GREGOR JOHANN MENDEL.
Died: 1884, Brno, Czechoslovakia
Major Work; Versuche uber Pflanzenhybriden (Experiments in Plant Hybridization) (1866)
The inheritance of characteristics is governed by pairs of discrete elements derived from each parent. These parental elements pass into the germ cells of the offspring without influencing each other; this is the law of segregation.
The inheritance of one element does not govern the inheritance of any other element; this is the law of independent assortment.
The painstaking work of Johann Mendel (he adopted the name Gregor only on entering the Augustinian monastery in Brno), carried out over many years and presented straightforwardly in his major scientific paper, explicated the mechanism of heredity and the process of evolution, and thus laid the basis for all further work in genetics. His work removed the inheritance of traits from the realm of speculation and made it the subject of statistical analysis. Unknown to the scientific world during his lifetime, his contributions were not recognized until twenty years after his death when several experimenters had begun to duplicate his work.
Born of a peasant family, Mendel showed exceptional intellectual abilities and was sent to a secondary school and then to a Gymnasium in Troppau (now Opava), Moravia, where he completed the course in 1840. After a period of illness and with financial help from his sister, he attended the University at Olmutz. (now Olomouc), completing the course in philosophy, which also included physics, mathematics. and statistics. In 1843, although he felt no vocation for holy orders, he entered the Augustinian monastery in Brno, largely because he saw no other way of supporting himself. His entry, however, was propitious, for it was there, freed from money worries, that he was able to pursue the studies for which he is known.
The monastery was a center of scientific activity, particularly research in agriculture. One of the teachers there, Matthew Klacel, directed an experimental garden and was investigating the causes of genetic variation in plants. Mendel's abilities were appreciated at the monastery, and he was sent to the University of Vienna to study physics, chemistry, and biology. Among his teachers was Franz Unger, who had studied fossil plants and the variability of cultivated plants, and who surmised that new plants evolved by combining simple elements within their cells, although he had no concrete evidence of the existence of these elements.
In 1856, after his return from Vienna, Mendel began his famous series of experiments with peas. From family experience (his father had been a professional gardener) and from his reading in Vienna, Mendel was familiar with hybridization, the chief method for improving cultivated plants. Following Unger's surmise, Mendel devised an experimental method for finding the simple elements of inheritance; Over a ten-year period, he cultivated about 30,000 pea plants, analyzing seven pairs of seed or plant characteristics. These pairs included smooth vs. wrinkled seed, green vs. yellow seed, tall vs. dwarf plants, and various pod and/or flower placements. His hypothesis was that in a hybrid, these characteristics are due to elements derived from each parent plant. The hybrid inherits one element from each parent, for example, from one. the element for smooth seeds; from the other, the element for wrinkled seeds; and neither element modifies the other. This is Mendel's law of segregation. However, the first generation of hybrids all display only one of the characteristics, the dominant one. (The other is recessive.) In the second generation, both ancestral traits appear again: Some plants will have smooth seeds, some will have wrinkled.
Mendel reduced the variables in his experiments to a minimum in order to get meaningful statistics. He cultivated 14,000 pea plants that differed in only one characteristic and found that in the second generation, the ancestral traits appear in the ratio 3 dominant to 1 recessive. His results are shown in the following chart, where S is smooth (dominant) and W is wrinkled (recessive): The second generation types appear in equal numbers, but since S is dominant, 3/4 of the plants have smooth seeds.
Mendel cultivated plants which differed in two, three, or more traits and found that the inheritance of one trait implied nothing about the inheritance of another. He found that all seven characteristics occurred independently, yielding 128 ([2.sup.7]) different types in the second generation. This is his law of independent assortment. He was able to prove his two laws because he (1) carefully selected his experimental subjects for a limited number of traits and (2) had a large population, which allowed him to treat the result statistically. Such a method of handling raw data was new.
(Mendel's elements were, of course, genes, a name coined by Johannsen in 1909. Mendel was unaware of chromosomes, which were first observed in 1873. The connection between genes and chromosomes was made by the American biologist T. H. Morgan, who, beginning in 1910, made extensive studies of the genetics of Drosophila, the fruit fly. Morgan discovered that Mendel's law of independent assortment is true only for genes that occur on different chromosomes--which happened to be the case for the seven pea elements. Genes that occur on the same chromosome are usually inherited together.)
Mendel presented his results in 1865 in lectures presented to the Brno Natural Sciences Society and in his paper "Versuche uber Pflanzenhybriden," published in 1866. Disappointingly, there was no response. Mendel attempted to verify his results with peas by doing similar experiments with Hieracium (a genus of common European weed), but without great success. Moreover, in 1868 he was elected abbot of the monastery, and the rest of his life was devoted primarily to official business and to a lengthy conflict with the Austrian authorities over taxes on religious property. He did publish a paper on Hieracium and several papers on meteorology. Beginning in 1877, with his support, weather forecasts for Moravia were issued, the first in central Europe.
Mendel read Darwin's works, but the reverse was not true; so Darwin, who searched for an explanation of the causes of variations, never knew that Mendel's results could supply an answer. Only in 1900, when three scholars (de Vries, Correns, and von Tschermak) each working independently duplicated Mendel's results, was he rediscovered. Each found, just before his work was completed, that he had been anticipated thirty-five years before by an Augustinian monk. Mendel was given posthumous credit as the founder of modern genetics.
[Mendel's original documents are preserved in the Mendelianum, a museum located in the former Augustinian monastery in Brno.]
Bowler, Peter. The Mendelian Revolution. London: Athlone Press, 1989. A description of the influence of Mendel's work on later scientific and cultural thought.
Iltis, H. Life of Mendel. Translated by E. and C. Paul. 2d ed. New York: Hafner Publishers, 1966. A translation of Gregor Johann Mendel. Leben, Werk, und Wirkung. Berlin: J. Springer, 1924. The standard biography of Mendel.
Stern, C., and E. R. Sherwood. The Origins of Genetics. A Mendel Source Book. San Francisco: W H. Freeman, 1966. This text includes Mendel's chief papers and his letters to other scientists.