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History of Genetics - Gregor Johann Mendel, Rediscovery, The Chromosome Theory Of Heredity, Culture Of Heredity, Fine Structure Of The Gene

variation view offspring doctrine

Genetics as a discipline is young, but the concept that forms its subject—inheritance—stretches back in time. The word has been formed from the adjective genetic, found in the sciences of the nineteenth and twentieth centuries—for example, biogenetic law, genetic affinity, genetic psychology—and meaning, according to the Oxford English Dictionary, "pertaining to, or having references to, origin." Not until 1906 was the noun genetics publicly proposed to cover those labors that, in the words of its author, William Bateson, "are devoted to the elucidation of the phenomena of heredity and variation: in other words to the physiology of descent, with implied bearing on the theoretical problems of the evolutionist and the systematist, and application to the practical problems of breeders, whether of animals or plants."

We begin, therefore, by commenting briefly on the long history of notions of heredity and variation, reflecting the while on the significance of cultural and economic factors that both drew attention to them and shaped them. Next we turn to the father figure of genetics, Gregor Mendel (1822–1884), and his introduction of the Mendelian experiment. With the rediscovery of his work in 1900 we explore the contribution of the early "Mendelians," the melding of Mendelian heredity with the theory of the chromosome, a synthesis that revealed a geography of heredity in the cell but did not answer the question, "What is the identity of the genetic material?" Only with the introduction of the Watson-Crick structure for DNA in 1953 was a window opened through which to glimpse the terrain that was to become "molecular genetics."

Heredity and variation are today considered as two sides of the same coin. Thus variation among sibs results from the varied commingling and expression of the hereditary determinants from two parents. Spontaneous changes in the hereditary material (mutations) give rise to variations, and these are inherited. In other words there is heredity, variation, and the heredity of variation, and they belong together. Prior to the latter half of the nineteenth century this conceptual framework did not exist. There still lingered relics of the ancient view of heredity as the result of the reproduction of the type, whereas any deviance from the type was ascribed to the effects of the mother's imagination, changed conditions of life, and so on. Moreover both heredity and variation were situated within the broad topic of "generation." This included the regeneration of lost and damaged organs, the development of the embryo from the fertilized egg, and all forms of reproduction, both sexual and asexual. But as attention directed to human heredity increased during the nineteenth century the subject of the transmission of hereditary characters began to acquire a separate conceptual status.

In the eighteenth century two rival conceptions of the phenomenon of inheritance were in play: the doctrine of preformation claimed that like generates like because the offspring are in some way already present in the germ and have only to unfold or "evolve" to yield offspring like the parents. A literal view of this doctrine was held by some who claimed that hidden miniatures of future generations must have existed from the time of creation, all nested one within another, like a set of Russian dolls, hence the term preexistence for this version of the doctrine. Procreation became, as it were, the act of revealing what had been created long before. Opposed to this was the doctrine of epigenesis, according to which the parts of the offspring are formed successively, and in the process the embryo undergoes a series of transformations. Support for this view came from descriptions of the development of the fertilized egg. Experimental support came also from the hybridization experiments of Joseph Kölreuter, who in 1766 described how he had succeeded in transforming one species of plant into another. How, he asked, was this possible if the offspring is already preformed? He also found that no matter which parent supplied the "male seed" (pollen) and which the female "seed" (ovules), the resulting hybrids were the same. How could these results be accounted for on the basis of any theory of preformation—whether, as the animalculists claimed, the preformed miniatures reside in the spermatozoa, or as the ovists urged, in the egg? A further difficulty arose for preformationists when nineteenth-century microscopists developed a cell theory according to which the fundamental unit of life is the cell, not some preformed rudiments of the organism.

These two resources—hybridization and the cell theory—were to form the basis for Gregor Mendel's research. Prior to his work, however, the study of hybridization had been carried out under different assumptions and in a much more limited manner. The number of hybrid offspring grown in an experiment was limited, and the prevalence of the effect of hybridization in diluting character differences supported the view that heredity is a blending process, like the mixing of ink and water. The conception of the organism too was generally holistic: that is, the type of the species acts as a whole, its "essential" characteristics—flower color, habit, leaf shape, and so on—being but expressions of the type. With this point of view, effective analysis of hereditary transmission is impossible.

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over 5 years ago

i m in search of mendal law of segregation and independent assaortment