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Gregor Mendel (1822–1884)

Mendel was an Augustinian friar and naturalist whose experiments on pea plants, published in 1866, described the fundamental laws of inheritance: traits are transmitted as discrete factors (later called genes) that segregate independently and combine according to predictable ratios. The work was largely ignored for thirty-four years. Its rediscovery in 1900 — independently by Hugo de Vries, Carl Correns, and Erich von Tschermak — initiated the science of genetics and set up the problem that population genetics would resolve: how to reconcile Mendel’s discrete factors with Darwin’s gradualist theory of natural selection. The reconciliation, achieved by Fisher, Haldane, and Wright in the 1920s and 1930s, demonstrated that many Mendelian genes of small effect produce exactly the continuous variation Darwinian selection requires. Mendel’s laws became the substrate on which the Modern Synthesis was built.

Life

Born Johann Mendel on 20 July 1822 in Heinzendorf bei Odrau, Austrian Silesia (now Hynčice, Czech Republic), into a German-speaking farming family. His father Anton was a smallholder; the family was not well-off. His academic ability was recognised early, and he was sent to the Gymnasium at Troppau (Opava) and then to the Philosophical Institute at Olomouc. Financial difficulty forced interruptions in his education.

Entered the Augustinian Abbey of St. Thomas in Brünn (Brno) in 1843, taking the religious name Gregor. The abbey was a centre of scientific and scholarly activity — Abbot Cyrill Napp encouraged research, and the monastic community included several naturalists. Mendel was ordained in 1847. Sent by the abbey to the University of Vienna (1851–53) to study physics, mathematics, and natural science under Christian Doppler (physics) and Franz Unger (plant physiology). The training in physics and mathematics — particularly in combinatorics and probability — shaped his experimental approach to inheritance, which was quantitative in a way that contemporary biology was not.

Returned to Brno and taught natural science at the Realschule (1854–68). Conducted the pea plant experiments in the monastery garden from 1856 to 1863. Presented the results in two lectures to the Natural History Society of Brünn in 1865; published as “Versuche über Pflanzenhybriden” (“Experiments on Plant Hybrids”) in the Society’s Proceedings in 1866. Elected Abbot of St. Thomas in 1868 — an administrative position that consumed the remaining sixteen years of his life in a protracted dispute with the civil authorities over monastery taxation. He conducted little further scientific work. Died 6 January 1884 in Brno, aged sixty-one.

The pea plant experiments

Mendel’s experimental programme ran from 1856 to 1863 in the monastery garden. He worked with the garden pea (Pisum sativum), chosen for its self-fertilising habit (which allows controlled crosses), its clearly distinguishable trait variants (tall/short, round/wrinkled, green/yellow, and others), and its short generation time. Over eight years he grew roughly 29,000 pea plants, crossing varieties and recording the ratios of trait variants in successive generations.

The results:

The law of segregation. Each organism carries two copies of each hereditary factor (one from each parent). The two copies segregate during the formation of reproductive cells, so that each gamete carries only one copy. When an organism inherits one dominant and one recessive factor, the dominant factor determines the visible trait (the phenotype), but the recessive factor is not destroyed — it can reappear in subsequent generations. The crossing of two hybrid plants (each carrying one dominant and one recessive factor) produces offspring in a 3:1 ratio of dominant to recessive phenotypes.

The law of independent assortment. Different hereditary factors segregate independently of one another. The inheritance of seed colour is independent of the inheritance of plant height. This implies that each factor has its own physical basis, transmitted independently.

Dominance and recessiveness. In hybrids, one trait variant masks the other. The masking variant is dominant; the masked is recessive. The recessive variant reappears unchanged when two hybrids are crossed — it has not been blended or diluted.

The significance of these findings was not recognised at the time. The 1866 paper was cited occasionally (Mendel sent reprints to several biologists, including Karl von Nägeli, who corresponded with him but did not appreciate the work’s importance) but had no measurable impact on biology until its rediscovery in 1900.

The neglect and the rediscovery

Why was the 1866 paper ignored? Several factors: it was published in a provincial journal with limited circulation; its quantitative, ratio-based approach was foreign to the descriptive botany of the period; Darwin’s own theory of inheritance (pangenesis) dominated thinking about heredity; and the significance of Mendel’s discrete factors for evolution was not apparent without the population-genetic mathematics that would be developed only decades later.

The rediscovery came in 1900, when three botanists — Hugo de Vries in the Netherlands, Carl Correns in Germany, and Erich von Tschermak in Austria — independently replicated Mendel’s results and found his 1866 paper in the literature. Each acknowledged Mendel’s priority. William Bateson in England became Mendel’s most vigorous advocate, coining the term “genetics” in 1905.

The rediscovery created a problem rather than solving one. Mendelian inheritance appeared to involve discrete, all-or-nothing factors — a pea is either round or wrinkled. Darwinian natural selection seemed to require continuous, graded variation. The Mendelian-biometrician debate that followed (1900–1918, roughly) was bitter, with Bateson’s Mendelians and Karl Pearson’s biometricians each convinced the other’s framework was wrong. The resolution came with Fisher’s 1918 paper “The correlation between relatives on the supposition of Mendelian inheritance,” which demonstrated mathematically that continuous variation arises from the combined effects of many Mendelian genes, each of small effect. The integration is treated fully at The Integration with Genetics.

The too-good-data question

Fisher, in a 1936 paper, noted that Mendel’s reported ratios were suspiciously close to their theoretical expectations — closer than random sampling would typically produce. The statistical probability of obtaining results as good as Mendel’s, across all his experiments, was low. Fisher suggested (diplomatically) that an assistant may have unconsciously biased the counts. The question has been debated by historians and statisticians since. Some defend Mendel’s data as falling within acceptable limits; others accept that some unconscious or deliberate selection of data likely occurred. The issue does not affect the validity of Mendel’s laws — they have been confirmed by thousands of subsequent experiments — but it complicates the historical record.

Where Mendel stops

Mendel described the laws of inheritance but could not identify the physical basis of the hereditary factors he postulated. What the factors are, where they reside, how they replicate, and how they produce their effects on the organism — all this lay beyond the reach of 1860s biology. The identification of chromosomes as the carriers of hereditary information came with Theodor Boveri and Walter Sutton around 1902–03. The identification of DNA as the molecular substrate came in 1944 (Avery, MacLeod, and McCarty) and 1953 (Watson and Crick). Mendel’s contribution is the logic of inheritance — the combinatorial rules by which factors segregate and assort. The material basis, the mechanism of expression, and the relationship between genotype and phenotype were questions his framework posed but could not answer.

Key works

See also: Darwinism · Fisher · Darwin · Haldane · Wright