding culminated at the annual meetings in Brno in 1836 and 1837, with the formulation of the physiological research question: what is inherited and how? One of the active participants in these discussions was Napp, who would admit Mendel to his monastery in 1843.
Staffan Müller-Wille and Orel (2007) offer a detailed analysis of Mendel’s relationship to botanical hybridizers and his innovative concern with transmission of single traits. Those earlier hybridizers (unlike twentieth-century geneticists) were concerned with species evolution, not inheritance of single traits. Mendel built on hybridizers’ concepts and questions, yet developed creative new methods and answers that would later be powerful tools for the Mendelians. His main innovation was his transition from questions about inheritance and transformation of species to answers involving transmission and distribution of individual traits.
Müller-Wille and Orel start with Carl Linnaeus, who in 1751 defined species as lineages that (in the same environment) always produced identical progeny down the generations. Varieties, in contrast, manifested differences caused by external differences in temperature, wind, soil, and so on. Soon, however, Linnaeus encountered what he called constant varieties— plants that did not seem to fit with the strict distinction of species and varieties, and which he interpreted as resulting from hybridizations. Over the next century botanists investigated hybrids, seeking laws of inheritance and classifying types of progeny in terms of species as whole bundles of traits. Could new species be formed by hybrid crosses between existing species? Were species so stable and distinct that hybrids would either be sterile or inevitably revert to one parental type? Or were species so fluid that they were immensely malleable, with chaotically varying progeny?
In 1849 Carl Friedrich Gärtner attributed the variability resulting from hybridization to the interaction of the “inner natures” of the parent species. Gärtner believed a formative force (Bildungskraft) caused the form of a species, and in hybridization the forces of the different species interacted; often over time one force would predominate, and over generations the progeny would gradually return to that force’s type. If two specimens appeared similar, the way to discern whether they belonged to the same species was to cross them with yet another species. If both specimens gave rise to similar progeny when out-crossed, that proved the specimens belonged to the same species, i.e., had the same inner nature. If the specimens produced different progeny when outcrossed, they belonged to different species. Upon further crossing of hybrids, he found bewilderingly many combinations of traits, for which he vainly sought a law.
Taking up the search for that law, Mendel, like Linnaeus, defined a species (Art) as a lineage that, under equal environmental conditions, produced identical offspring. For instance, his tall pea plants were one species, his short ones another. (English translations of Mendel’s paper obscure this point, as they variously render Art as “species,” “variety,” “stock,” or “strain.”) Here we see his crucial innovation: crossing species that differed only in one easily distinguishable trait simplified the problem and enabled him to discern basic patterns, such as the 3 : 1 ratio from the self-pollination of hybrids.
Crossing two parental species produced pea plants that resembled one parent, but these were variable hybrids because their descendants varied. In the next generation, one quarter of the plants resembled one grandparent (e.g., tall) and bred true when self-pollinated (all progeny were tall): they belonged to the same species as that grandparent. Another quarter resembled the other grandparent (e.g., short) and bred true, which showed they belonged to the same species as that grandparent. The remaining 50 percent were variable hybrids.
In further generations, ever more of the progeny belonged to one or the other ancestral species, because the constant species members bred true, plus some of the offspring of hybrids belonged to constant species. This explained the “reversion” to ancestral types. But unlike his predecessors, Mendel saw no gradual process: each pea plant was either a member of a constant species or a variable hybrid. Mendel also worked out the more complex mathematical patterns when crossing species that differed in two or more traits.
In experiments with other plant species Mendel wished to determine whether the laws of development discovered for Pisum were also valid for other plants. He mentioned that one experiment with Phaseolus, regarding the shape of the plant, was in agreement with the same law. The second experiment, about change of coloration, he explained in terms of a composite series made up of independent colors A = A1 + A2 + ...
Thus, like Gärtner, Mendel used hybridization to determine the inner nature that revealed if a plant was an exemplar of a species or a hybrid. But, to explain those phenomena, Mendel wrote neither of species forms (like earlier hybridizers) nor of pairs of genes (like later geneticists); rather he referred to discrete types of reproductive cells that were associated with the regular development of certain characteristics, and thus with different types or species of plants.
Each egg or pollen cell had a certain constitution (Mendel used the somewhat general term Beschaffenheit). Members of pure species produced germ cells with all the same constitution, for instance the potential to make short plants. When they combined to make a new plant, all the cells shared that constitution, hence the same species continued (all the progeny were short). Hybrid plants produced two types of germ cells. When different germ cells met in hybridization, one trait would dominate (e.g., tall), but the hybrid plants produced equal numbers of each kind of pollen and eggs (e.g., tall and short). The constitution of the germ cell was the cause underlying the mathematical laws describing the patterns of variable hybrids, and the constancy of pure species (see second part for more).
After 1865 Mendel paid greatest attention to experiments with Hieracium, for which he found different results from peas. When he crossed two species, the appearance of members of the first generation varied greatly, yet the next generation appeared uniform and so did further descendants. Mendel called these constant hybrids, that is, new, true-breeding species. He believed that in such hybridizations the constitutions of the pollen and egg were “entirely and permanently accommodated together” and the resulting plant would produce germ cells that all shared the same constitution. Thus it was possible for hybridization to produce new species, but not in the lawless indefinite variety some earlier hybridizers had thought possible.
When Mendel delivered his lectures on his pea plant experiments to the newly founded Brno Natural Science Society in 1865, his audience did not grasp his answers to the research questions formulated in Brno thirty years before (Orel and Wood, 2005). Nor was his work taken up widely. The figure of Mendel as an ignored scientific genius has become so conventional that in 2000 the sociologist of science Steve Fuller used him as the standard case of the unrecognized discoverer who proposes a scientific novelty and suffers neglect. However, rather than suffering, Mendel’s feeling (recorded by Abbot F. Barina, who was the last novice Mendel accepted into the monastery) was as follows:
Though I have had to live through many bitter moments in my life, I must admit with gratitude that the beautiful and good prevailed. My scientific work brought me much satisfaction, and I am sure it will soon be recognized by the whole world. (Křríženecky, 1965, p. 6)
Augustine Brannigan has argued that Mendel was not completely forgotten, but that his contemporaries understood him to be talking about species hybridization, and claiming that the descendants of hybrids tend to revert to ancestral types. Around 1900 the “rediscoverers” of Mendel instead picked up on his mathematical laws of transmission of particular traits. Jan Sapp has analyzed how various Mendelians interpreted Mendel in light of controversies in twentieth-century genetics and evolution. There are also ongoing controversies over the level at which Mendel understood the implications of his experiments (Monaghan and Corcos, 1990; Falk and Sarkar, 1991; Orel and Hartl, 1999).
See also Olby’s bibliography, below. The translations of Mendel’s paper into other languages are summarized in Folia Mendeliana 8 (1973) and 9 (1979). English and German texts can be found at http://www.mendelweb.org/.
WORKS BY MENDEL
“Versuche über Pflanzenhybriden.” Verhandlungen des naturforschenden Vereines in Brünn 4, Abhandlungen (1866): 3–47. Reprinted with corrections from a copy of the manuscript in: Fundamenta Genetica, edited by Jaroslav Krízenecky. Prague: Czechoslovak Academy of Sciences, 1965a. Translated in The Origins of Genetics: A Mendel Source Book, edited by Curt Stern and Eva R. Sherwood. San Francisco: Freeman, 1966.
“Ueber einige aus künstlicher Befruchtung gewonnenen Hieracium-Bastarde.” Verhandlungen des naturforschenden Vereines in Brünn 8, Abhandlungen (1869): 26–31. Translated as: “On Hieracium-Hybrids Obtained by Artificial Fertilisation” in Stern and Sherwood.
Brannigan, Augustine. The Social Basis of Scientific Discoveries. Cambridge, U.K.: Cambridge University Press, 1981.
Corcos, Alain F., and Floyd V. Monaghan. Gregor Mendel’s Experiments on Plant Hybrids: A Guided Study. New Brunswick, NJ: Rutgers University Press, 1993.
Dunn, L. C. “Mendel, His Work and His Place in History.” Commemoration of the Publication of Gregor Mendel’s Pioneer Experiments in Genetics, special issue of Proceedings of the American Philosophical Society 109 (1965): 189–198.
Fairbanks, Daniel J., and Bryce Rytting. “Mendelian Controversies: A Botanical and Historical Review.” American Journal of Botany 88 (2001): 737–752.
Falk, Raphael, and Sahotra Sarkar. “The Real Objective of Mendel’s Paper: A Response to Monaghan and Corcos.” Biology and Philosophy 6 (1991): 447–451.
Frolov, Ivan Timofeevich, and S. A. Pastusnyi. Mendel’, mendelizm I dialektika. Moscow: Mysl’, 1972.
Fuller, Steve. Thomas Kuhn: A Philosophical History for Our Times. Chicago: University of Chicago Press, 2000.
Gliboff, Sander. “Gregor Mendel and the Laws of Evolution.” History of Science 37 (1999): 217–235.
Kříženecky, Jaroslav. Gregor Johann Mendel: 1822–1884. Texte und Quellen zu seinem Wirken und Leben. Festgabe der Deutschen Akademie Naturforscher Leopoldina zum Mendel Memorial Symposium 1865–1965, August 1965 in Brünn. Leipzig, Germany: J. A. Barth, 1965.
Matalová, Anna. “Response to Mendel’s Death in 1884.” Folia Mendeliana 19 (1984): 217–221.
Monaghan, Floyd V., and Alain F. Corcos. “The Real Objective of Mendel’s Paper.” Biology and Philosophy 5 (1990): 267–292.
Müller-Wille, Staffan, and Vít\zslav Orel. “From Linnaean Species to Mendelian Factors: Elements of Hybridism,1751–1870.” Annals of Science 64, no. 2 (April 2007): 171–215.
Orel, Vítězslav. Gregor Mendel: The First Geneticist. Oxford: Oxford University Press, 1996.
_____. “Contested Memory: Debates over the Nature of Mendel’s Paradigm.” Hereditas 142 (2005): 98–102.
_____, and I. Cetl. The Secret of Mendel’s Discovery. In Japanese. Tokyo, 1973.
_____, and Daniel L. Hartl. “Controversies in the Interpretation of Mendel’s Discovery.” History and Philosophy of the Life Sciences 16 (1994): 423–464.
_____, Françoise Robert, and Jean Robert Armogathe. Mendel,1822–1884: Un inconnu célèbre. Un savant, une époque. Paris: Belin, 1986.
Sapp, Jan. “The Nine Lives of Gregor Mendel.” In Experimental Inquiries, edited by H. E. Le Grand. Dordrecht, Netherlands: Kluwer Academic Publishers, 1990. Available from http://www.mendelweb.org/MWsapp.html.
Stubbe, Hans. Kurze Geschichte der Genetik bis zur Wiederentdeckung der Vererbungsregeln Gregor Mendels. Jena, German Democratic Republic: Fischer, 1963. Translated as History of Genetics, from Prehistoric Times to the Rediscovery of Mendel’s Laws. Cambridge, MA: MIT Press, 1972.
Wood, Roger J., and Vítězslav Orel. Genetic Prehistory in Selective Breeding: A Prelude to Mendel. Oxford: Oxford University Press, 2001.
_____, and Vítězslav Orel. “Scientific Breeding in Central Europe during the Early Nineteenth Century: Background to Mendel’s Later Work.” Journal of the History of Biology 38 (2005): 239–272.
Relationship of Mendel to Mendelian Genetics . The name Gregor Mendel has long been closely associated with the study of heredity and variation, a field of studies to which the name “genetics” was given in 1906. Four decades earlier Mendel’s research on hybridization was published, and by 1906 he had long been dead. How strong, then, is the claim for Mendel as founder of this great science? The British evolutionist, Sir Gavin de Beer, had no doubt on the matter. In 1965 he declared, “There is not known another example of a science which sprang fully formed from the brain of one man.” To an audience at the Royal Society that year he explained: “It is not often possible to pinpoint the origin of a whole new branch of science accurately in time and place ... But genetics is an exception, for it owes its origin to one man, Gregor Mendel, who expounded its basic principles at Brno on 8 February and 8 March 1865” (De Beer, p. 154).
Clearly Mendel had nothing to do with the establishment of the institutions associated with genetics. Rather, sixteen years after his death, it was William Bateson, aided by the Royal Horticultural Society, who, in the English-speaking world, began to map out the discipline that in 1906 he christened “genetics,” and in 1909 published the first full scale English text, Mendel’s Principles of Heredity. The same year appeared Wilhelm Johannsen’s German text, Elemente der exakten Erblichkeitslehre. Was it then an exaggeration on De Beer’s part to claim that the science “sprang fully formed” from Mendel’s brain? To address this question let us summarize briefly the core features of the Mendelian genetics established in the early years of the twentieth century, then anatomize Mendel’s paper of 1865, in order to assess the extent of the dependence of the former on the latter.
Early Mendelian Genetics . The central concept of Mendelian genetics was that of the “factor” or “gene.” In the simplest case, it was claimed, an hereditary trait is determined by two such factors, one from each parent. For a given hereditary trait, every cell in the organism has one paternal and one maternal copy of each. Bateson called them “allelomorphs,” later abbreviated to alleles. In the germ cells, however, only one of the two alleles would be present, because in forming these cells the two factors or alleles would have been “segregated” into different daughter germ cells. Subsequent fusion of germ cells in fertilization would restore the two-fold constitution from which arise the cells of the new organism. Neither Bateson nor Johannsen wished to identify their proposed hereditary units with specific particles in the cell.
By 1915, however, Thomas Hunt Morgan and his group in New York had created maps showing the relative locations of these hereditary determinants on the chromosomes of the fruit fly. The concept of the gene as a material particle was thus introduced. Meanwhile Johannsen, wishing to put an end to confusions between the observed characteristics of an organism and its genetic constitution, had introduced the terms gene, genotype, and phenotype. Mendelian factors were henceforth and increasingly referred to as genes, and hereditary transmission was used to refer to genes rather than to characteristics.
Although it was soon recognized that Mendelian heredity governs characteristics that blend and have small variations, as well as those that are nonblending and have large variations, the early Mendelians concentrated their research on the latter. These, they claimed, give little if any opportunity for natural selection to play its creative, adaptive role. That role requires the variations to be small so that they can be accumulated in a gradual stepwise manner. Acting on large nonblending variations, natural selection would be no more than a sieve, weeding out the nonadaptive. Supporters of Darwinian evolution therefore considered Mendelian research a diversion, irrelevant to the problems of evolution. As a result it took some three decades to achieve a synthesis between Darwinism and Mendelian genetics. What, then, were Mendel’s views on hereditary particles and Darwinian evolution, and how close was his research to that of the early Mendelians?
Mendel’s Theoretical and Experimental Contributions . Mendel’s research was carried out before chromosomes had been described, but not before it was established that every cell comes from the division of a preexisting cell, and that fertilization is the union of two germ cells. He accepted these conclusions as some did not, Charles Darwin for one. Furthermore, Mendel believed that the development of the organism proceeds “in accord with a constant law based on the material composition [Beschaffenheit] and arrangement of the elements that attained a viable union in the cell” (Stern and Sherwood, p. 42; Kříženeck, p. 88). His was a materialist account. But nowhere did he speak of just two elements or factors for an hereditary trait. Most often he used phenotypic language. Thus he described “traits that pass into hybrid association ... unchanged” (Stern and Sherwood, p. 9; Kříženeck p. 63) rather than elements or factors. Germ cells he described as “endowed with the potential [Anlage] for creating identical individuals” (Stern and Sherwood, p. 24; Kříženeck p. 74). Here Anlage translated as “potential,” “disposition,” or “rudiment” bespeaks Mendel’s appreciation of the nineteenth century distinction between “hereditary constitution” and the observed characters or traits. The other location for such language is in a letter to Carl Nägeli in 1870. In discussing the determination of sex he referred to the Anlage for the functional development of the sex organs, and suggested that the germ cells are “different as regards the sex anlage” (Stern and Sherwood, p. 97). Anlage, admittedly, has multiple meanings, from blueprint to potential and predisposition, but these two occasions in which Mendel used the word make clear that when attending to the level of cells, he could stop the characteristic talk and turn to the language of hereditary constitution. The early Mendelians, for the most part, were no more consistent than Mendel in this respect.
Mendel was one among several who in the nineteenth century used letter notation to represent hybridizations, among them Nägeli and Max Wichura. Mendel knew their work, but had rightly rejected the fractional law of inheritance upon which they relied. The referents of Mendel’s letters, however, were, like theirs, the “forms” of the plants. When he turned to the reproductive cells of the hybrids, though, Mendel used such notation to refer to the germ cells. Thus his representation of the pairs of germ cells that come together when the hybrid Aa reproduces and of the resulting offspring is:
The letters on the right side of the equation clearly refer to the foundation cells (zygotes) produced, their kinds, and the proportions between them, in terms of the kinds of plants they yield. Had he been referring to factors, the expression on the right side of the equation would be: AA + 2Aa + aa.
Mendel, it seems, did not picture cells, whether of hybrids or of pure species, as possessing two elements per trait that go their separate ways when the germ cells are formed. Such a conception would have meant that a separation between pairs of elements occurs whether the elements are like or unlike. But in forming the germ cells he suggested: “all elements present participate in completely free and uniform fashion, and only those that differ separate from each other” (Stern and Sherwood, p. 43;Kříženeck, p. 88). His language conjures up the separation of pepper grains from a mixture with mustard seeds rather than paired elements in the cell. And why not? Bateson rightly called the segregating cell division “out of which each gamete comes sensibly pure in respect of the allelomorph it carries,” Mendel’s “essential discovery” (Bateson, 1928, p. 245).
Mendel’s interpretation of his results in terms of the cell theory was just one of the many brilliant features of his 1865 paper. Although the idea of pairs of elements in the cells is missing, the notion of pairs is contained in one central and indispensable feature of his experimental design, namely his “constant differing traits” (konstant differierende Merkmale). These were his chosen seven traits, each of which differed in its expression in different strains, not in a “more or less” manner, but “decisively.” Thus for the trait height, strains were either tall or short, for seed color, either green or yellow etc. These were the “differing characters” or character-pairs (je zwei differierende Merkmale). His success in establishing their independence from one another in hereditary transmission gave him the confidence to deny that species are unified entities that only act as wholes. Thus prepared, he could unravel the confused accounts of the behavior of hybrid descendants in the literature, and explain the alleged greater variability of cultivated plants over their wild relatives in terms of the frequency of spontaneous hybridization and the ensuing recombination of independently transmitted characters.
Mendel’s experimental procedure was unique in its day. Here he drew inspiration from physics and, Gliboff believes, from botany, possibly from the work of Unger (Gliboff, 1999). Mendel tested the true-breeding quality of thirty-four varieties of pea for two seasons (1854–1856) before he started the hybridizations. He grew twenty-two of these varieties as controls throughout the experiment (1856–1862) to check that they continued true when no crossbreeding occurred. Some plants in the experiment were grown under glass to see if protection from insects gave results differing from those obtained from plants grown outside where insects might have interfered. He harvested peas in their thousands in order to achieve statistically significant results.
His interpretation of the results was also unprecedented. Having been trained in combinatorial mathematics he was well aware of the numerical relations between the terms in the expansion of a binomial. Thus (A + a)2=A2+ 2Aa + a2—the three terms related as 1 : 2 : 1 (when Aa cannot be distinguished from A, the ratio becomes 3 : 1). This meant that he did not just report data showing a three-fold predominance of one class of offspring over the other, but he saw the significance of the approximation of his data to 3 : 1. Such a ratio would be produced if the germ cells of the hybrid Aa are “pure,” i.e., are of the type A or a but not Aa, and if both are produced in equal numbers, and all combinations in fertilization are realized equally.
Historical research has revealed that two of the three “rediscoverers” of Mendelian heredity, Hugo de Vries and Carl Correns, had read Mendel in the 1890s, Correns before he carried out his hybridization experiments (Rheinberger, 1995), De Vries only after he had arrived at the 3 : 1 ratio independently (Stamhuis et al., 1999; Meijer, 1995). But it was reading Mendel’s paper possibly for the second time that alerted him to the great significance of these ratios. The third rediscoverer, Erich Tschermak, had completed his experiments before reading Mendel, but he did not arrive at the full Mendelian explanation of his results (Olby, 1985, pp. 120–123).
However, all the rediscoverers’ papers were eclipsed by Mendel’s stunning achievement. It was clearly Mendel who first set out the design of the hybridization experiments subsequently carried out by his rediscoverers. His remarkable experimental methodology, as Müller-Wille and Orel have shown, belongs in the long tradition of plant hybridization going back to Linnaeus (Müller-Wille and Orel, 2007). Yet Mendel’s introduction of the “character-pair” as an independent unit in hybridization, his statistical sophistication, and his creative application of cytology were major innovations. They constituted major tools of the Mendelism that followed. Moreover, even in 1900 Mendel’s results were more impressive than those of his rediscoverers, and this they acknowledged.
On these grounds Mendel stands out as the preeminent figure in the prehistory of Mendelism. However, if the attribution of founder status to Mendel is accepted in De Beer’s terms, it should not imply the presence in Mendel’s work of features only introduced in the following century.
Mendel and Evolution . Some historians have argued that Mendel’s experiments and their theoretical explanation were directed against Darwin’s theory of evolution by natural selection (Callender 1988; Bishop 1996). Mendel encountered species transformism first in the rather speculative tradition of Naturphilosophie when he became friendly with a fellow Augustinian, Franz Matthaeus Klacel. According to the Naturphilosophen a “world spirit” has worked within organisms, transforming them into higher and more complex forms over long periods of time. Mendel again met with transformism in the teaching of Unger at the University of Vienna. There he learned of the work of the German plant hybridists, Joseph Kölreuter and Gärtner, and of the debate as to whether new species might have arisen by hybridization. Already in the winter of 1853 he must have been considering a project with Pisum on the lines of the experiments for which he is now famous. And as the introductory and the concluding sections show, he was, like Gärtner, concerned to address the question of whether there are hybrids that remain as constant as do their originating species.
Although Mendel visited London in 1862 as a member of Brno’s delegation to the International Exhibition there, he never met or corresponded with Darwin. We know that Mendel read and wrote marginalia in his own copy of the 1863 German translation of The Origin of Species. These, like the concluding section of his 1865 paper, show clearly that he did not accept Darwin’s views on the source of variation (Stern and Sherwood, pp. 37–38; Kříženecky, pp. 84–85). This negative response was not on account of religious concerns, for Mendel, though a devout Christian, took a very liberal view of Christianity and was certainly not a supporter of the biblical creation story. When he referred publicly to “the spirit of the Darwinian teaching” (Stern and Sherwood, p. 51), he did not criticize or denigrate it.
But Darwin’s claim that all variation is directly or indirectly traceable to changes in the conditions of life Mendel rejected, as he surely did Darwin’s claim that the role of hybridization in generating variation has been “greatly exaggerated” (Darwin, 1861, p. 20). Both Mendel and, earlier, his teacher Unger, had conducted transplant experiments to test the alleged lasting effects of changed conditions and had not found them. Because Darwin believed crossbreeding dilutes new variations, he saw it as a deterrent to their establishment. But Mendel knew from his experiments that the disappearance of some characters following crossbreeding does not necessarily lead to their loss. For Mendel, therefore, Darwin’s theory needed fundamental revision. (But Darwin could have responded to Mendel that the genetic recombination generated by hybridization would not alone supply sufficient novel variations. Hence arose the suggestion of abrupt, hereditary variations independent of hybridization, as in the work of Bateson  and the mutation theory of De Vries .)
To describe Mendel as a convinced Darwinian would thus be misleading. Rather, what little evidence we have would suggest that he was a transformist who, unlike Darwin, perceived in the genetic recombination following hybridization a rich source of variation upon which selection could act. To read more into his work seems unjustified.
See also Müller-Wille and Orel’s bibliography above.
Bateson, William. Materials for the Study of Variation Treated with Especial Regard to Discontinuity in the Origin of Species. London: Macmillan, 1894.
_____. “Presidential Address to Section D.” Report of the British Association 1904. Reprinted in Bateson, Beatrice. William Bateson, F.R.S. Naturalist. His Essays and Addresses Together with a Short Account of His Life. Cambridge, U.K.: Cambridge University Press, 1928.
_____. Mendel’s Principles of Heredity. Cambridge, U.K.: Cambridge University Press, 1909.
Bishop, B. E. “Mendel’s Opposition to Evolution and to Darwin.” Journal of Heredity 87 (1996): 205–213.
Callender, L. A. “Gregor Mendel—An Opponent of Descent with Modification.” History of Science 26 (1988): 41–75.
Correns, C. “G. Mendel’s Regel über das Verhalten der Nachkommenschaft der Rassenbastarde.” Berichte der deutschen botanischen Gesellschaft 18 (1900): 158–168.
Darwin, Charles R. On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. 3rd. ed. London, 1861. Translated by Victor Carus, Über die Entstehung der Arten im Tier und Pflanzenreiche durch natürliche Züchtung. Stuttgart, 1863. Mendel’s copy.
De Beer, Gavin. “Genetics: The Centre of Science.” Proceedings of the Royal Society of London, Series B, Biological Sciences 164 (1966): 154–166.
De Vries, Hugo. “Sur la loi de disjonction des hybrides.” Comptes rendus hebdomadaire des séances de l’Académie des sciences 130 (1900): 845–847.
_____. Die Mutationstheorie: Versuche und Beobachtungen über die Entstehung von Arten im Pflanzenreich. Leipzig, Germany: Veit, 1901. Translated by J. B. Farmer and A. D. Darbishire as The Mutation Theory: Experiments and Observations on the Origin of Species in the Vegetable Kingdom. 2 vols. Chicago: Open Court, 1909–1910.
Heimans, J. “Mendel’s Ideas on the Nature of Hereditary Characters: The Explanation of Fragmentary Records of Mendel’s Hybridizing Experiments.” Folia Mendeliana 6 (1971): 91–98.
Henig, Robin Marantz. The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Genetics. New York: Houghton Mifflin, 2000.
Iltis, Hugo. Gregor Johann Mendel: Leben, Werk und Wirkung.Berlin: Julius Springer, 1924. Translated by Eden and Cedar Paul. Life of Mendel. London: George Allen & Unwin, 1932. Reprinted 1966. Note that pp. 207–408 of the German edition were excluded from the English translation.
Johannsen, Wilhelm. Elemente der exakten Erblichkeitslehre. Jena,Germany: Fischer, 1909.
_____. “The Genotype Conception of Heredity.” American Naturalist 45 (1911): 129–159.
Kimmelman, Barbara A. “Organisms and Interests in Scientific Research: R. A. Emerson’s Claims for the Unique Contributions of Agricultural Genetics.” In The Right Tools for the Job: At Work in Twentieth-Century Life Sciences, edited by Adele E. Clarke and Joan H. Fujimura. Princeton, NJ: Princeton University Press, 1992.
Mawer, Simon. Gregor Mendel: Planting the Seeds of Genetics. New York: Abrams/Field Museum, Chicago, 2006.
Meijer, Onno. “Hugo de Vries No Mendelian?” Annals of Science 42 (1985): 189–232.
Olby, Robert. “Mendel No Mendelian?” History of Science 17 (1979): 53–72. Reprinted with minor changes in Olby, The Origins of Mendelism. 2nd ed. Chicago: Chicago University Press, 1985.
_____. “Horticulture: The Font for the Baptism of Genetics.” Nature Reviews: Genetics 1 (2000): 65–70.
Paul, Diane, and Barbara Kimmelman. “Mendel in America:Theory and Practice, 1900–1919.” In The American Development of Biology, edited by Ronald Rainger, Keith R. Benson, and Jane Maienschein. Philadelphia: University of Pennsylvania Press, 1988.
Rheinberger, Hans-Jörg. “When Did Carl Correns Read Gregor Mendel’s Paper?” Isis 86 (1995): 612–618.
Roberts, H. F. Plant Hybridization before Mendel. Princeton, NJ: Princeton University Press, 1929. Reprinted New York: Hafner Publishing Co., 1965.
Stamhuis, Ida, Onno Meijer, and E. J. Zevenhuizen. “Hugo de Vries on Heredity: Statistics, Mendelian Laws, Pangenesis, Mutation.” Isis 90 (1999): 238–267.
Weiling, Franz. “Descent of the Prelate Cyrill Napp, the Spiritual Mentor of J. G. Mendel.” Sudhoffs Archiv 55, no. 1 (1971): 80–85.
_____. “Historical Study: Johann Gregor Mendel, 1822–1884.” American Journal of Medical Genetics 40, no. 1 (1991): 1–25; discussion 26.
Tschermak, Erich. “Über künstliche Kreuzung bei Pisum sativum.” Zeitschrift für das landwirtschaftliche Versuchswesen in Oesterreich 3, no. 5 (1900): 465–555.
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Mendel, Johann Gregor
MENDEL, JOHANN GREGOR
(b. Heinzendorf, Austria [now Hynčice, Czechostovakia], 22 July 1822; d. Brno, Austria [now Czehostovakia], 6 January 1884)
At the February and March 1865 meetings of the Natural Sciences Society of Brno, J. G. Mendel first presented an account of his eight years of experimental work on artificial plant hybridization. His paper was published in the Society’s Verhandlungen in 1866 but went unnoticed. In 1900, within a two-month period, there appeared three preliminary reports by Hugo de Vries, Carl Correns, and Erich von Tschermak, who, working independently in Amsterdam, Tübingen, and Vienna respecthely, attained the same results almost simultaneously. Each of them stated that just before completing his work, he learned that he had been preceded, by several decades, by a virtually unknown monk. Mendel’s experimental work, designed after tong contemplation of the problem, painstakingly executed on an extenshe scale, intelligently analyzed and interpreted, and presented straightforwardly and clearly, yielded results of such general and far-reaching significance that his paper became the basis of the science of genetics.
Mendel’s father, Anton, was a peasant. He served in the army during the Napoleonic Wars and later was able to turn his experience acquired in other regions to the improvement of his farm. Mendel’s mother was the daughter of a village gardener, and other of his ancestors were professional gardeners in service at the local manor. Heinzendorf was on the border between the Czech- and German-speaking areas; the Mendel family spoke German, but about one-fourth of their ancestors were of Czech extraction. Mendel himself later lived on excellent terms with both. After the death of two infant girls, a daughter, Veronica, was born to the couple (1820), followed by Johann (1822), and another daughter, Theresia (1829). In the primary school the enlightened Reverend Schreiber, vicar of the neighboring village, taught the children natural science in addition to elementary subjects, and encouraged the cultivation of fruit trees both at the school and by the parishioners. Mendel also helped his father in grafting fruit trees in their orchar.
Since Mendel showed exceptional abilities, his parents, on the advice of the vicar and the village schoolmaster, sent the boy in 1833 to the Piarist secondary school in nearby Leipnik (Lipnik), and a year later to the Gymnasium in Troppau (Opava), where he spent six years. He left with a certificate primae classis cum eminentia, the designation referring to his industriousness, knowledge, and ability. In 1838 Menders father suffered serious injuries during his statutory labor, and had to retire and turn over his farm to his son-in-law. Mendel had to earn his living by private tutoring. The physical and mental strain affected his health so much that in his fifth year at the Gymnasium he had to interrupt his studies for several months. He recovered, completed his secondary studies, and in 1840 enrolled in the Philosophy course, as preparation for higher studies, at the University of Olmütz (Otomouc). His efforts to find private pupils were in vain, however, because he had no references. This new distress and frustration brought on further illness, this time for a longer period, so that Mendel had to spend a year with his parents to recover. But he refused to give up his studies and become a farmer. His younger sister, Theresia, offered him a part of her dowry to help him return to Olmütz and complete the two-year Philosophy course. He accepted and thus became acquainted with the elements of Philosophy, physics, and mathematics, including—in a course given by J. Fnx—the principles of combinatorial operations, which he later employed with great success in his research. His physics professor, F. Franz, in 1843 recommended the admission of this “young man of very solid character, almost the best in his own branch,” to the Augustinian monastery in Brno, where Franz himself had stayed for nearly twenty years.
Mendel entered the monastery on 9 October 1843 with the name Gregor. He did so out of necessity and without feeling in himself a vocation for holy orders. But he soon realized that now he was free of all financial worries and that he had found the best possible conditions for pursuing his studies. The monastery was a center of learning and scientific endeavor, and many of its members were teachers at the Gymnasium or at the Philosophical Institute. The monastery was supported mainly by the income from its estates. F. C. Napp (1792–1867), who was the abbot from 1824, devoted much energy to the improvement of agriculture; he was a member of the Central Board of the Moravian Agricultural Society and later its president. He wanted one of his monks to teach natural sciences and agriculture at the Philosophical Institute, and he established the tradition of experimenting with plants in the monastery garden. When Mendel entered the order, Matthew Klácel, a teacher of Philosophy (1808–1882), was directing the experimental garden and investigating variation, heredity, and evolution in plants. He enjoyed a high reputation among the Brno botanists, and drew upon his experience in natural history to formulate ideas on the Hegelian Philosophy of gradual development that ultimately led to his dismissal and emigration to America. Klácel guided Mendel in his first studies in science and later put him in charge of the experimental garden.
During his theological studies (1844–1848) Mendel, in accordance with the abbot’s interests, also attended courses at the Philosophical Institute in agriculture, pomology, and viticulture given by F. Diehl (17701859), who, in his textbook of plant production, Abhandlungen aus der Landwirtschaftskunde für Landwirthe … (2 vols., Brno, 1835), had described artificial pollination as the main method of plant improvement. In these lectures Mendel also learned of the methods of sheep breeding introduced by F. Geisslern (1751–1824), Diebl was, with Napp, among the main organizers of the Congress of German Agriculturists at Brno in 1840, where hybridization as a method of fruit-tree breeding was discusse.
After he finished his theological studies, Mendel was appointed chaplain to the parish served by the monastery, his duty being to see to the spiritual welfare of the patients in the neighboring hospital. But Mendel was extremely sensitive, and could not bear to witness stitfermg; he was overcome by fear and shyness and again became very depressed, almost to the point of illness. The sympathetic abbot relieved him of this duty and sent him as a substitute teacher to the grammar school at Znojmo in southern Moravia.
Mendel enjoyed teaching and so impressed both his pupils and his colleagues that at the end of his first year the headmaster recommended him for the university examination for teachers of natural sciences, which would allow him a regular appointment. Mendel passed well in physics and meteorology, but failed in geology and zoology. Since the failure seemed to be due largely to Mendel’s lack of a university education, Andreas Baumgartner, the professor of physics, advised Napp to send Mendel to the University of Vienna.
At the university Mendel attended lectures on experimental physics by Doppler and on the construction and use of physical apparatus by Andreas von fttinghausen (1796 1878), who had earlier published a book on combinatorial analysis, Die combinatorische Analysis (Vienna, 1826). It is clear that some of the methods described there later influenced Mendel’s derivation of series in the hybrid progeny. Mendel also attended lectures on paleontology botany, zoology, and chemistry, and was especially influenced by the professor of plant physiology, Franz Unger, who also gave a practical course on organizing botanical experiments. In his research Unger turned from the investigation of forms of fossil plants, to the influence of soil upon plants, to the causes of variation, He was known for his views on evolution, and in his lectures emphasized that sexual generation was the basis of the origin of the great variety in cultured plants. He sought to explain the evolution of new plant forms by the combination of the simplest elements in the cell, surmising their existence but unable to prove it. In Vienna, Mendel also thoroughly studied Gaertner’s Versuche und Beobachtungen über die Bastardzeugung im Pflanzcnreich (Stuttgart, 1849), in which nearly 10,000 separate experiments with 700 plant species yielding hybrids were described. In his copy, preserved in the Mendelianum, Mendel marked pages where pea hybrids are mentioned and also made notes on pairs of Pisum characteristics, some of which later appeared in his experimental program.
During his studies Mendel became a member of the Zoologisch-botanischer Verein in Vienna and published his first two short communications in its Verhandhungen (1853, 1854). They concerned damage to plant cultures by some insects.
After his return to Brno, Mendel was appointed substitute teacher of physics and natural history at the Brno Technical School. His superior was A. Zawadski (1798– 1868), previously professor of physics and applied mathematics and dean of the Philosophical Faculty at the University of Lvov. He had come to Brno in 1854, after being dismissed for alleged responsibility for student uprisings in 1848. Zawadski was a man of wide scientific interests, ranging from botany, zoology, and paleontology to evolution. On his nomination Mendel became a member of the natural science section of the Agricultural Society in Brno.
As a teacher Mendel was highly appreciated both by students and colleagues. His task was not easy, for the classes were large, some with over 100 students. After his first year his headmaster reported that Mendel was a good experimentalist and, with rather scanty equipment, was able to give excellent demonstrations in both physics and natural history.
After his return from Vienna, Mendel also began experimenting with peas. The most arduous aspect was artificial pollination. He began the work in 1856, when he was also preparing for his second university examination for teachers of natural science, which took place in May 1856. Once again the instability of his psychological constitution betrayed him. He broke down during the written examination, withdrew from the other parts, and returned to Brno. There he became so seriously ill that his father and uncle came all the way from Silesia to see him. It was their only visit to him in Brno. Indirect evidence suggests that his indisposition derived from the stress of his studies and preoccupation with his research problems, compounded by the memory of his experience with the previous examination. After this failure he attempted no further degrees, and remained a substitute teacher until 1868, when he gave up teaching.
In 1868 Mendel was elected abbot of the monastery, which involved many official duties. He also became a member of the Central Board of the Agricultural Society and was entrusted with distribution of subsidies for promoting farming; from 1870 he was frequently elected to its executive committee. He took an active part in the organization of the first statistical service for agriculture. Later he also reported on scientific literature and cooperated with the editorial board of the society’s journal, Mittheilungen der K. K. Mährisch-schlesischen Gesellschaft zur Beförderung des Ackerbaues, der Natur- und Landeskunde. Near the end of his life he was considered for the society’s presidency, but he refused to accept this honor because of his poor health. From 1863 Mendel was also a member of the Brno Horticultural Society and after 1870 he belonged to the Society of Apiculturists, and influenced the development of both these fields.
Mendel soon became known for his liberal views, which he demonstrated by public support for the nominees of the Liberal Party in the general election of 1871. When the victorious Liberal government issued a law requiring a large contribution by the monastery to the religious fund, Mendel refused to pay the new taxes. After 1875 he was thus involved in a lengthy conflict with the authorities, which led to the sequestration of monastery land. In a last attempt to regain Mendel’s support for the Liberal party, he was offered a place on the board of directors of the Moravian Mortgage Bank in 1876, and was even proposed for the office of its governor in 1881. Mendel persisted in his opposition, however, convinced that he was fighting for the rights of the monastery. The tension eventually had a deleterious effect on Mendel’s health; he died of chronic inflammation of the kidneys with edema, uremia, and cardiac hypertrophy. At his funeral nobody was aware that an outstanding scientist was being buried, even though Mendel’s experiments on Pisum were remembered by the fruit growers of Brno and in the obituary notices of local newspapers.
Work. Meteorological Studies. Mendel began his meteorological studies in 1856 and was soon recognized as the only authority on this subject in Moravia. In his first meteorological paper, published in 1863, he summarized graphically the results of observations at Brno, using the statistical principle to compare the data for a given year with average conditions of the previous fifteen years. Between 1863 and 1869, the paper was followed by five similar communications concerned with the whole of Moravia. Later Mendel published three meteorological reports describing exceptional storm phenomena. He also devoted much time to the observation of sunspots, assuming that they had some relation to the weather. In 1877, with his support, weather forecasts for farmers in Moravia were issued, the first in central Europe.
As always, there was a practical aspect to Mendel’s pursuits and interests, but the primary motivation was scientific. The best example is his paper on the tornado that he observed at Brno in 1870. He began with a careful description and followed it with a new interpretation, which was that the observed phenomena were vortices engendered by encounters between conflicting air currents. This interpretation was overlooked, as were his hybridization experiments, even by those who advanced similar explanations many years later. These studies, although remote from his main work and far less important, have much in common methodologically with his studies of hybridization. They grew out of his habit of scruputously collecting and recording data, thinking in quantitative terms, and subjecting observational data to statistical treatment.
Plant Hybridization. Mendel’s principal work was the outcome of ten years of tedious experiments in plant growing and crossing; seed gathering and careful labeling: and observing, sorting, and counting almost 30,000 plants. It is hardly conceivable that it could have been accomplished without a precise plan and a preconceived idea of the results to be expected. There is evidence that Mendel began with an inductive hypothesis carefully framed so as to be testable in his experimental program.
From his previous experience Mendel was familiar with methods for improvement of cultivated plants. Both his teachers, Diebel and Unger, had pointed to hybridization as the source for this improvement. But hybridization was an empirical procedure, and Unger therefore stressed the necessity of studying the nature of variability. He suggested that it might be possible to find an explanation in the combination of hypothetical elements within the cells. Mendel’s approach was the common one of reducing the problem to an elementary level and formulating a hypothesis that could be proved or disproved by experiments.
Between 1856 and 1863 Mendel cultivated and tested at least 28,000 plants, carefully analyzing seven pairs of seed and plant characteristics, This was his main experimental program. His original idea was that heredity is particulate, contrary to the model of “blending inheritance” generally accepted at that time. In the pea plants hereditary particles to he investigated are in pairs. Mendel called them “elements” and attributed them to the respective parents. From one parent plant comes an element determining, for instance, round seed shape; from the other parent, an element governing the development of the angular shape. In the first generation all hybrids are alike, exhibiting one of the parental characteristics (round seed shape) in unchanged form. Mendel called such a characteristic “dominant”; the other (angular shape), which remains latent and appears in the next generation, he called “recessive,” The “elements” determining each paired character pass in the germ cells of the hybrids. without influencing each other, so that one of each pair of “elements” passes in every pollen (sperm) and in every egg (ovule) cell In fertilization, the element marked by Mendel A, denoting dominant round seed shape, and the clement a, denoting the recessive angular shape, meet at random, the resulting combination of “elements” being
In hybrid progeny both parental forms appear again; and Mendel’s explanation of this segregation of parental traits was called, after 1900, Mendel’s law (or principle) of segregation.
In his simplest experiments with crossing pea plants that differed in only one trait pair. Mendel cultivated nearly 14,000 plants and explained the progeny of the hybrid in terms of the series A + 2 Aa + a. At the same time he conformed to the view of K. F. von Gaertner and J. G. Koelreuter that hybrids have a tendency to revert to the parental forms. Mendel then called his explanation of hybrid progeny “the law of development thus found,” which he tested further in a case “when several different traits are united in the hybrid through fertilization.” Hereditary elements Belonging to different pairs of traits, for example A and a for the round and angular seed shapes and B and b for the yellow and green seed colors, recombine the individual series A + 2 Aa + a and B + 2 Bb + b, resulting in terms of a combination series:
AB + Ab + aB + ab + 2 A Bb + 2 a Bb
+ 2 AaB + 2 Aab + AaBb.
In his paper Mendel also illustrated a recombination of three trait pairs, showing every expected combination of characteristics and relevant elements in actual counts of offspring. He also observed that 128 constant associations of seven alternative and mutually exclusive characteristics were actually obtained—that being the expansion of 27, and the maximum number theoretically possible. His conclusion was that the “behavior of each of different traits in a hybrid association is independent of all other differences in the two parental plants,” which principle was later called Mendel’s law of independent assortment.
The generalization of Mendel’s explanation was that “if n denotes the number of characteristic differences in two parental plants, then 3n is the number of terms in the combination series, 4n the number of individuals that betong to the series, and 2n the number of combinations that remain constant.”
In his second lecture to the Natural Sciences Society of Brno (March 1865) Mendel presented his hypothesis explaining “the development of hybrids in separate generations,” and furnished both a theoretical and experimental proof of his assumption by crossing hybrids with constant dominant and constant recessive forms. His explanation was “that hybrids form germinal and pollen cells that in their composition correspond in equal numbers to all the constant forms resulting from the combination of traits united through fertilization.”
At this meeting Mendel also described briefly his experiments with other plant species, the object of which was to determine “whether the law of development discovered for Pisum is also valid for hybrids of other plants.” He predicted that “through this approach we can learn to understand the extraordinary diversity in the coloration of our ornamental flowers.” In his concluding remarks he compared the observations made on Pisum with the results obtained by Koelreuter and Gaertner, especially) those on hybrid characteristics. Mendel could not agree with the assumption that in some cases hybrid offspring remain “exactly like the hybrid and propagate unchanged,” as Gaertner believed. Mendel also touched on the experimental transformation of one species into another by artificial fertilization. In a simple experiment with Pisum he exhibited transformation from the viewpoint of his theory, explaining why some transformations took longer than others. He noted that Koelreuter’s transformation experiments proceeded” in a manner similar to that in Pisum,” and that in this way “the entire process of transformation would have a rather simple explanation.” In this connection Mendel opposed Gaertner’s view “that a species has fixed limits beyond which it cannot change.” and he thus asserted his conviction that his theory favored the assumption of continuous evolution.
In the opening paragraph of his paper Mendel stated that his experiments were initiated by “artificial fertilization undertaken on ornamental plants to obtain new color variants,” and he emphasized the significance of such investigations in establishing “the evolutionary history of organic forms.” Subsequently he often mentioned that “law of development” but without using the terms “inheritance” or “hereditary,” By the “law of development” he certainly meant the law governing the evolution of cultured plants. He also assumed that his theory was valid in generality because “no basic difference could exist in important matters, since unity in the plan of development of organic life is beyond doubt.”
In comparison with his predecessors, Mendel was original in his approach, in his method, and in his interpretation of experimental results. He reduced the hitherto extremely complex problem of crossing and heredity to an elementary level appropriate to exact analysis. He left nothing to chance. The choice of Pisum as his main experimental material resulted both from his study of the literature and from numerous preliminary experiments, He very carefully selected varieties whose purity had been assured by several years of cultivation under strictly controlled conditions. The hybridized varieties differed in only a few characteristics, and those that did not allow a clear distinction were discarded. Limiting the characteristics to a small number enabled Mendel to distinguish all possible combinations. His introduction of simple symbols that permitted comparing the experimental results definitively with the theory was very important. Altogether new was his use of large populations of experimental plants, which allowed him to express his experimental results in numbers and subject them to mathematical treatment. By the statistical analysis of large numbers Mendel succeeded in extracting “laws” from seemingly random phenomena. This method, quite common today, was then entirely novel Mendel, inspired by physical sciences, was the first to apply it to the solution of a basic biological problem and to explain the significance of a numerical ratio. His great powers of abstraction enabled him to synthesize the raw experimental data and to reveal the basic principles operating in nature.
Mendel’s manuscript, as read at the 1865 meetings, was published without change in the Natural Sciences Society’s Verhandlungen in 1866. The other members, however, could hardly have grasped either the main idea or the great significance of his discoveries. The Verhandlungen was distributed to 134 scientific institutions in various countries, including those in New York, Chicago, and Washington. Mendel commissioned forty reprints, two of which have been found in Brno and five others elsewhere; one was sent to Naegeli and another to Anton Kerner, two contemporary authorities on hybridization.
Further Research. The main results of Mendel’s experiments and their interpretation, which constituted his whole theory, were reported in “Versuche über Pflanzenhybriden” (1866). This memoir was his magnum opus, one of the most important papers in the history of biology, and the foundation of genetic studies. Mendel had confidence in his experimental work and its rational interpretation; but he did observe “that the results I obtained were not easily compatible with our contemporary scientific knowledge and that under [such] circumstances publication of one such isolated experiment was doubly dangerous, dangerous for the experimenter and for the cause he represented. Thus I made every effort to verify the results obtained with Pisum.” He was aware that there would be difficulty in “finding plants suitable for another extended series of experiments and that under unfavorable circumstances years might elapse without my obtaining the desired information,” Nevertheless he tried very hard to confirm, as far as possible, the general validity of the experiments, first with other genera of plants (some experiments extending to four to six generations) and then with animals. After 1866, however, he published only a single short paper on Hieracium hybrids (1869), But the great efforts he devoted to this goal are evident from his letters written from 1866 to 1873 to Naegeli. They amount to scientific reports containing many details of his work, of the problems he encountered, and of the results he obtained. The discussion is extremely sober, objective, and scientific—a patient reaction to Naegeli’s “mistrustful caution” regarding his experiments. The letters also convey Mendel’s personality: his sincere endeavor to reach the truth, his truly scientific spirit, his modesty in the calm defense of his viewpoint, This correspondence remained unknown until 1905, when it was published by Naegeli’s pupil, Carl Correns.
Mendel’s experiments demonstrated that hybrids of Matthiola, Zea, and Mirabilis (like those of Phaseolus reported in the first paper) “behave exactly like those of Pisum.” There still remained the question “whether variable hybrids of other plant species show complete agreement in their behavior with hybrids of Pisum.” In the relevant contemporary literature it was reported that some hybrids (such as Aa) remain constant (A), which contradicted the generalization of Mendel’s results. The genus Hieracium (hawkweed) seemed to Mendel most suitable for solving this question.
Mendel’s Hieracium research project was also connected with some taxonomical questions, since the transitional forms of a highly polymorphic genus like Hieracium were very difficult to classify. The results of his four years’ work, reported at the meeting of the Natural Sciences Society in Brno on 9 June 1869 and published in the society’s Verhandlungen in 1870, were disappointing. He had to admit that in his Hieracium experiments “the exactly opposite phenomenon seems to he exhibited” as compared with Pisum Subsequently, however, he carefully added that the whole matter “is still an open question, which may well be raised but not as yet answered,” These experiments were extremely laborious and delicate because of the minuteness of the flowers and their particular structure; Mendel succeeded in obtaining only six hybrids, and only one to three specimens of each. Another obstacle was to be explained only in 1903: that Hieracium reproduces partly by apogamy, so that in many instances offspring are not formed by cross-pollination and are all alike, as though derived from cuttings.
Mendel discussed these experiments and the problems involved in more detail in his letters to Naegeli. The small number of Hieracium hybrids he obtained did not allow any definite conclusion, and it is surprising thai eventually he found the theoretical explanation even in this case. Notes in Menders handwriting brought to light only recently in the Mendelianum indicate that he insisted on his idea of variable hybrids and, assuming polygene action, he tried to explain that in Salix, as in Hieracium a nulltifactor crossing takes place and that the segregation of their hybrids follows the same principle as in Pisum. According to this assumption, the reported constancy exhibited by the extremely variable Hieracium hybrids would be only apparent.
Mendel centered his efforts on proving that a certain system operates in nature and that its laws could be formulated. It required a great capacity for abstraction and simplification of the extremely complex set of observed phenomena. He had to focus his attention on the main issues; otherwise he would have become lost in the complexities of nature, as had all his predecessors who found many isolated phenomena but did not synthesize them into a coherent system. Many potentially interesting observations had to be left out of consideration in the first phase. Thus, it has been often overlooked that besides the main findings, Mendel noted several phenomena attributed, after 1900, to other scientists: the intermediate forms of heredity, the additive action of his “elements,” like that of genes, and complete linkage. He also described, in principle, the frequencies of “elements” in the population. Later, as his extant fragmentary notes show, he imagined the existence of the interaction of the “elements” and of an action like that later called polygenic. He also suggested that egg cells and pollen cells contain different hereditary units for the development of sex.
After 1871 Mendel conducted hybridization experiments on bees, hoping to prove his theory in the animal kingdom. He kept about fifty bee varieties, which he attempted to cross in order to obtain “a new synthetic race.” He was not successful however, because of the complex problem of the controlled mating of queen bees. In these experiments he also proved the hybrid effect on fertility of bees.
Mendel must have been greatly disappointed that there was no recognition of his scientific work and that even Naegeli missed its essential feature and did not grasp the historical significance of his theory. Naegeli’s attention was focused on other problems, and Mendel’s findings did not lit into his manner of thinking. Thus he raised objections—in fact not relevant—to Mendel’s experiments and rejected his rational conclusions. Mendel was not understood in his time. Only in the Following decades did the discoveries of the material basis of what was later called Mendelian—behavior of the nucleus in cell division, constancy in each species of the number of chromosomes, the longitudinal splitting of chromosomes, the reduction division during the maturation of germ cells, and the restitution of the number of chromosomes in fertilization—prepare the way for understanding the cytological basis of Mendelian inheritance and for its general acceptance.
The absence of response and recognition was one of the reasons that Mendel stopped publishing the results of his later experiments and observations. He did, nonetheless, take satisfaction and pleasure in the application of his theory in the breeding of new varieties of fruit trees and in propagating the idea of hybridization among local gardeners and horticulturists.
Mendel and Darwin . After Mendel’s rediscovery in 1900, Mendelism was often opposed to the Darwinian theory of natural selection, and unfortunately so, for the apparent opposition was based on misunderstanding, In fact the modern theory of descent and heredity has two foundations, one laid by Darwin and the other by Mendel, and both indispensable. It was in 1926, however, that Chetwerikov attempted a synthesis of Darwinian and Mendelian theories, a move completed in 1930–1932 by R. A. Fisher, Sewall Wright, and J. B. S. Haldane. The importance of Mendel for the theory of evolution rests in his demonstration of the mechanism that is the primary source of variability in plant and animal populations, on which natural selection subsequently operates.
When he wrote his paper, Mendel was already acquainted with Darwin’s On the Origin of Species. A copy of its German translation with Mendel’s marginalia, preserved in the Mendelianum in Brno, shows Mendel’s deep interest in this work. Similar marginalia are in the Brno copies of other Darwin books. Mendel’s notes show his readiness to accept the theory of natural selection, He rejected the Darwinian provisional hypothesis of pangenesis, however, as contradicting in principle his own interpretation of the formation and development of hybrids, like Darwin, Mendel was convinced that “it is impossible to draw a sharp line between species and variations.”
On the other hand, Darwin, looking for the causes of variations, seems never to have realized that the clue to this problem was in the hybridization experiments. He never learned about Mendel’s work, although almost the only book in which it was cited. S. O. Focke’s Pflanzenmischlinge … (1881), is known to have passed through his hands.
Mendel was a lonely, unrecognized genius. Yet the rediscovery of his work brought to a ctose an era of speculation on heredity, which then became a subject of scientific analysis. He opened a new path to the study of heredity and revealed a new mechanism operating in the process of evolution. Every generation of biologists has found something new in his fundamental experiments. The science of genetics, which had boih its origins and a powerful impetus in Mendel’s work, has advanced with prodigious speed, linking many branches of biology (cytology in particular) with mathematics physics, and chemistry. This development has led to a deeper understanding of man and nature with far-reaching theoretical implications and practical consequences.
Original Worcks. Mendel published thirteen papers, two of which were on plant -damaging insects, nine on meteorology, and two-the most important on plant hybrids. Most of them, including seven on meteorology and the two on plant hybrids, were published in Verhandlungen des Naturforschenden Vereins in Brünn,1 (1863) to 9 (1871). A list of them was published by J. Křiženecky in Gregor Johann Mendel 1822–1884. Texte und Quellen zu seinem Wirken und Leben (Leipzig, 1965), along with the text of the 1865 paper, revised according to the original MS. Another critical ed. of the 1865 paper was edited, with the text of the 1869 paper on Hieracium hybrids, an introduction, and commentaries, by F. Weiling as Ostwalds Klassiker der Exakten Wissenschaften, n.s. VI (Brunswick, 1970). There are numerous trans, in many languages of Mendel’s magnum opus, “Versuche über Pfanzenhybriden.” Most are listed in M. Jakubiček and J. Kubiček, Bibliographia Mendeliana (Brno, 1965); and M. Jakubiček, Bibliographia Mendeliana. Supplementum 1965–1969 (Brno, 1970). Trans. into English have been published several times since C. T. Druery’s trans, was modified and corrected by W. Bateson in Mendel’s Principles of Heredity. A Defence (Cambridge, 1902); this was republished by J. H. Bennett as Experiments in Plant Hybridization-Gregor Mendel (Edinburgh, 1965), A new English trans, was edited by C. Stern and E. R. Sherwood, The Origins of Genetics. A Mendel Source Book (San Francisco-London, 1966), which includes the paper on Hieracium hybrids and ten of Mendel’s letters to Naegeli. The letters were first published by Carl Correns, “Gregor Mendel’s Briefe an Carl Nägeli 1866–1873,” in Abhandlungen der Königlichern sächsischen Gesellschaft der Wissenschaften, Math-phys. Kl, 29 (1905), 189–265, and their English trans, in Genetics, 35 (1950), 1–29.
Besides the thirteen full-length papers by Mendel over twenty minor publications, mostly book reviews from 1870 1882 signed with the initial “M” or “m” have been identifie.
II. Secondary Literature. The first—and still the best-detailed biography is H. Iltis, Gregor Johann Mendel Leben, Werck und Wirkung (Berlin, 1924), translated into English as Life of Mendel (New York, 1932; 2nd ed., 1966), Additional information was published by O. Richter, Johann Gregor Mendel wie er wirklich war (Brno, 1943). Literature on Mendel’s work and his importance in the history of biology is extremely plentiful. Over 800 titles are listed in Jakubiček and kubiček’s Bibliographia Mendeliana and in its supp. (see above). The most important recent literature includes “Commemoration of the Publication of Gregor Mendel’s Pioneer Experiments in Genetics,” in Proceeding of the American Philosophical Society, 109 , no. 4 (1965) 189–248; F. A. E. Crew, The foundations of Genetics (Oxford, 1966); L. C. Dunn, Genetics in the 20th Century Essays on the Progress of Genetics During Its first 50 Years (New York, 1951); and A Short History of Genetics (New York, 1965); R. A. Fisher, “Has Mendel’s Work Been Rediscovered?” in Annals of Science, 1 (1936), 115–137; A. E. Gaissinovich, Zarozhdenie genetiky (Moscow, 1967); J. Křiženecky, ed., Fundementa genetica (Brno, 1965); M. Sosna, ed., G. Mendel Memorial Symposium (Brno. 1965) (Prague, 1966); R. C. Olby, Origins of Mendelism (London, 1966); H Stubbe, Kurze Geschichte der Genetik bis zur Wiederentdeckung der Vererbungsregeln Gregor Mendels (Jena, 1963); and A. H. Sturtevanrt, A History of Genetics (New York, 1965).
Over 700 original documents relating to Mendel have been preserved in the Mendelianum, established in 1964 by the Moravim Museum in the former Augustinian monastery at Brno. Since 1966 it has published annually the series Folia Mendeliama; no, 6 (1971) contains important papers, including an elucidation of the problem of the triple rediscovery of Mendel’s work, presented at the international Gregor Mendel Coltoquium, held at Brno, 29 June-3 July 1970. The rediscovery of Mendel’s work is further discussed in V. Orel The Secret of Mendel’s Discovery (Tokyo, 1973), in Japanese.
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Johann Gregor Mendel
Johann Gregor Mendel
The Moravian natural scientist and Augustinian abbot Johann Gregor Mendel (1822-1884) laid the foundations of modern genetics with his paper dealing with the hybridization of peas.
Gregor Mendel was born on July 22, 1822, at Hynčice, Czechoslovakia (then Heinzendorf, Austrian Silesia). His ancestors were farmers, and his father still had to work three days a week as a serf. Mendel displayed a great love for nature all his life.
Years of Preparation and Education
In 1831 Mendel was sent to the Piarist school in Lipník (Leipnik) and at the age of 12 to the grammar school in Opava (Troppau). In 1840 he enrolled at the Institute of Philosophy in Olomouc (Olmütz).
Mendel was admitted to the Augustinian order in Brno (Brünn) in 1843. The Augustinians taught philosophy, foreign languages, mathematics, and natural sciences at secondary schools and universities. Abbot Napp, the head of the monastery, devoted all his energy to the economic development of the monastery and to the scientific education of the members of the order. Surrounded by an atmosphere of dynamic activity, Mendel found optimum conditions for his studies and later for his research work. Along with his theological studies Mendel took courses in agriculture, pomiculture, and vine growing at the Institute of Philosophy in Brno. In 1847 he was ordained a priest and served for a short time as vicar at the Old Brno Monastery.
In 1849 Mendel became a teacher of mathematics and Greek at the grammar school in Znojmo (Znaim). After a year the headmaster recommended him for the university examination. Together with his application for admission to the examination Mendel enclosed his autobiography, which is the only authentic preserved document. Mendel failed the examination, probably because he lacked a complete university education. Only his written test on meteorology satisfied his examiner, and, on the latter's recommendation, Abbot Napp sent Mendel to study natural sciences at the University of Vienna (1851-1853). He heard F. Unger lecture on plant anatomy and physiology, the use of the microscope, and the practical organization of experiments. Unger was known for his views on evolution and had investigated the problem of the origin of plant variability by means of transplanting experiments. Mendel later performed these experiments also. It is now assumed that Unger's views deeply influenced Mendel in the formation of his ideas before he performed his experiments with edible peas (Pisum).
On his return to Brno in 1854 Mendel was appointed a teacher of physics and natural history in the Technical School. In 1856 he prepared himself for the university examination again, but he became seriously ill and did not take it. By this time, however, Mendel was fully occupied with his hybridizing experiments with Pisum. He remained a teacher till 1868, when he was elected abbot of the monastery.
Mendel started his extensive program of hybridizing experiments in 1854. He focused his energy on the problem of the origin of plant variability. For two years he tested the purity of selected varieties of Pisum and then began experimenting with artificial fertilization. A new reconstruction of Mendel's experimental data illustrates that he must have tested about 28,000 Pisum plants during the years 1856-1863.
Mendel summarized the experimental results in a paper, "Experiments on Plant Hybrids," which he read at two meetings of the Natural Science Society in Brno in 1865; the paper was published in the proceedings of the society's journal. Though prominent natural scientists were present at the meeting, no one understood Mendel's ideas or the significance of his work. The proceedings were distributed to 134 scientific institutions in Europe and the United States, but the published paper failed to arouse interest.
Mendel's original idea, that heredity is particulate, was contrary to the theory of "blending heredity" that was generally accepted at that time. In the plants that Mendel tested (and in biparental-reproducing organisms generally), the hereditary particles (called elements by Mendel) from each parent are members of pairs. In forming the reproductive cells, the pair members segregate in different pollen or sperm nuclei and in different eggs or ovules to transmit the hereditary determinants. From one parent comes one particle determining, for example, the round shape of the seed (A), and from the other parent that representing the wrinkled shape (a). Mendel called the trait passing entirely unchanged into hybrid (derived from unlike parents) association "dominant," and the trait becoming latent in hybrids "recessive." The particles meet (recombine) in the offspring (Aa) but do not influence each other.
Suppose the pair members of these hybrid offspring now segregate in forming reproductive cells, producing two types of sperm or egg, namely Aor a, and that these particles meet at random in fertilization. The resulting combination series of relevant particles is: ¼ AA, ¼ Aa, ¼ aA, ¼ aa, or AA ¼2 Aa ¼ aa. That is, there are four genetic types of offspring from the hybrids, each type represented by 25 percent of the total. In this way, in the hybrid progeny the parental forms appear again; after 1900 this segregation of the hereditary units (in 1909 termed genes) was called Mendel's law of segregation.
Mendel found that hereditary particles belonging to different trait pairs, for example, A, a for the seed shape and B, b for the seed coloration, formed the combination series in recombining without influencing each other. The combination series could be predicted by combining the simple series AA □ 2Aa □ aa; BB □ 2Bb □ bb, resulting in the combination series AABB □ AAbb □ aaBB □ aabb □ 2AABb □ 2aaBb □ 2AaBB □ 2Aabb □ 4AaBb. In his paper Mendel actually illustrated such a recombination in crossing peas differing in two and three trait pairs. Expected particle recombinations were realized in actual counts of the offspring. The recombination of the hereditary particles was called Mendel's law of independent assortment.
Mendel gave the impulse for his experiments in the first sentence of his paper: "Artificial fertilization undertaken on ornamental plants to obtain new color variants initiated the experiments to be discussed." His task was to find "the generally applicable law of the formation and development of hybrids as a way of finally reaching the solution to a question whose significance for the evolutionary history of organic forms must not be underestimated." In his paper he expressed the opinion that "the distinguishing traits of two plants can, after all, becaused only by differences in the composition among grouping of the elements existing in dynamic interaction in the primordial cells." He assumed the general validity of his theory because, according to him, "unity in the plan of development of organic life is beyond doubt."
Being interested in the development of hybrid forms, Mendel also explained that the population descending from hybrids tends to revert to the pure parental forms, resulting in diminishing the hybrid's form. Thus, as a consequence of Mendelian segregation, Mendel also laid the basis for the interpretation of the effect of inbreeding.
Mendel continued his hybridizing experiments, crossing various forms of 22 other genera of plants, to prove the general validity of his theory in the plant kingdom. He also cultivated wild plants in the garden with the aim of investigating Lamarck's views concerning the influence of environment upon plant variability; he could not agree with Lamarck. He was convinced, like Darwin, that it was impossible to draw a hard-and-fast line between species and varieties, and in the conclusion of his Pisum paper he expressed the conviction that the variability of cultivated plants could be explained by his theory.
After 1871 Mendel also tried to carry out hybridizing experiments with bees. He bred about 50 bee races which he tried crossing to obtain new cultural breeds. His crossing experiments could not be successful, however, because of the complex problem of the controlled mating of queens. For this reason Mendel focused his activity on research of the technological aspects of apiculture, such as the hibernation of bees.
As a member of the Natural Science Section of the Agricultural Society in Brno and as a respected meteorologist, Mendel summarized the results of meteorological observations in 1856 and published them in six reports (1862-1869). He also published three papers on extraordinary storms (1870-1872). He was a member of the Central Board of the Agricultural Society from 1870, and he supported the first weather forecasts for farmers in 1878. In 1861 he helped found the Natural Science Society of Brno.
Taxation and the Monastery
After Mendel was elected abbot of the monastery in 1868, he had little time for his experimental activities, although they never came to a total stop. In 1874 the government proclaimed a new law relating to the contribution of the cloisters to the religious fund. Mendel refused to pay the high assessed taxes and thus, from the end of 1875, got himself into trouble with the provincial government and with the Ministry of Education in Vienna. The result of this conflict was the lasting sequestration of the landed monasterial property. In an attempt to win Mendel over and stop his opposition to the taxation law, the government appointed him to the Board of Directors of the Moravian Mortgage Bank. In 1876 he became the vice-governor of the bank and in 1881 the governor. Nevertheless, Mendel never agreed to the taxation law.
The long struggle over taxation had a serious effect on Mendel's health. He died on Jan. 6, 1884, without any public recognition of his outstanding scientific achievements.
Contributions to Genetics
Mendel's paper of 1865 went unnoticed except for an occasional reference in scientific literature. In 1900 it was rediscovered by scientists, when his theory was generalized as Mendel's laws of heredity. That date also marked the beginning of the science of heredity, which in 1906 was named genetics. Not even after 1900 was Mendel's theory acknowledged as being generally valid, and the Darwinian selection theory was often considered to oppose the Mendelian theory.
Later, it was demonstrated that Mendel had also observed such phenomena as intermediate inheritance, complete linkage, additive gene action, and gene interaction, and that he himself appreciated the Darwinian selection theory and refused to accept the hypothesis of pangenesis. The synthesis of the Darwinian and Mendelian theories was first proved by S. S. Tchetverikoff in 1926 and finally by R. A. Fisher in 1930, Sewall Wright in 1931, and J. B. S. Haldane in 1932.
Since that time Mendel's work has been reappraised. His hypothesis of hereditary particles turned out to be quite general and provided the elementary principle of heredity in all forms of life from viruses to man. From this viewpoint his laws of heredity appear to be only the subordinate principles of Mendel's main discovery, which furnishes proof of the existence of genes as determining the whole character of each organism.
The best biography of Mendel is Hugo Iltis, Life of Mendel (1924; trans. 1932). Mendel's papers on hybridization are published in English in J. H. Bennett, ed., Experiments in Plant Hybridization (1965). Curt Stern and Eva R. Sherwood, eds., The Origin of Genetics: A Mendel Source Book (1966), is a translation of Mendel's papers. It also contains 11 letters that Mendel wrote to Karl Nägeli, which give basic information on Mendel's experiments with different plant species. Information on Mendel and on the early development of genetics is published in the series "Folia Mendeliana Musei Moraviae." The historical development of Mendelism is treated in Robert C. Olby, Origins of Mendelism (1966). The historical development of modern genetics is outlined in L. C. Dunn, ed., Genetics in the 20th Century: Essays on the Progress of Genetics during its First Fifty Years (1951). □
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Although some leading scientists in the late nineteenth century considered religion to be an impediment to progress in science, the life of the monk Gregor Mendel serves as an important counter-example. The fact that a monk initiated one of the greatest advances in biology demonstrates the poverty of the notion of there being a perpetual war between science and religion. In Mendel's case, rather than hindering science, religious institutions promoted scientific knowledge, experimentation, and progress.
Early life and influences
On July 22, 1822, Mendel was born in the village of Heinzendorf (now Hyncice) in northern Moravia (in the present-day Czech Republic), a part of the Austrian Empire that was culturally German. Mendel was originally named Johann, but took the name Gregor in 1843 upon entering the Augustinian order of the Roman Catholic Church. His father was a peasant farmer with a keen interest in improving agriculture. A priest in his community, Father Schreiber, used his knowledge of fruit trees to help his parishioners practically. He studied the latest techniques for improving fruit yields, practiced artificial fertilization, and distributed grafts to community members, including the Mendel family.
Mendel's intellectual abilities were recognized early in his life, and his family sent him to school first in Leipnik (Lipnik) and then to Gymnasium in Troppau (Opava). After graduating from Gymnasium, he attended a two-year course of study at the Philosophical Institute in Olmütz (Olomouc), which was interrupted for a year due to illness. He graduated from the Philosophical Institute in 1843, having studied religion, philosophy, ethics, mathematics, and physics, in order to prepare for further studies in natural science at a university. While in Olmütz, Mendel had grave financial difficulties because his father was incapacitated from work as a result of an injury, and Mendel had difficulty finding tutoring jobs. His poverty probably brought on his illness and caused him continual travail.
Upon the recommendation of one of his teachers, Mendel entered the Augustinian monastery in Brno in 1843. He had begun contemplating entering the Catholic priesthood about three years earlier, but it is not known how seriously or deeply he felt a religious calling. Mendel's own account of entering the monastery emphasized his need to escape from poverty rather than an inner religious motivation. Mendel also knew that the monastery in Brno would be a hospitable environment for pursuing studies in the natural sciences.
Indeed, the Brno monastery, under the leadership of Abbott F. C. Napp, attracted a number of talented men interested in science. Napp himself studied horticulture and wrote a manual about improving plant varieties. He set up a nursery in the monastery where new plant varieties could be developed. Thus, the monastery provided a very propitious environment for the young Mendel, who was encouraged to teach science in nearby schools. The monastery also allowed him to attend the University of Vienna from 1851 to 1853 to study natural science so he could pass the exam to qualify him to teach in a Gymnasium. Mendel never passed this exam, however.
Although the monastery was a stimulating place for the study of natural science, the religious training and exercise in the Brno monastery seems to have been perfunctory. The bishop of Brno criticized Napp and the monastery for devoting so much attention to science, while neglecting the spiritual dimension of monastic life. Shortly after Mendel arrived, a monk there was stripped of his authority to teach because he was accused of introducing Hegelian and pantheistic doctrines into his scientific writings. Napp tried to defend this monk, but to no avail. Mendel never challenged the Catholic Church or its teachings, but his energies were clearly devoted more to scientific pursuits than to religious ones.
Experiments with peas
From 1854 to 1863 Mendel carried out his pea experiments, which later became famous for laying the groundwork for the modern science of genetics. Because Mendel relied on statistics to analyze the results of his work, his background in physics and mathematics provided him insight in developing these experiments. To perform his experiments, Mendel selected twenty-two varieties of pea plants that bred true (i.e., each was a pure variety that, when crossed with its own variety, always had offspring with the same traits as the parents). Each variety was crossed with another with which it differed in an obvious way, such as seed color, pod shape, position of flowers, or length of stem. For example, he crossed one pea variety that had round seeds with another variety that had angular seeds. In the first generation hybrids Mendel observed that all the offspring had the trait of only one of the parent varieties. The hybrid, between peas with round seeds and those with angular seeds produced all round seeds in the first generation. Mendel called the trait that appeared in the first generation the dominant trait. This demonstrated that hereditary characters did not blend, as many scientists of the time supposed they did, but rather they were discrete factors.
Mendel continued his experiment by self-fertilizing the first generation hybrids. He discovered that both original traits reemerged in a ratio approximating three (for the dominant trait) to one (for the recessive trait). In the case of the round and angular seeds, Mendel's actual data showed 5474 round seeds and 1850 angular seeds in the second generation. Mendel concluded from his experiments that each plant had two hereditary characters. Each parent would pass only one of its characters on to its offspring. These characters segregate randomly, leading to the ratios Mendel found. This explanation is known as Mendel's Law of Independent Assortment.
Mendel published the results of his pea experiments in 1866 in the Proceedings of the Natural Science Society of Brno, but even though some botanists cited his work subsequently, none recognized the full significance of his experiments before 1900. Mendel even corresponded with Karl Nägeli (1817–1891), a prominent botanist, but despite his interest in Mendel's work, Nägeli never realized how important it was. When Mendel died on January 6, 1884, he was almost unknown, though he did express confidence late in his life that his work would be recognized in the future.
Historians still debate the significance of biological evolution for Mendel's work. Charles Darwin (1809–1882) had not yet published his theory of evolution when Mendel began his experiments, but Mendel was already conversant with Charles Lyell's (1797–1875) uniformitarian geology, which had been a formative influence on Darwin. Mendel also studied botany at the University of Vienna under Franz Unger (1800–1870), who embraced a pre-Darwinian evolutionary theory. Mendel was thus fully aware of debates about biological variation and speciation, and he may have hoped that his hybridization experiments would shed light on the evolutionary process.
The rediscovery of Mendel's work in 1900 by three different scientists—Hugo de Vries, Carl Correns, and Erich von Tschermak—occurred in the context of debates over evolution. Biological evolution was widely accepted by European scientists by 1900, but scientists did not have a satisfactory explanation as to how variation occurs or what the mechanisms of heredity are. Mendelian genetics provided new insights about heredity, but also posed new problems for evolutionary theory. De Vries argued that Mendelian genetics provided support for discontinuous variation rather than Darwinian gradualism. On the basis of his misinterpretation of primrose hybridization experiments he thought that mutations—the sudden emergence of new characters—drove the evolutionary process. These new characters were then passed on in Mendelian fashion. Other scientists opposed de Vries's mutation theory and continued arguing for gradual variations. The dispute over gradualism versus discontinuous variation was only settled in the 1930s and 1940s with the integration, known as the neo-Darwinian synthesis, of Darwinian natural selection theory with Mendelian genetics.
Implications for religion
The religious implications of Mendel's theory were minimal, so no significant religious opposition to Mendelian genetics arose. However, in the early twentieth century, many eugenics proponents began using Mendelian genetics to promote various programs to control human heredity, including sterilization, birth control, incarceration, and regulation of marriage certificates. The Roman Catholic Church and some conservative Protestants opposed eugenics, but they did not criticize Mendelian genetics. Rather they rejected eugenics as a misuse of genetics.
Probably the most significant connection between Mendelian genetics and religion was the use of Mendelian genetics by creationists. Many creationists hailed Mendel's theory of heredity as a proof for biological stasis. The variations that Mendel (and de Vries) observed only involved the reshuffling of hereditary characters (genes) that were already present, not the introduction of new traits. They rejected the neo-Darwinian synthesis, which argued that micro-mutations could accumulate to produce speciation.
Mendel's life and the reception of his ideas demonstrates the way that religious communities and individuals in nineteenth and early twentieth-century Europe often nurtured scientific discovery. They were not only open to new scientific ideas, but in some cases actively cultivated them.
See also Creation Science; Eugenics; Evolution; Genetics
bowler, peter j. the mendelian revolution: the emergence of hereditarian concepts in modern science and society. baltimore, md.: johns hopkins university press, 1989.
callender, l. a. "gregor mendel: an opponent of descent with modification." history of science 26 (1988): 41–75.
iltis, hugo. life of mendel. new york: hafner, 1966.
olby, robert. origins of mendelism, 2nd edition. chicago: university of chicago press, 1985.
orel, vitezslav. gregor mendel: the first geneticist, trans. stephen finn. oxford: oxford university press, 1996.
orel, vitezslav. mendel, trans. stephen finn. oxford: oxford university press, 1984.
stern, curt, and, sherwood, eva r. the origin of genetics: a mendel source book. san francisco: w. h. freeman, 1966.
wallace, bruce. the search for the gene. ithaca, n.y.: cornell university press, 1992.
richard c. weikart
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Natural Scientist 1822-1884
Gregor Mendel laid the foundation for the modern understanding of inheritance with his experiments on transmission of traits in garden peas. The ideas he developed are still in use today, and his essential insights into the physical nature of inheritance led directly to the understanding of the gene as a physical entity within the cell.
Education and Training
Mendel was born into a farming family in Heinzendorf, Austria (now Hyncice, Czech Republic). He attended university in Olmutz, but financial difficulties soon persuaded him to enter the Augustinian monastery in Brno, where he received both theological and agricultural training. Mendel remained affiliated with the monastery for the rest of his life. He served briefly as a parish chaplain in the region, and for many years served as a popular and successful teacher at the technical school in Brno. His training in agricultural experimentation, obtained at the University of Vienna, beginning in 1851, prepared him for the experiments that he began in 1856 on peas.
Experiments on Peas
Mendel's experiments were designed to investigate the most widely accepted model of inheritance, blending, which held that the traits of an offspring would be a blend of the parental traits. For example, the theory of blending predicts that a tall and short parent would give rise to a medium-height offspring. Mendel's results showed that for many simple traits, at least, this model was wrong. Instead, the offspring displayed traits in exactly the same form as they appeared in one or the other of the parents.
Mendel chose to study a small group of traits that occur in either of two forms, such as round versus wrinkled pea shape. He began by developing "pure-breeding" lines of each form. In a pure-breeding line, crossing two members gives only offspring that are identical to the parents for that trait. Mendel then crossed pure-breeding parents who had different forms of a trait. For example, he crossed a pea plant that produced only round peas with one that produced only wrinkled peas. All the offspring from this cross-developed only round peas; no wrinkled peas were found. When these off-spring were crossed among themselves, however, both round and wrinkled were observed, in a numerical ratio of three round-pea plants for every one wrinkled-pea plant.
Mendel explained these results by proposing that each visible trait is governed by the presence of two "factors," which may be the same or different in any individual. One of these factors is "dominant," while the other is "recessive." In the above example, the round-producing factor is dominant, and the wrinkled-producing factor is recessive. If two recessive factors are present, the organism will display the recessive trait. If the organism has two dominant factors, or one dominant and one recessive, the dominant trait will be displayed.
Laws of Inheritance
To explain the numerical relationships he obtained, Mendel developed the Law of Segregation. He proposed that during the process of egg and sperm formation, the two factors separate, or segregate, so that each egg or sperm contain only one factor. For a parent containing one of each type of factor, this means that half the sperm (or eggs) will contain the dominant factor, and half the recessive factor. During fertilization, these randomly pair up, so that some offspring will have two dominants, some two recessives, and some one of each. Simple algebra shows that the ratio of offspring in such a cross will be 3:1, just as Mendel found.
To show how this works, let 0.5D be the proportion of dominant factors and 0.5r be the proportion of recessive factors. Multiplying (0.5D + 0.5r) times itself gives the offspring ratios, 0.25D2 + 0.5Dr + 0.25r2. In this expression, 0.25D2 indicates that one-quarter of the offspring will have both dominant factors, 0.5Dr means half will have one of each type, and 0.25r2means one-quarter will have both recessive factors. Since both the D2 and Dr organisms will show the dominant trait, the ratio of dominant to recessive traits in the offspring will be 0.75:0.25, or 3:1.
Mendel went on to study crosses between peas with multiple sets of traits, such as round seeds plus tall plants crossed with wrinkled seeds plus short plants. He found that the factors for each trait acted independently, so that the offspring of these crosses showed all possible combinations of traits. From the results of these experiments, he formulated his second principle, known as the Law of Independent Assortment, which states that the members of factor pairs assort (segregate) independently of each other during sperm and egg formation, and combine again randomly.
Mendel's Scientific Legacy
While neither Mendel nor anyone else in his day knew anything about chromosomes or genes, the laws of inheritance he discovered predicted exactly how genes behave on chromosomes during the reproductive process. Indeed, the factors he discovered are genes, which come in pairs and segregate on separate chromosomes during sperm and egg production, just as he suggested. Gene pairs located on different sets of chromosomes will assort independently during the process. While most genes do not exhibit simple dominance-recessiveness relations, and most traits are governed by more than one gene, it is to Mendel's credit that he began by trying to understand simple systems in order to develop generalizable laws.
Mendel published the results of his experiments, "Versuche über Pflanzenhybriden" ("Attempts at Plant Hybridization") in 1866. He did little scientific work after he became abbot of the monastery two years later. His work was ignored by the larger scientific community, in part because it was not published in a widely read journal, and in part because it tackled a problem, the physical basis of heredity, that few other scientists were thinking deeply about at that time.
That changed shortly afterward, when microscopic studies of cells revealed that chromosomes divided when cells divided, provoking speculation that they might be involved in inheritance. Mendel's studies were redis-covered in 1900, sixteen years after his death, by three biologists studying similar phenomena. The importance of his theory of inheritance was immediately recognized and widely accepted, and became the starting point for further investigations of the nature of inheritance that were carried out by Thomas Hunt Morgan, Alfred Sturtevant, and other twentieth-century geneticists. Mendelism, as the theory was called, was merged with Darwinism in the 1930s to form the "New Synthesis," which explained evolutionary theory in modern genetic terms.
see also Chromosomal Theory of Inheritance, History; Inheritance Patterns; Mendelian Genetics; Nature of the Gene, History; Probability.
Henig, Robin Marantz. The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Genetics. Boston: Houghton Mifflin, 2000.
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Gregor Mendel, the father of genetics , was born on July 22, 1822, in Heinzendorf, Austria. He died on January 6, 1884. He was the first person to propose the idea of genes and to apply mathematics to genetics. Although his work was initially ignored by scientists, it proved to be the basis of modern genetics.
Mendel's interest in natural science developed early. He studied at the Philosophical Institute at Olmütz for two years. In 1843 he entered the monastery in Brünn, becoming a priest in 1847. Then he went to the University of Vienna, where he studied science from 1851 to 1853. In 1854 Mendel returned to Brünn and taught natural science in the technical high school there until 1868.
Seeking to learn how plants inherit different traits, Mendel began his experiments with garden peas in the small monastery garden in 1856. From 1856 to 1863 Mendel grew almost 30,000 specimens of garden peas. These plants had sharply contrasting characteristics (tall versus short, smooth seed versus wrinkled seed, and so on). He studied seven pairs of alternative characteristics, making hundreds of crosses by artificial pollination .
Mendel kept very careful records of the plants that he crossed and the resulting offspring. He noted that the occurrence of the alternative characteristics in the crossed varieties of plants followed simple statistical, mathematical laws. For example, Mendel crossed species that produced tall plants with those that produced short plants. Then, he counted the numbers of tall and short plants that appeared in subsequent generations. In the first generation, all of the plant offspring were tall. The next generation had some tall plants and some short plants in proportions of three (tall) to one (short). This showed that no blending of traits occurred (no medium-height plants). Further, if allowed to self-pollinate (fertilize themselves), the short plants always had short offspring. Mendel proposed that each plant received one character from each of its parents. Tallness was dominant and shortness was recessive , appearing only in later generations. Mendel also showed that when several pairs of alternative characteristics are observed, the several pairs enter into all possible combinations in the subsequent generations. In the pea plants he studied, he observed that the seven alternative characteristics recombined at random. He worked out the statistics of these combinations and confirmed his predictions by experiment.
Mendel developed three theories to explain the results of his experiments. His first law is the principle of segregation. It states that during the formation of sex cells (egg and sperm), paired factors are segregated (separated). Therefore, a sperm or egg may contain either a tallness factor or a shortness factor, but cannot contain both. The second law, the principle of independent assortment, states that characteristics are inherited independently of one another. Thus, the fact that the tallness factor is inherited does not determine which alternative of any other pair of characteristics is inherited. The law of dominance, which is the third theory, states that each inherited characteristic is determined by the interaction of two hereditary factors (now called genes). One factor always dominates the other (for example, tallness always dominates shortness). Mendel was the first to understand that trait units are physical particles passed from one generation to another by reproduction. This is remarkable, since at that time knowledge about cell structure was limited.
It is now known that Mendel's second principle applies only to genes that are transmitted in different linkage groups. Also, the appearance (or dominance) in hybrid offspring of one of the alternative characteristics has now been proven not to be true for all alternative characteristics. However, these limitations do not affect the fundamental truth of Mendel's findings. Mendel's system, called Mendelism, is one of the basic principles of biology.
Mendel presented his findings to his fellow scientists in 1865, but they failed to see the revolutionary nature of his work. When he was promoted to head of the monastery in Brünn in 1868, Mendel turned his focus away from science to concentrate on his duties at the monastery. He did, however, continue work in botany, bee culture, and the weather until his death.
Mendel was widely respected and loved, but went unrecognized as the great scientific thinker that he was. Fame and due credit came to Mendel only after his death. In 1900, three other European scientists independently obtained results similar to Mendel's. The researchers realized that he had already published both the experimental data laying out his results and a general theory explaining them nearly thirty-five years earlier.
see also Biological Evolution; Genes; Genetics.
The New Encyclopedia Britannica, 15th ed. Chicago: Encyclopedia Britannica, Inc., 1993.
The World Book Encyclopedia. Chicago: World Book, Inc., 1995.
The Year 2000 Grolier Multimedia Encyclopedia. Danbury, CT: Grolier Interactive Inc., 1999.
MendelWeb. University of Washington, Seattle. <http://www.netspace.org/MendelWeb/>.
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Mendel, Gregor Johann
Gregor Johann Mendel (grā´gôr yō´hän mĕn´dəl), 1822–84, Austrian monk noted for his experimental work on heredity. He entered the Augustinian monastery in Brno in 1843, taught at a local secondary school, and carried out independent scientific investigations on garden peas and other plants until his election as prelate in 1868. Failing eyesight and his duties as prelate somewhat curtailed his researches; although he anticipated Oscar Hertwig's discovery that fertilization of an egg involved only one male sex cell, these findings went unpublished.
Mendel was the first to fashion, by means of a controlled pollination technique and careful statistical analysis of his results, a clear, analytic picture of heredity. His account of the experiments and his conclusions, published in 1866 (tr. Experiments in Plant Hybridization, 1926), were ignored during his lifetime. Rediscovered by three separate investigators (Correns, de Vries, and Tschermak) in 1900, Mendel's conclusions have become the basic tenets of genetics and a notable influence in plant and animal breeding.
Mendelism is the system of heredity formulated from Mendel's conclusions. Briefly summarized, as we understand it today by means of the science of genetics, the Mendelian system states that an inherited characteristic is determined by the combination of a pair of hereditary units, or genes, one from each of the parental reproductive cells, or gametes. In the body cells each pair of genes determines a particular hereditary characteristic (e.g., in the pea plant, a pair determining tallness or dwarfness).
Mendel's First Law
The law of segregation (Mendel's first law) states that in the process of the formation of the gametes (see meiosis) the pairs separate, one going to each gamete, and that each gene remains completely uninfluenced by the other. Mendel found that when a pure strain of peas bearing one form of a gene (that is, a strain in which both members of the gene pair being studied are the same), inbred for many generations, was crossed with a pure strain carrying an alternative form of the gene, one of these forms consistently prevailed over the other in determining the visible characteristics of the offspring; he therefore termed the two forms dominant and recessive, and called the phenomenon itself the law of dominance. Given A as the dominant factor and a as the recessive, the offspring of the purebred strains having genes of the form AA and aa are hybrids, individuals each being Aa. When the hybrids are crossed, the offspring exhibit the characteristic in question in a ratio of three dominant to one recessive; i.e., the four possible combinations of the genes in Aa and Aa are AA,aA,Aa, and aa. By the same rule, when a hybrid is crossed with a purebred recessive (Aa with aa) the ratio is one to one. Breeders often use these ratios to trace the hybrid or purebred nature of the parent stock.
Mendel's Second Law
The law of independent assortment (Mendel's second law) states that characteristics are inherited independently of each other; e.g., the dominant trait of yellow seed color in pea plants can appear in combination with either the dominant trait of plant tallness or the recessive trait of dwarfness. This law has been modified by the discovery of linkage in genetics.
See biography of Mendel by V. Ore (1984); see also R. C. Olby, The Origins of Mendelism (2d ed. 1985).
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Austrian Natural Scientist 1822-1884
Gregor Mendel elucidated the theory of particulate inheritance, which forms the basis of the current understanding of genes as the hereditary material. Born in Heinzendorf, Austria, in 1822, Johann Gregor Mendel was the fourth of five children in a family of farmers. He attended the primary school in a neighboring village, which taught elementary subjects as well as the natural sciences. Mendel showed superior abilities, and in 1833, at the advice of his teacher, his parents sent him to the secondary school in Leipnik, then to the gymnasium in Troppau. There he attempted to support himself by private tutoring, but his lack of the necessary financial support made the years that Mendel spent in school extremely stressful for him. His younger sister gave him part of her dowry and, in 1840, he enrolled in the University of Olmütz, where he studied physics, philosophy, and mathematics. In 1843 he was admitted into the Augustinian monastery in Brno, where he stayed for almost two decades. Originally, Mendel was not interested in religious life, but joining the monastery freed him from the financial concerns that plagued him and allowed him to pursue his interests in the natural sciences.
Under the leadership of its abbot, F. C. Napp (1792-1867), the monastery in Brno integrated higher learning and agriculture by arranging for monks to teach natural sciences at the Philosophical Institute. Napp encouraged Matthew Klácel to conduct investigations of variation and heredity on the garden's plants. Klácel, a philosopher by training, integrated natural history and Hegelian philosophy to formulate a theory of gradual development. This work eventually led to his dismissal, and he immigrated to the United States. Mendel was put in charge of the garden after Klácel's departure.
From 1844 through 1848 Mendel took theological training as well as agricultural courses at the Philosophical Institute, where he learned about artificial pollination as a method for plant improvement. After he finished his theological studies, Mendel served a brief and unsuccessful stint as parish chaplain before he was sent to a grammar school in southern Moravia as a substitute teacher. His success as a teacher qualified him for the university examination for teachers of natural sciences, which he failed because of his lack of formal education in zoology and geology. To prepare himself to retake the test, he went to the University of Vienna, where he enrolled in courses in various natural sciences and was introduced to botanical experimentation. After completing his university training he returned to Brno and was appointed substitute teacher of physics and natural history at the Brno technical school.
Mendel was an excellent teacher, and he often taught large classes. In 1856 he began botanical experiments with peas (Pisum ), using artificial pollination to create hybrids . Hoping to continue his education, he once again took the university examination, but failed and suffered an emotional and physical breakdown. His second failure spelled the end of his career as a student, but he remained a substitute teacher until 1868, when he was elected abbot of the monastery. Mendel stayed in Brno, serving the monastery, performing botanical experiments, and collecting meteorological information until he died of kidney failure in 1884. At the time of his death, he was well known for his liberal views and his conflict with secular authorities over the setting aside of monastery land; at this time, only the local fruit growers knew him for his botanical research.
Experiments on Inheritance
While his contemporaries knew little of his scientific work, Mendel's historical significance lies almost entirely in his experimental work with the hybridization of plants and his theory of inheritance. Beginning in 1856 and continuing through 1863, Mendel cultivated nearly thirty thousand plants and recorded their physical characteristics. Beginning with a hypothesis about the relationship between characteristics in parents and off-spring, Mendel formulated an experimental program.
Mendel believed that heredity was particulate, that attributes were passed from parents to offspring as complete characters. His notions of heredity were contrary to the belief in blending inheritance, which was generally accepted at the time and explained the attributes of an organism as a blended combination of its parents' characters. Instead of viewing an organism's individual characteristics as composites of its predecessors, Mendel asserted that organisms inherited entire characters from either one or the other parent. To test his theory, he chose seven plant and seed characteristics, such as the shape of the seed or the color of the flower, and traced the inheritance of the characters through several generations of pea plants.
As he crossed thousands of pea plants and recorded the seven characteristics, Mendel found that certain traits were passed from parent to offspring in a lawlike fashion. Just as he had hypothesized, certain traits regularly appeared when he crossed plants with different combinations of characteristics. He used the term "dominant" in reference to those traits that were passed from the parent to the offspring and the term "recessive" in reference to those traits that were exhibited in at least one of the parents, but not in its offspring. Mendel denoted plants with dominant traits by recording two capital letters, such as AA, and those that expressed recessive traits with lower case letters, like aa. In the first generation of offspring from crosses of AA with aa, dominant traits always appeared and recessive traits never appeared.
Mendel's system of denoting dominant and recessive traits with two letters allowed him to trace dominant and recessive characters through successive generations. The crossing of AA with aa would result in the production of individuals with traits represented by Aa, with the dominant trait always appearing, but not the recessive trait. By crossing two Aa individuals, Mendel found that the dominant trait appeared three times for every one time that the recessive trait appeared. Mendel explained that the crossing of two Aa individuals resulted in the production of the following combinations:
AA Aa Aa aa
Because the dominant trait always decided the characteristic, any organism with at least one A would express the dominant trait. Recessive characteristics would appear only in those individuals with aa.
Mendel's 1866 "Versuche über Pflanzenhybriden" (Attempts at Plant Hybridization) presented his entire theory of inheritance and has become one of the most significant papers in the history of biology. He explained that his results "were not easily compatible with contemporary scientific knowledge" and, as such, "publication of one such isolated experiment was doubly dangerous, dangerous for the experimenter and for the cause he represented." In an attempt to bolster his case, Mendel experimented on several other plants and then with animals. However, after 1866 he published only one more short article on the subject.
Rediscovery of Mendel's Work
Mendel's painstaking experimental work on plant hybridization and heredity sat virtually unnoticed for thirty-five years before three natural scientists simultaneously rediscovered it at the turn of the twentieth century. His 1865 paper, presented at the Natural Sciences Society of Brno and published in the Society's Verhandlungen in 1866, received little notice from his contemporaries. However, in 1900 Carl Correns, Erich von Tschermak, and Hugo DeVries, each working independently, found Mendel's paper while they were each in the process of completing similar experiments. In the hands of a new generation of natural scientists, Mendel's work was immediately and widely accepted, and he was touted as the epitome of a scientist.
Mendelism, as his work was called, was often posited in opposition with the Darwinian theory of natural selection. Many early twentieth-century Mendelians and Darwinians believed that the two theories were incompatible with one another, in part because of Darwin's reliance on the theory of pangenesis and because contemporary biologists, who also viewed Darwinism in conflict with DeVries's mutationism, associated Mendel's work with mutationism.
Despite the debates over the relationship between Mendelism and Darwinism, Mendel's work immediately received widespread support, and it served as the basis for work in genetics as well as plant and animal breeding. Beginning around 1900, Mendelism also provided a substantial boost to the growing science of eugenics, the genetic improvement of humans by encouraging "high-quality" individuals to have children while discouraging "low-quality" people from reproducing. By scientifically explaining inheritance, Mendelism bolstered the eugenicists' claim that "good begets good and bad begets bad." Later geneticists distanced themselves from eugenics by arguing that, while Mendelism easily explained simple traits like eye color or blood type, it did not apply to more complicated traits like intelligence or industriousness.
Beginning in the late 1930s, yet another generation of natural scientists reinterpreted Mendelism and Darwinism, and they concluded that they were mutually reinforcing scientific theories. R. A. Fisher, Sewall Wright, J. B. S. Haldane, and other so-called synthesis biologists argued that Mendelism provided the explanation for one facet of evolution, inheritance, while Darwinism explained another, selection. Viewed in this light, Mendel's work complemented Darwin's theory of natural selection, and the two have served as the principal basis for modern biological thought since the mid-twentieth century.
see also Chromosomes; Darwin, Charles; Genetic Mechanisms and Development.
Mark A. Largent
DeVries, Hugo, Carl Correns, and Armin von Tschermak. The Birth of Genetics. Brooklyn, NY: Brooklyn Botanic Garden, 1950.
Iltis, Hugo, Eden Paul, and Cedar Paul. Life of Mendel. London: G. Allen & Unwin, 1932.
Kruta, V., and V. Orel. "Johann Gregor Mendel." In Dictionary of Scientific Biography, Vol. 9. New York: Charles Scribner's Sons, 1974.
Olby, Robert. The Origins of Mendelism. New York: Schocken Books, 1966.
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