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Radiotherapy => Deoxyribonucleic Acid => Heredity
Heredity
INTRODUCTION
Heredity, study of all those qualities of organisms that are governed by certain biologically active elements derived from the parents. Although the scientific and experimental study of heredity, called genetics, developed only in the early 20th century, theories about inheritance date from ancient Greece. Even before the so-called founder of modern genetics, the 19th-century Austrian monk Gregor Mendel, carried out his important work on inheritance in pea plants, hundreds of theories concerning fertilization and hybridization in plants-and, in the 18th century, animals as well-had been proposed. These theories helped to lay the foundation for the development of modern genetic theory.
BACKGROUND
In the early 19th century questions concerning evolutionary change focused on two issues: how characteristics are passed on faithfully from one generation to the next, and how variations in such characteristics occur and are passed on. One explanatory mechanism was developed by the French zoologist Jean-Baptiste de Lamarck, who believed that as the world's geographic and climatic areas change, new influences affect plant and animal life, and that these new influences, in turn, trigger new needs. As a result, new structures arise and old structures are modified. He believed that these modified structures, otherwise known as acquired characteristics, can be passed on to succeeding generations and are therefore inherited. Lamarck assumed that the results of such inheritance were cumulative from generation to generation, eventually producing new species.
In the later 19th century, theories of heredity were developed by such scientists as Charles Darwin, the German biologist Ernst Haeckel, the Dutch botanist Hugo De Vries, and the German biologist August Weismann. A major reason for this interest was that Darwin's theory of natural selection, first published in 1859, lacked a workable concept of heredity. Consequently, biologists became keenly aware of the need to understand how variations occur and to learn which variations are passed on to succeeding generations. Darwin himself postulated a theory of inheritance, called pangenesis, which maintained that the cells of the body produce minute particles-pangenes-that circulate through the body and pass into the male and female gametes, ultimately producing the cells of the next generation. In short, according to Darwin, each part of the body contributes a fraction of itself to the semen. In addition, Darwin as well as Lamarck believed in the inheritance of acquired characteristics. Weismann, by contrast, believed that the cells of the ovaries and testes, which give rise to egg and sperm, are not affected by changes in those tissues making up the rest of the body, and that acquired traits cannot be transmitted from parent to offspring. Weismann also postulated, incorrectly, the existence of a hierarchy of hereditary particles that are released successively during embryonic differentiation and compete with one another for supremacy.
An important approach to the problem of heredity before 1900 was biometrics-the measurement and statistical analysis of variation within populations. Biometricians emphasized the importance of quantitative measurements of physical characteristics and their distribution throughout populations, not only at a single point in time but over the course of two or more generations. Two of the main founders of biometrics were the British scientist Sir Francis Galton and his student Karl Pearson. Biometrics provided a way of analysing the inheritance of traits in a population without the use of breeding experiments.
20TH CENTURY
The rediscovery in 1900 of Mendel's 1866 paper on patterns of inheritance in the pea plant provided an important source of new ideas of heredity. From his work on crossing pea plants, Mendel arrived at two generalizations. The first was the law of segregation: in the formation of germ cells, the two factors (alleles) for any characteristic are always separated from each other and end up in a different egg or sperm. When the egg and sperm come together in a new individual the alleles form a gene pair often with a dominant gene and a recessive gene. The second generalization, later called the law of independent assortment, stated that the maternal and paternal factors for any set of characteristics segregate independently from those of any other set of characteristics; the expression of a gene for any single characteristic is not usually influenced by the expression of another characteristic.
One of the most important distinctions aiding in the development of heredity studies in general and of Mendelian principles in particular was that drawn between genotype and phenotype, as enunciated by the Danish botanist Wilhelm Johannsen in 1911. Genotype refers to the actual genes that an organism carries and is capable of passing on to the next generation. Phenotype refers to the actual appearance (in terms of traits) that an organism displays. Sometimes, but not always, phenotype reflects genotype, as in the case of doubly recessive genes; but if an organism has one dominant and one recessive gene, the phenotype is that of the dominant trait, thereby masking the presence of the recessive gene. The importance of this distinction lay in its stress on the fact that the only way to determine genotype is through breeding experiments, not simply through examination of an organism's phenotype.
After several years of experimenting with Drosophila melanogaster (the fruit fly) at Columbia University, the American scientist Thomas Hunt Morgan-along with three brilliant graduate students, Alfred Henry Sturtevant, Calvin Blackman Bridges, and Hermann Joseph Muller-helped to establish the chromosome theory of heredity. The Morgan group proposed that Mendelian factors are arranged in a linear fashion on chromosomes, thereby establishing the physical reality of the genes as discrete particles. A good portion of the work done on heredity between 1910 and 1925 was devoted to the complex relationships among these invisible chromosomes.
After World War II the study of heredity reached a high point of development, as biologists began to delve into the nature of the gene itself. That nucleic acids are the principal substances of heredity and that they seem to work by guiding the synthesis of protein became clear in the 1940s and 1950s. The Watson-Crick model of deoxyribonucleic acid (DNA), first proposed in 1953, accounted beautifully for the major genetic, biochemical, and structural characteristics of the hereditary material. The bulk of work on heredity since 1953 has focused on the enormous intricacies and complexities involving the function of DNA, including its self-regulatory processes, and its evolution.
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