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Mendel's Principles of Heredity
By William Bateson, Gregor Mendel
Dover Publications, Inc.Copyright © 2010 Dover Publications, Inc.
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INTRODUCTORY. MENDEL'S DISCOVERY.
Introductory—Some pre-Mendelian Writings—Mendel's Discovery—Dominant and Recessive—Segregation. Allelomorphism—Homozygote and Heterozygote. Purity of Type.
AMONG the biological sciences the study of heredity occupies a central position. Whether we be zoologists, botanists, or physiologists, the facts of heredity concern us. Upon this physiological function all the rest in some degree depend. Every advance in knowledge of that central function must affect the course of thought along each several line of biological inquiry.
Moreover though, as naturalists, we are not directly concerned with the applications of science, we must perceive that in no region of knowledge is research more likely to increase man's power over nature. The science of sociology, and in many of its developments the science of medicine also, must of necessity form working hypotheses respecting the course of heredity, and we cannot doubt that a perception of the truth in regard to the function of transmission will greatly contribute to the progress of these sciences. Lastly, to the industrial arts of the breeder of plants or animals, the knowledge we are attempting to provide is of such direct importance that upon this consideration no special emphasis is required. In studying heredity, therefore, we are examining a vital problem of no mean consequence, and those who engage in that work are happy in the thought that they are assisting one of the main advances in natural knowledge.
But though we may approach this study of genetics—to use the modern designation—from so many different sides, it is especially in their bearing on the problem of the evolution of species that the facts have hitherto been most profitably investigated. It was in the attempt to ascertain the interrelationships between species that experiments in genetics were first made. The words "evolution" and "origin of species" are now so intimately associated with the name of Darwin that we are apt to forget that the idea of a common descent had been prominent in the minds of naturalists before he wrote, and that, for more than half a century, zealous investigators had been devoting themselves to the experimental study of that possibility. Prominent among this group of experimenters may be mentioned Koelreuter, John Hunter, Herbert, Knight, Gaertner, Jordan, Naudin, Godron, Lecoq, Wichura—men whose names are familiar to every reader of Animals and Plants under Domestication. If we could ask those men to define the object of their experiments, their answer would be that they were seeking to determine the laws of hereditary transmission with the purpose of discovering the interrelationships of species. In addition to the observation of the visible structures and habits of plants and animals they attempted by experiment to ascertain those hidden properties of living things which we may speak of as genetic, properties which breeding tests can alone reveal. The vast mass of observation thus accumulated contains much that is of permanent value, hints that if followed might have saved their successors years of wasted effort, and not a few indications which in the light of later discovery will greatly accelerate our own progress.
Yet in surveying the work of this school we are conscious of a feeling of disappointment at the outcome. There are signs that the workers themselves shared this disappointment. As we now know, they missed the clue without which the evidence so laboriously collected remained an inscrutable medley of contradictions.
While the experimental study of the species problem was in full activity the Darwinian writings appeared. Evolution, from being an unsupported hypothesis, was at length shown to be so plainly deducible from ordinary experience that the reality of the process was no longer doubtful. With the triumph of the evolutionary idea, curiosity as to the significance of specific differences was satisfied. The Origin was published in 1859. During the following decade, while the new views were on trial, the experimental breeders continued their work, but before 1870 the field was practically abandoned.
In all that concerns the problem of species the next thirty years are marked by the apathy characteristic of an age of faith. Evolution became the exercising-ground of essayists. The number indeed of naturalists increased tenfold, but their activities were directed elsewhere. Darwin's achievement so far exceeded anything that was thought possible before, that what should have been hailed as a long-expected beginning was taken for the completed work. I well remember receiving from one of the most earnest of my seniors the friendly warning that it was waste of time to study variation, for "Darwin had swept the field."
Parenthetically we may notice that though scientific opinion in general became rapidly converted to the doctrine of pure selection, there was one remarkable exception. Systematists for the most part kept aloof. Everyone was convinced that natural selection operating in a continuously varying population was a sufficient account of the origin of species except the one class of scientific workers whose labours familiarised them with the phenomenon of specific difference. From that time the systematists became, as they still in great measure remain, a class apart.
A separation has thus been effected between those who lead theoretical opinion and those who by taste or necessity have retained an acquaintance with the facts. The consequences of that separation have been many and grievous. To it are to be traced the extraordinary misapprehensions as to the fundamental phenomena of specific difference which are now prevalent.
If species had really arisen by the natural selection for impalpable differences, intermediate forms should abound, and the limits between species should be on the whole indefinite. As this conclusion follows necessarily from the premisses, the selectionists believe and declare that it represents the facts of nature. Differences between species being by axiom indefinite, the differences between varieties must be supposed to be still less definite. Consequently the conclusion that evolution must proceed by insensible transformation of masses of individuals has become an established dogma. Systematists, entomologists or botanists for example, are daily witnesses to variation occurring as an individual and discontinuous phenomenon, but they stand aside from the debate ; and whoever in a discussion of evolutionary theory appeals to the definiteness of varietal distinctions in colour for instance, or in form, as recognizable by common observation without mechanical aid, must be prepared to meet a charge of want of intelligence or candour. This is no doubt a passing phase and will end so soon as interest in the problems of evolution is combined with some knowledge of variation and heredity.
Genetic experiment was first undertaken, as we have seen, in the hope that it would elucidate the problem of species. The time has now come when appeals for the vigorous prosecution of this method should rather be based on other grounds. It is as directly contributing to the advancement of pure physiological science that genetics can present the strongest claim. We have an eye always on the evolution-problem. We know that the facts we are collecting will help in its solution; but for a period we shall perhaps do well to direct our search more especially to the immediate problems of genetic physiology, the laws of heredity, the nature of variation, the significance of sex and of other manifestations of dimorphism, willing to postpone the application of the results to wider problems as a task more suited to a maturer stage. When the magnitude and definiteness of the advances already made in genetics come to be more generally known, it is to be anticipated that workers in various departments of biology will realise that here at last is common ground. As we now know, the conceptions on which both the systematists and the speculative biologists have based their methods need complete revision in the light of the new facts, and till the possibilities of genetic research are more fully explored the task of reconstruction can hardly be begun. In that work of exploration all classes of naturalists will alike find interest. The methods are definite and exact, so we need not fear the alienation of those systematists to whom all theoretical inquiry is repulsive. They are also wide in their scope, and those who would turn from the details of classification as offering matter too trivial for their attention may engage in genetic inquiries with great confidence that every fragment of solid evidence thus discovered will quickly take its place in the development of a coordinated structure.
Some pre-Mendelian Writings.
Of the contributions made during the essayist period three call for notice: Weismann deserves mention for his useful work in asking for the proof that "acquired characters"—or, to speak more precisely, parental experience—can really be transmitted to the offspring. The occurrence of progressive adaptation by transmission of the effects of use had seemed so natural to Darwin and his contemporaries that no proof of the physiological reality of the phenomenon was thought necessary. Weismann's challenge revealed the utter inadequacy of the evidence on which these beliefs were based. There are doubtless isolated observations which may be interpreted as favouring the belief in these transmissions, but such meagre indications as exist are by general consent admitted to be too slight to be of much assistance in the attempt to understand how the more complex adaptative mechanisms arose. Nevertheless it was for the purpose of elucidating them that the appeal to inherited experience was made. Weismann's contribution, though negative, has greatly simplified the practical investigation of genetic problems.
Though it attracted little attention at the time of its appearance, an honourable place in the history of our science must be accorded to the paper published by de Vries (1889) under the title Intracellulare Pangenesis. This essay is remarkable as a clear foreshadowing of that conception of unit-characters which is destined to play so large a part in the development of genetics.
The supreme importance of an exact knowledge of heredity was urged by Galton in various writings published during the period of which I am speaking. He pointed out that the phenomena manifested regularity, and he made the first comprehensive attempt to determine the rules they obey. It was through his work and influence that the existence of some order pervading the facts became generally recognized. In 1897 he definitely enunciated his now famous "Law" of heredity, which declared that to the total heritage of the offspring the parents on an average contribute ½, the grandparents ¼, and the great-grandparents 1/8, and so on, the total heritage being taken as unity. To this conclusion he had been led by several series of data, but the evidence upon which he especially relied was that of the pedigrees of Basset Hounds furnished him by the late Sir Everett Millais. In that instance the character considered was the presence or absence of black in addition to yellow and white. The colours were spoken of as tri-colour and non-tri-colour, and the truth of the law was tested by the average numbers of the respective colours which resulted from the various matings of dogs of known ancestral composition. These numbers corresponded so well with the expectations given by the law as to leave no reasonable doubt that the results of calculation were in general harmony with natural fact.
There are features in this important case which need special consideration, and to these I will return. Meanwhile we may note that though there was admittedly a statistical accord between Galton's theory and some facts of heredity, yet no one familiar with breeding or even with the literature of breeding could possibly accept that theory as a literal or adequate presentation of the facts. Galton himself in promulgating it made some reservations; but in the practice of breeding, so many classes of unconformable phenomena were already known, that while recognizing the value of his achievement, we could not from the first regard it as more than an adumbration of the truth. As we now know, Galton's method failed for want of analysis. His formula should in all probability be looked upon rather as an occasional consequence of the actual laws of heredity than in any proper sense one of those laws.
Of the so-called investigations of heredity pursued by extensions of Galton's non-analytical method and promoted by Professor Pearson and the English Biometrical school it is now scarcely necessary to speak. That such work may ultimately contribute to the development of statistical theory cannot be denied, but as applied to the problems of heredity the effort has resulted only in the concealment of that order which it was ostensibly undertaken to reveal. A preliminary acquaintance with the natural history of heredity and variation was sufficient to throw doubt on the foundations of these elaborate researches. To those who hereafter may study this episode in the history of biological science it will appear inexplicable that work so unsound in construction should have been respectfully received by the scientific world. With the discovery of segregation it became obvious that methods dispensing with individual analysis of the material are useless. The only alternatives open to the inventors of those methods were either to abandon their delusion or to deny the truth of Mendelian facts. In choosing the latter course they have certainly succeeded in delaying recognition of the value of Mendelism, but with the lapse of time the number of persons who have themselves witnessed the phenomena has increased so much that these denials have lost their dangerous character and may be regarded as merely formal.
Rediscovery of Mendel: his Method.
With the year 1900 a new era begins. In the spring of that year there appeared, within a few weeks of each other, the three papers of de Vries, Correns, and Tschermak, giving the substance of Mendel's long-forgotten treatise. Each of these three writers was able from his own experience to confirm Mendel's conclusions, and to extend them to other cases. There could therefore, from the first, be no question as to the truth of the facts. To appreciate what Mendel did the reader should refer to the original paper, which is a model of lucidity and expository skill. His success is due to the clearness with which he thought out the problem. Being familiar with the works of Gaertner and the other experimental breeders he surmised that their failure to reach definite and consistent conclusions was due to a want of precise and continued analysis. In order to obtain a clear result he saw that it was absolutely necessary to start with pure-breeding, homogeneous materials, to consider each character separately, and on no account to confuse the different generations together. Lastly he realised that the progeny from distinct individuals must be separately recorded. All these ideas were entirely new in his day. When such precautions had been observed he anticipated that a regular result would be attainable if the experiments were carried out on a sufficient scale.
After several preliminary trials he chose the edible Pea (Pisum sativum) for his subject. Varieties in cultivation are distinguished by striking characters recognizable without trouble. The plants are habitually self-fertilised, a feature which obviates numerous difficulties.
Following his idea that the heredity of each character must be separately investigated, he chose a number of pairs of characters, and made crosses between varieties differing markedly in respect of one pair of characters. The case which illustrates Mendelian methods in the simplest way is that in which heredity in respect of height was studied. Mendel took a pair of varieties of which one was tall, being 6—7 feet high, and the other was dwarf, ¾ to I ½ feet. These two were then crossed together. In peas this is an easy operation. The unbroken anthers can be picked out of a bud with a pair of fine forceps and the pollen of the plant chosen for the father may be at once applied to the stigma of the emasculated flower. The cross-bred seeds thus produced grew into plants which were always tall, having a height not sensibly different from that of the pure tall variety. In our modern terminology such a cross-bred, the first filial generation, is called F1. From the fact that the character, tallness, appears in the cross-bred to the exclusion of the opposite character, Mendel called it a dominant character; dwarfness, which disappears in the F1 plant, he called recessive.
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