Gregor Mendel Botaniker

Theoretische Interpretation

Mendel bezog seine Ergebnisse auf die Zelltheorie der Befruchtung, nach der aus der Fusion zweier Zellen ein neuer Organismus entsteht. Damit reine Brutformen sowohl des dominanten als auch des rezessiven Typs in den Hybrid gebracht werden konnten, mussten die beiden unterschiedlichen Charaktere im Hybrid vorübergehend untergebracht werden sowie ein Trennungsprozess bei der Bildung der Pollenzellen und die Eizellen. Mit anderen Worten, der Hybrid muss Keimzellen bilden, die das Potenzial haben, entweder das eine oder das andere Merkmal zu ergeben. Dies wurde seitdem als das Gesetz der Segregation oder die Lehre von der Reinheit der Keimzellen beschrieben. Da eine Pollenzelle mit einer Eizelle fusioniert, würden alle möglichen Kombinationen der unterschiedlichen Pollen- und Eizellen nur die Ergebnisse liefern, die von Mendels kombinatorischer Theorie vorgeschlagen werden.

Mendel präsentierte seine Ergebnisse erstmals 1865 in zwei separaten Vorträgen vor der Naturwissenschaftlichen Gesellschaft in Brünn. Seine Arbeit „Experimente an Pflanzenhybriden“ wurde im folgenden Jahr in der Zeitschrift Verhandlungen des naturforschenden Vereins in Brünn veröffentlicht. Es erregte wenig Aufmerksamkeit, obwohl viele Bibliotheken es erhielten und Nachdrucke verschickt wurden. Die Tendenz derjenigen, die es lasen, war zu dem Schluss zu kommen, dass Mendel einfach genauer gezeigt hatte, was bereits allgemein angenommen wurde - nämlich, dass hybride Nachkommen zu ihren Ursprungsformen zurückkehren. Sie übersahen das Variabilitätspotential und die evolutionären Implikationen, die sein Nachweis der Rekombination von Merkmalen ermöglichte. Vor allem der Schweizer Botaniker Karl Wilhelm von Nägeli korrespondierte tatsächlich mit Mendel,trotz skeptischer Haltung hinsichtlich der Bedeutung seiner Ergebnisse und Zweifel, dass die Keimzellen in Hybriden rein sein könnten.

Latter years

Mendel appears to have made no effort to publicize his work, and it is not known how many reprints of his paper he distributed. He had ordered 40 reprints, the whereabouts of only eight of which are known. Other than the journal that published his paper, 15 sources are known from the 19th century in which Mendel is mentioned in the context of plant hybridization. Few of these provide a clear picture of his achievement, and most are very brief.

By 1871 Mendel had only enough time to continue his meteorological and apicultural work. He traveled little, and his only visit to England was to see the Industrial Exhibition in 1862. Bright disease made his last years painful, and he died at the age of 61. Mendel’s funeral was attended by many mourners and proceeded from the monastery to the monastery’s burial plot in the town’s central cemetery, where his grave can be seen today. He was survived by two sisters and three nephews.

Rediscovery

In 1900 Dutch botanist and geneticist Hugo de Vries, German botanist and geneticist Carl Erich Correns, and Austrian botanist Erich Tschermak von Seysenegg independently reported results of hybridization experiments similar to Mendel’s, though each later claimed not to have known of Mendel’s work while doing their own experiments. However, both de Vries and Correns had read Mendel earlier—Correns even made detailed notes on the subject—but had forgotten. De Vries had a diversity of results in 1899, but it was not until he reread Mendel in 1900 that he was able to select and organize his data into a rational system. Tschermak had not read Mendel before obtaining his results, and his first account of his data offers an interpretation in terms of hereditary potency. He described the 3:1 ratio as an “unequal valancy” (Wertigkeit). In subsequent papers he incorporated the Mendelian theory of segregation and the purity of the germ cells into his text.

In Great Britain, biologist William Bateson became the leading proponent of Mendel’s theory. Around him gathered an enthusiastic band of followers. However, Darwinian evolution was assumed to be based chiefly on the selection of small, blending variations, whereas Mendel worked with clearly nonblending variations. Bateson soon found that championing Mendel aroused opposition from Darwinians. He and his supporters were called Mendelians, and their work was considered irrelevant to evolution. It took some three decades before the Mendelian theory was sufficiently developed to find its rightful place in evolutionary theory.

The distinction between a characteristic and its determinant was not consistently made by Mendel or by his successors, the early Mendelians. In 1909 Danish botanist and geneticist Wilhelm Johannsen clarified this point and named the determinants genes. Four years later American zoologist and geneticist Thomas Hunt Morgan located the genes on the chromosomes, and the popular picture of them as beads on a string emerged. This discovery had implications for Mendel’s claim of an independent transmission of traits, for genes close together on the same chromosome are not transmitted independently. Moreover, as genetic studies pushed the analysis down to smaller and smaller dimensions, the Mendelian gene appeared to fragment. Molecular genetics has thus challenged any attempts to achieve a unified conception of the gene as the elementary unit of heredity. Today the gene is defined in several ways, depending upon the nature of the investigation. Genetic material can be synthesized, manipulated, and hybridized with genetic material from other species, but to fully understand its functions in the whole organism, an understanding of Mendelian inheritance is necessary. As the architect of genetic experimental and statistical analysis, Mendel remains the acknowledged father of genetics.