Indian J Physiol Pharmacol 2001; 45(2) : 133-135

EditorialEMERGENCE OF MENDELISM:

PAGES FROM HISTORY

It is a common knowledge that Gregor Mendel (1822-1884) performed hisexperiments on ordinary garden pea in a monastery at Brunn in Moravia,Czechoslovakia during the period 1856-1863. He presented his observationsto the Brunn Society of Natural Sciences in 1865, and published his resultsin the Proceedings of the Brunn Society of Natural Sciences in 1866. Theresearch phase of his career was not long; it effectively ended in 1868 whenhe was elected abbot of the monastery. The demands of his newresponsibilities, especially in the face of a tax dispute raised against themonastery, did not allow him to continue his genetic studies in plants. In1884, Gregor Johann Mendel died of a kidney disorder.

On the basis of rather simple, but precisely performed breedingexperiments and quantitative analytic reasoning, Mendel proposed thatdiscrete, particulate units of heredity exit, and explained how they aretransmitted from one generation to the next generation. This was trulyoutstanding scientific achievement. However, Mendel's observations remainedby and large unappreciated, and his paper was poorly disseminated amongscientists till 1900 when, well after his death, at least three biologists -Hugo DeVries, Karl Correns and Eric Von Tschermak - independentlydiscovered the significance of Mendel's observations, which are nowconsidered the foundation of modern genetics. The delayed recognition ofMendel's work has no simple explanation, but an analysis of the historicalfacts might help.

Mendel was not the first to make an attempt to understand the principlesof inheritance using breeding experiments. Joseph Gottlieb Kolreuter (1733-1806), a German botanist, performed crossbreeding experiments with tobaccoplants. He observed that a new hybrid form resulted from crossbreeding oftwo groups, and on repeated backcrosses, one of the parental forms sometimesreappeared. He also observed segregation of traits in his breedingexperiments with carnations. Kolreuter did not however realize thesignificance of his observations, because he believed in special creation andfixity of species. Similarly, Karl Friedrich Gaertner (1772 -1850) performedbreeding experiments with peas and obtained results not unlike those ofMendel. Gaertner, however, did not analyse the individual traits. Mendel,on the other hand, worked with seven visible features (unit characters) of

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garden peas, and each of these features wasrepresented by two highly contrasting traits.Unlike his predecessors, Mendel used a validexperimental model, an elegant studydesign, sound methodology, and quantitativeanalysis: all these are essential for goodexperimental biology. His approach toexp er im en tal genetics was very differentfrom that of other scientists of his time. Asa result, his work, the work of a monk, wastreated as if it did not belong to the worldof science.

Furthermore, Mendel's work wasmarginalized because. of newly foundinterest in the theory of natural selectionof Darwin and Wallace, in which why certainphenotypes survive preferentially wasconsidered more important than howphenotypic variations are transmitted. It isthe latter question which Mendel's studyaddressed at a time when it was notconsidered important.

There was an even more seriousconceptual problem which hamperedMendel's idea in his lifetime. Somecontemporary biologists were indeedintrigued by the underlying mechanism ofnatural variation. As Darwin and Wallacebelieved, the predominant concept inthis regard was the blending theory ofinheritance, that the 'hereditary fluid' fromtwo' parents meet together and blends,forming an inseparable mixture giving riseto continuous variation. On the contrary,Mendel presented, based on hisobservations, that hereditary transmissionoccurred through particulate, discreteunits or factors, which resulted indiscontinuous variations. Given the over-powering influence of Darwin's theory,

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Mendel's thesis did not fit well with thecontemporary biologists' belief about naturalvariation.

Mendel's theory was furthermarginalized because of a lack of anycytological basis for his observation. Duringthe period from 1865 to 1900, it becameprogressively evident that Weissmann'sgermplasm houses the genetic materialwhich contains the informationfor heredity and development. In 1879,Walter Flemming discovered chromosomesin nuclei of salamander cells. Against thisbackground, Mendel's theory returned witha bang in 1900. This was a paradigm shiftwhen new observations, new data, new toolsand new minds converged at a point whichwent beyond the existing rules andparadigm. In 1902, Walter Sutton andTheodor Boveri independently discoveredthe behaviour of chromosomes duringmeiosis, which corroborated Mendel'sprinciples of segregation and independentassortment.

However, all these explanations do notfully solve the mystery as to why at leastsome contemporary biologists failed toappreciate the significance of Mendel'sobservations, which in Kuhn's terminologyconstitute a small revolution when a limitedgroup of scientists in a specific sub specialitystarts finding anomaly in existing rules andparadigm in view of newly observed data.An additional reason could be that peoplewho mattered in genetics at Mendel's timeand place did not promote his findings, forwhatever reasons. In 1866, Mendel soughtthe help of the famous Swiss biologist KarlVon Nageli (1817-1891), who himself wasengaged in plant breeding experiments. Von

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Nageli, however, was not impressed withMendel's theory and suggested that Mendelshould grow some more peas. Mendel hadalready recorded observations on some13000 hand-bred specimens of pea plants.Mendel sent him 140 packets of pea seedsto grow them at the Botanical Gardensin Munich. Von Nageli never plantedthe seeds and did not refer toMendel's findings in his major treatise(Mechanisch- physiologische Theorie derAbstam-munglehre) published in 1884.Because of concentration of power in thehand of a few self-opinionated scientists,their tubular vision about progress inscience, conceptual inertia and other diversepsychological blocks, such dismissal of apotentially great idea has not been a rarephenomenon in the history of science.

Above everything else, Mendel isremembered for his exemplary unbiasedexperimentation and meticulous analysisof observed data with a clear, free mind.Besides being the gold standard of researchmethodology In biological sciences,Mendel laid the foundation stones for twobranches of modern biology: genetics andbiometry.

Four postulates form the basic tenets ofMendelian genetics. First, geneticcharacters are controlled by unit factorspresent in pairs in individual organism.Second, presentation of a single characterin a single individual is determined by

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relative dominance and recessiveness of unitfactors in pair. Third, the paired unit factorssegregate randomly during gameteformation. Finally, segregating pairs ofunit factors assort independently of eachother during gamete formation. Clearly,independent assortment of genetic materialforms the basis of wide spectrum of geneticvariation, which appears to be important inthe process of organic evolution in allorganisms. Thus, Mendelian geneticsreplaced the Darwin's blending theory ofinheritance through continuous variation.

Furthermore, Mendel's method ofhandling data was the beginning ofstatistical analysis in biological sciences,which stimulated a new generation ofscientists like Karl Pearson and RonaldFisher. Somewhere, between 1892 and 1900,Karl Pearson coined the term biometry,which literally means biologicalmeasurement, to define analysis andinterpretation of data with a view towardobjective evaluation of the reliability of theconclusions based on scientific data. In 1901,Karl Pearson published this term in theinaugural issue of his journal Biometrika.

IJPP is proud to analyse thecircumstances which delayed the recognitionof Mendel's monumental contribution bymore than thirty years. Now that the humangenome has been completely mapped, it iseasy to appreciate what a revolution Mendelinitiated.