Today the results and conclusions of MENDEL are seen as the starting point of modern genetics. It is therefore unavoidable to check his statements critically in order to mark out the range that they are operative in. MENDEL commented on this:
"The validity of the statements made of Pisum has still to be proven and a repeat of at least the most important experiments would therefore be desirable...(and)...whether the variable hybrids of other plant species display an according behaviour has also to be shown by experiments. But it can be suspected that no principle differences of the main points will occur, since the unity of the developmental plan of organic life cannot be doubted."
This last sentence is very interesting, because it describes a principle of continuity that seems commonly accepted nowadays, but was sharply opposed at MENDEL's time. Continuity is kept upright by heredity and constitutes one of the main pillars of the theory of evolution.
MENDEL's work was noted by only few of his contemporaries. One of the few people with whom he corresponded about it was the botanist C. v. NÄGELI. MENDEL reported the crossing of closely related wild types, certain strange conclusions that did not fit into the usual schemata and studies that pointed at the inheritance of sexes according to the principle of segregation. He saw problems arising with the heredity of flower colours, because the existence of numerous shades cannot be explained by single genes alone. So he postulated that a whole range of genes were participating and that each of them was responsible for a certain contribution to the shaping of the colour intensity. In contrast to Pisum he did find an intermediate inheritance in corn, that he found to be of no principal difference from the dominant-recessive ones.
All these results were never published by MENDEL. They are preserved in the correspondence with C. v. NÄGELI. C.CORRENS edited and published it in 1905. Supported by C. v. NÄGELI MENDEL did also work on Hieracium hybrids, especially such of the subgenera Pilosella and Archhieracium. The choice of these objects proved to be rather unhappy, since Hieracium is a genus that poses difficulties to botanists even today. Often stable transitional forms occur, whose existence MENDEL could not explain. Besides the technical problems (flowers of composites are never easy to work with) were his experiments disturbed by a phenomenon that was not understood until much later: The seeds of several species develop directly from the mother cells of the embryo sac or a cell of the surrounding tissue, meiosis does not take place. Such a way of seed development is called agamospermy. The mother plant develops seeds without being pollinated. These seeds behave like layers of the mother plant. Additionally species exist, where part of the seeds develop from agamospermy and part from pollinated egg cells. This leads to completely confusing ratios, because the preconditions upon which MENDEL's laws are founded are not fulfilled any more.
MENDEL's fundamental work was forgotten for 35 years. It was not before the year 1900 that it became known. The German C. CORRENS, the Dutchman HUGO de VRIES and the Austrian ERICH von TSCHERMAK-SEYSENEGG are regarded as its rediscoverers. Their articles were all published at nearly the same time, in spring 1900. They heard first of MENDEL's work, when their own work was nearly finished. H. de VRIES writes apologetically:
"This important work is hardly cited, so that I myself did not get to know it before I had finished most of my own experiments and had concluded the same laws as are mentioned in the text."
The priority of MENDEL was acknowledged without restriction by all three researchers. C. CORRENS recognized furthermore that not all characters can be freely combined, but that some of them are coupled and are thus always inherited together.
With the year 1900 a busy research activity began. The validity of the Mendelian laws was shown for numerous animal and plant species. But exceptions were also found and it was tried to explain them. The question of the mechanism of heredity gained an exceptional importance. The chromosome theory of inheritance offered at first an answer insufficiently proven by experiments. Today it is regarded as the second fundamental pillar of modern genetics besides MEMDEL's laws. It became a prerequisite for the understanding of heredity on a molecular level. And then, in the year 1944, O. T. AVERY, C. M. McLEOD and M. McCARTY (Rockefeller Institute, New York) discovered the desoxy ribonucleic acid (DNA) as the vehicle of genetic information. Nine years later J. D. WATSON and F. H. CRICK (1953, Cavendish Laboratory, Cambridge/England) presented their famous DNA double helix. Their publication ends nearly succinct with the sentence
"It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism of the genetic material."
The time around 1900 was not yet ripe for such results. Biochemical knowledge was still in its beginning and just as little was known about macromolecules as about weak forces and modern analytic methods (like Röntgen structure analysis = X-ray structure analysis). The German physicist W. C. RÖNTGEN detected the radiation that bears his name as soon as 1895, but it was not until much later that the technical devices for its use in the analysis of molecular structures and the necessary theoretical knowledge for the explanation of the refraction patterns were known.
The work done by MENDEL is today viewed as the fundament of modern genetics. The next big task that researchers concentrated on, was the mechanism of heredity. By and by the chromosome theory was proven and accepted.
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