Johann Gregor Mendel

Johann Gregor Mendel

The Moravian natural scientist and Augustinian abbot Johann Gregor Mendel (1822-1884) laid the foundations of modern genetics with his paper dealing with the hybridization of peas.

Gregor Mendel was born on July 22, 1822, at Hynčice, Czechoslovakia (then Heinzendorf, Austrian Silesia). His ancestors were farmers, and his father still had to work three days a week as a serf. Mendel displayed a great love for nature all his life.

Years of Preparation and Education

In 1831 Mendel was sent to the Piarist school in Lipník (Leipnik) and at the age of 12 to the grammar school in Opava (Troppau). In 1840 he enrolled at the Institute of Philosophy in Olomouc (Olmütz).

Mendel was admitted to the Augustinian order in Brno (Brünn) in 1843. The Augustinians taught philosophy, foreign languages, mathematics, and natural sciences at secondary schools and universities. Abbot Napp, the head of the monastery, devoted all his energy to the economic development of the monastery and to the scientific education of the members of the order. Surrounded by an atmosphere of dynamic activity, Mendel found optimum conditions for his studies and later for his research work. Along with his theological studies Mendel took courses in agriculture, pomiculture, and vine growing at the Institute of Philosophy in Brno. In 1847 he was ordained a priest and served for a short time as vicar at the Old Brno Monastery.

In 1849 Mendel became a teacher of mathematics and Greek at the grammar school in Znojmo (Znaim). After a year the headmaster recommended him for the university examination. Together with his application for admission to the examination Mendel enclosed his autobiography, which is the only authentic preserved document. Mendel failed the examination, probably because he lacked a complete university education. Only his written test on meteorology satisfied his examiner, and, on the latter's recommendation, Abbot Napp sent Mendel to study natural sciences at the University of Vienna (1851-1853). He heard F. Unger lecture on plant anatomy and physiology, the use of the microscope, and the practical organization of experiments. Unger was known for his views on evolution and had investigated the problem of the origin of plant variability by means of transplanting experiments. Mendel later performed these experiments also. It is now assumed that Unger's views deeply influenced Mendel in the formation of his ideas before he performed his experiments with edible peas (Pisum).

On his return to Brno in 1854 Mendel was appointed a teacher of physics and natural history in the Technical School. In 1856 he prepared himself for the university examination again, but he became seriously ill and did not take it. By this time, however, Mendel was fully occupied with his hybridizing experiments with Pisum. He remained a teacher till 1868, when he was elected abbot of the monastery.

Hybridizing Experiments

Mendel started his extensive program of hybridizing experiments in 1854. He focused his energy on the problem of the origin of plant variability. For two years he tested the purity of selected varieties of Pisum and then began experimenting with artificial fertilization. A new reconstruction of Mendel's experimental data illustrates that he must have tested about 28,000 Pisum plants during the years 1856-1863.

Mendel summarized the experimental results in a paper, "Experiments on Plant Hybrids," which he read at two meetings of the Natural Science Society in Brno in 1865; the paper was published in the proceedings of the society's journal. Though prominent natural scientists were present at the meeting, no one understood Mendel's ideas or the significance of his work. The proceedings were distributed to 134 scientific institutions in Europe and the United States, but the published paper failed to arouse interest.

Mendel's original idea, that heredity is particulate, was contrary to the theory of "blending heredity" that was generally accepted at that time. In the plants that Mendel tested (and in biparental-reproducing organisms generally), the hereditary particles (called elements by Mendel) from each parent are members of pairs. In forming the reproductive cells, the pair members segregate in different pollen or sperm nuclei and in different eggs or ovules to transmit the hereditary determinants. From one parent comes one particle determining, for example, the round shape of the seed (A), and from the other parent that representing the wrinkled shape (a). Mendel called the trait passing entirely unchanged into hybrid (derived from unlike parents) association "dominant," and the trait becoming latent in hybrids "recessive." The particles meet (recombine) in the offspring (Aa) but do not influence each other.

Suppose the pair members of these hybrid offspring now segregate in forming reproductive cells, producing two types of sperm or egg, namely Aor a, and that these particles meet at random in fertilization. The resulting combination series of relevant particles is: ¼ AA, ¼ Aa, ¼ aA, ¼ aa, or AA ¼2 Aa ¼ aa. That is, there are four genetic types of offspring from the hybrids, each type represented by 25 percent of the total. In this way, in the hybrid progeny the parental forms appear again; after 1900 this segregation of the hereditary units (in 1909 termed genes) was called Mendel's law of segregation.

Mendel found that hereditary particles belonging to different trait pairs, for example, A, a for the seed shape and B, b for the seed coloration, formed the combination series in recombining without influencing each other. The combination series could be predicted by combining the simple series AA □ 2Aa □ aa; BB □ 2Bb □ bb, resulting in the combination series AABB □ AAbb □ aaBB □ aabb □ 2AABb □ 2aaBb □ 2AaBB □ 2Aabb □ 4AaBb. In his paper Mendel actually illustrated such a recombination in crossing peas differing in two and three trait pairs. Expected particle recombinations were realized in actual counts of the offspring. The recombination of the hereditary particles was called Mendel's law of independent assortment.

Mendel gave the impulse for his experiments in the first sentence of his paper: "Artificial fertilization undertaken on ornamental plants to obtain new color variants initiated the experiments to be discussed." His task was to find "the generally applicable law of the formation and development of hybrids as a way of finally reaching the solution to a question whose significance for the evolutionary history of organic forms must not be underestimated." In his paper he expressed the opinion that "the distinguishing traits of two plants can, after all, becaused only by differences in the composition among grouping of the elements existing in dynamic interaction in the primordial cells." He assumed the general validity of his theory because, according to him, "unity in the plan of development of organic life is beyond doubt."

Being interested in the development of hybrid forms, Mendel also explained that the population descending from hybrids tends to revert to the pure parental forms, resulting in diminishing the hybrid's form. Thus, as a consequence of Mendelian segregation, Mendel also laid the basis for the interpretation of the effect of inbreeding.

Mendel continued his hybridizing experiments, crossing various forms of 22 other genera of plants, to prove the general validity of his theory in the plant kingdom. He also cultivated wild plants in the garden with the aim of investigating Lamarck's views concerning the influence of environment upon plant variability; he could not agree with Lamarck. He was convinced, like Darwin, that it was impossible to draw a hard-and-fast line between species and varieties, and in the conclusion of his Pisum paper he expressed the conviction that the variability of cultivated plants could be explained by his theory.

After 1871 Mendel also tried to carry out hybridizing experiments with bees. He bred about 50 bee races which he tried crossing to obtain new cultural breeds. His crossing experiments could not be successful, however, because of the complex problem of the controlled mating of queens. For this reason Mendel focused his activity on research of the technological aspects of apiculture, such as the hibernation of bees.

As a member of the Natural Science Section of the Agricultural Society in Brno and as a respected meteorologist, Mendel summarized the results of meteorological observations in 1856 and published them in six reports (1862-1869). He also published three papers on extraordinary storms (1870-1872). He was a member of the Central Board of the Agricultural Society from 1870, and he supported the first weather forecasts for farmers in 1878. In 1861 he helped found the Natural Science Society of Brno.

Taxation and the Monastery

After Mendel was elected abbot of the monastery in 1868, he had little time for his experimental activities, although they never came to a total stop. In 1874 the government proclaimed a new law relating to the contribution of the cloisters to the religious fund. Mendel refused to pay the high assessed taxes and thus, from the end of 1875, got himself into trouble with the provincial government and with the Ministry of Education in Vienna. The result of this conflict was the lasting sequestration of the landed monasterial property. In an attempt to win Mendel over and stop his opposition to the taxation law, the government appointed him to the Board of Directors of the Moravian Mortgage Bank. In 1876 he became the vice-governor of the bank and in 1881 the governor. Nevertheless, Mendel never agreed to the taxation law.

The long struggle over taxation had a serious effect on Mendel's health. He died on Jan. 6, 1884, without any public recognition of his outstanding scientific achievements.

Contributions to Genetics

Mendel's paper of 1865 went unnoticed except for an occasional reference in scientific literature. In 1900 it was rediscovered by scientists, when his theory was generalized as Mendel's laws of heredity. That date also marked the beginning of the science of heredity, which in 1906 was named genetics. Not even after 1900 was Mendel's theory acknowledged as being generally valid, and the Darwinian selection theory was often considered to oppose the Mendelian theory.

Later, it was demonstrated that Mendel had also observed such phenomena as intermediate inheritance, complete linkage, additive gene action, and gene interaction, and that he himself appreciated the Darwinian selection theory and refused to accept the hypothesis of pangenesis. The synthesis of the Darwinian and Mendelian theories was first proved by S. S. Tchetverikoff in 1926 and finally by R. A. Fisher in 1930, Sewall Wright in 1931, and J. B. S. Haldane in 1932.

Since that time Mendel's work has been reappraised. His hypothesis of hereditary particles turned out to be quite general and provided the elementary principle of heredity in all forms of life from viruses to man. From this viewpoint his laws of heredity appear to be only the subordinate principles of Mendel's main discovery, which furnishes proof of the existence of genes as determining the whole character of each organism.