Five-minute digest
Experiments in Plant Hybridisation (in Bateson's Mendel's Principles of Heredity)
Summary
Gregor Mendel's 'Experiments in Plant Hybridisation' was read to the Natural History Society of Brünn (now Brno, in the Czech Republic) at its meetings of 8 February and 8 March 1865 and printed in the Society's proceedings, which appeared in 1866. Mendel, an Augustinian friar and schoolteacher, reported eight years of crossing varieties of the garden pea in his monastery garden. The paper was little noticed until 1900, when Hugo de Vries, Carl Correns and Erich von Tschermak independently reached similar results and drew attention to it. The library text is the volume in which William Bateson brought Mendel to English readers: Mendel's Principles of Heredity: A Defence (Cambridge, 1902). It contains Bateson's preface and introductory lecture, a revised Royal Horticultural Society translation of the pea paper, a translation of Mendel's later paper on Hieracium (hawkweed), and Bateson's long reply to the biometrician W. F. R. Weldon. Mendel chose the pea because its varieties differ in sharp, constant characters, its flowers protect against stray pollen, and its hybrids are fertile. He followed seven pairs of characters, among them round or angular seeds, yellow or green seed contents, tall or dwarf stems and the position of the flowers. In every first cross the hybrid resembled one parent: Mendel called that character dominant and the one that disappeared recessive. When the hybrids were self-fertilised, the recessive character reappeared in about a quarter of the offspring; in one trial 5,474 round seeds stood to 1,850 angular, and over all the experiments the average ratio was 2.98 to 1, or 3 to 1. Further generations showed that the dominant-looking plants were of two kinds, one-third breeding true and two-thirds behaving again like hybrids, giving the series A + 2Aa + a. When two or three pairs of characters were combined, each pair followed its own ratio independently, producing all possible combinations in predictable numbers. Mendel explained these results by supposing that hybrids form egg and pollen cells carrying each parental character in equal numbers, and he confirmed the theory by crossing the hybrids back to their parents. Briefer experiments with beans (Phaseolus) and a concluding comparison with the work of Kölreuter and Gärtner close the paper. The 1869 Hieracium paper reports that hawkweed hybrids, unlike peas, bred true. Bateson's introduction explains Mendel's principle of the purity of germ cells and contrasts it with Francis Galton's law of ancestral heredity, introducing the terms allelomorph, heterozygote and homozygote. His Defence answers Weldon's objections point by point, examining the colours and shapes of peas, the records of earlier breeders such as Knight and Laxton, and apparent exceptions, and ends predicting that Mendel's work will transform every science of living things.
Top Lessons
Choose the right material
Mendel begins by explaining that an experiment is only as good as the material chosen for it. By selecting a plant with sharp, constant characters, controlled pollination and fertile hybrids, he made it possible to count and classify every offspring.
Counting reveals hidden order
Single crosses had shown earlier observers only that hybrid offspring vary. By counting thousands of seeds and plants across generations, Mendel found the ratios 3 : 1 and 1 : 2 : 1 and the independent combination of characters. Patient numerical record turned a confusion into a law.
Hypothesis and test
Mendel did not stop at describing his ratios. He proposed that hybrids form equal numbers of reproductive cells of each type, deduced what crosses back to the parents should yield, and found the predictions confirmed.
Neglect, rediscovery and controversy
Mendel's paper lay largely unread for thirty-five years. When it was rediscovered, Bateson defended it fiercely against Weldon, who thought it too simple for the facts of heredity. The dispute between Mendelians and biometricians was later resolved when R. A. Fisher and others showed that Mendelian factors could account for continuous variation.
Verified Quotes
“an unknown man, Gregor Johann Mendel, had, alone, and unheeded, broken off from the rest--in the moment that Darwin was at work--and cut a way through.”
“It requires indeed some courage to undertake a labour of such far-reaching extent”
“The value and utility of any experiment are determined by the fitness of the material to the purpose for which it is used”
“The expression “recessive” has been chosen because the characters thereby designated withdraw or entirely disappear in the hybrids, but nevertheless reappear unchanged in their progeny”
“there is found, as between the number of forms with the dominant and recessive characters, an average ratio of 2·98 to 1, or 3 to 1.”
“Had Mendel’s work come into the hands of Darwin, it is not too much to say that the history of the development of evolutionary philosophy would have been very different from that which we have witnessed.”
“Each conception of life in which heredity bears a part--and which of them is exempt?--must change before the coming rush of facts.”
Medical Lens
Mendel's paper concerns peas, but it founded the genetics on which modern medical understanding of inherited disease rests. Bateson's introduction contrasts Mendel's principle of segregation with Galton's statistical law of ancestral heredity, built partly on studies of human stature, and forecasts that an exact knowledge of heredity will change man's power over nature. In the same year as this book, 1902, Archibald Garrod, in correspondence with Bateson, interpreted alkaptonuria as a recessive Mendelian trait, the beginning of the study of inborn errors of metabolism. Mendelian ideas were also taken up, in the following decades, by eugenics movements in several countries; the history of medicine treats that use as a separate chapter in which hereditary theory was applied to social policy. For psychiatry, the book stands at the turn from nineteenth-century theories of hereditary degeneration, which assumed blending and cumulative taint, to the particulate model later used in psychiatric genetics.
- Bateson, Introduction, opening and section on the law of ancestral heredity
Bateson's forecast that the laws of heredity will change man's outlook and power, and his comparison of Galton's ancestral law, drawn partly from human stature, with Mendel's principle.
- Mendel, The Reproductive Cells of the Hybrids
The hypothesis that hybrids form equal numbers of germ cells of each parental type, the basis of later recessive and dominant patterns of inherited disease.