Gregor Mendel botanicus

Theoretische interpretatie

Mendel bracht zijn resultaten vervolgens in verband met de celtheorie van bevruchting, volgens welke een nieuw organisme wordt gegenereerd uit de fusie van twee cellen. Om pure kweekvormen van zowel het dominante als het recessieve type in de hybride te kunnen brengen, moest er een tijdelijke aanpassing van de twee verschillende karakters in de hybride zijn, evenals een scheidingsproces bij de vorming van de pollencellen en de eicellen. Met andere woorden, de hybride moet kiemcellen vormen die de potentie hebben om het ene of het andere kenmerk op te leveren. Dit is sindsdien beschreven als de wet van segregatie, of de doctrine van de zuiverheid van de geslachtscellen. Aangezien één stuifmeelcel samensmelt met één eicel, zouden alle mogelijke combinaties van de verschillende stuifmeel- en eicellen slechts de resultaten opleveren die worden voorgesteld door de combinatorische theorie van Mendel.

Mendel presenteerde zijn resultaten voor het eerst in twee afzonderlijke lezingen in 1865 aan de Natural Science Society in Brünn. Zijn paper 'Experiments on Plant Hybrids' werd het volgende jaar gepubliceerd in het tijdschrift Verhandlungen des naturforschenden Vereines in Brünn. Het trok weinig aandacht, hoewel veel bibliotheken het ontvingen en herdrukken werden verstuurd. De neiging van degenen die het lazen was om te concluderen dat Mendel simpelweg nauwkeuriger had aangetoond wat al algemeen werd aangenomen - namelijk dat hybride nakomelingen terugkeren naar hun oorspronkelijke vormen. Ze over het hoofd gezien het potentieel voor variabiliteit en de evolutionaire implicaties die zijn demonstratie van de recombinatie van eigenschappen mogelijk maakte. Met name correspondeerde de Zwitserse botanicus Karl Wilhelm von Nägeli eigenlijk met Mendel,ondanks dat hij sceptisch bleef over de betekenis van zijn resultaten en betwijfelde of de kiemcellen in hybriden zuiver zouden kunnen zijn.

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.