History of Genetics: Ancient and Classical Views of Heredity
The term genetics was coined at the beginning of the twentieth century to separate new forms of scientific inquiry from previous studies of generation, inheritance, or heredity. Classical genetics was originally part broad area of science known as "generation," a term that originally encompassed the study of reproduction, embryology, development and differentiation, regeneration of parts, and genetics. As early as the sixth century B.C., Greek philosophers had begun to search for explanations of how and why the world and human beings came to be formed and organized as they were. The work of Socrates (470-399 B.C.), Plato (429-347 B.C.), and Aristotle (384-322 B.C.) established the foundations of Western science and philosophy. Aristotle's major biological works, including On the Generation of Animals, raised fundamental questions about reproduction, development, and heredity. By recognizing the value of studying organisms in detail, Aristotle initiated a fruitful approach to philosophy and biology.
One of Aristotle's goals was to create a natural scheme of classification by determining and weighing many characteristic traits. He concluded that the most important measure of biological affinities among diverse species was their method of reproduction: sexual and asexual reproduction, and spontaneous generation. According to Aristotle, the female parent contributed only unorganized matter to the new individual while the male provided the form. In attempting to explain how animals produce offspring like themselves, Aristotle examined two major models of development known as preformation and epigenesis. Preformationist theories hold that a miniature individual preexists in either the egg or the semen and begins to grow into its adult form when properly stimulated. The theory of epigenesis, which Aristotle favored, asserts that the new organism is gradually produced from an undifferentiated mass by the addition of parts.
Artistotle's theories about form and matter, male and female roles, epigenesis, and so forth formed the core of biological thought for hundreds of years. Indeed, ideas about generation changed very little change from the time of Aristotle to the publication of On the Generation of Animals (1651) by William Harvey (1578-1657), who was the first naturalist to argue that all living beings arose from eggs.
Preformationist theories enjoyed a prominent place in eighteenth century science, but preformationism allows only one parent to serve as the source of the new individual. Some naturalists objected to preformationist theory because of observations that indicated that both parents contributed to the traits of their offspring.
A new way of thinking about heredity, fertilization, and development was made possible by the establishment of cell theory in the 1830s. The establishment of cell theory is generally attributed to Matthias Jacob Schleiden (1804-1881) and Theodor Schwann (1810), who recognized the importance of Robert Brown's (1773-1858) discovery the cell nucleus. Further investigations during the last quarter of the nineteenth century provided many insights into the role played by the nucleus during cell division, and the recognition of fundamental cytological phenomena such as mitosis, maturation, and fertilization and important cellular organelles, such as mitochondria, chloroplasts, and the Golgi apparatus.
Cytological studies led to the discoveries that linked cytology to inheritance and development. Based on these studies, August Weismann (1834-1914) proposed the theory of the continuity of the germplasm and predicted the reduction division of the chromosomes during the formation of the germ cells. In Cell-Formation and Cell-Division (1875) Eduard Strasburger (1844-1912) described the division of plant cells. Walter Flemming's (1843-1905) Cell Substance, Nucleus, and Cell Division (1882) established a basic framework for the stages of cell division. Flemming used the term chromatin for the nuclear substance and gave the name mitosis to cell division. Heinrich W. G. Waldeyer (1836-1921) introduced the term chromosome in 1888.
Many scientists were interested in theories of heredity in connection with Charles Darwin's controversial theory of evolution. Darwin's own speculations about heredity raised interest in the problem of variation, but the work most closely associated with the development of modern genetics was the study of plant hybridization, a traditional approach used by both practical horticulturists and naturalists. Joseph Gottlieb Koelreuter (1733-1806) was one of the first botanists to systematically make and test hybrids. Koelreuter's work was extended by Carl Friedrich von Gaertner (1772-1850).
In the 1860s, Gregor Mendel carried out a remarkable series of hybridization experiments and systematically analyzed the results of his tests. Although Mendel is generally regarded as the founder of modern genetics, and the basic laws of genetics (segregation and independent assortment) are known as Mendel's laws, his work was ignored for almost 40 years. In a practical sense, classical genetics began not with the publication of Mendel's papers, but in 1900 with the rediscovery of his laws of inheritance by Hugo de Vries (1848-1935), Carl Correns (1864-1935), and Erik von Tschermak (1871-1962). During the intervening years, developments in the study of cell division, fertilization, and the behavior of subcellular structures had established a new framework capable of accommodating Mendel's statistical patterns.
Many of the terms now used by geneticists were introduced by William Bateson, including the word genetics (coined in 1905 from the Greek, for descent), allelomorph (allele), zygote, homozygote, and heterozygote. In 1909 Wilhelm L. Johannsen introduced the term gene to replace older terms like factor, trait, and character. To clarify other aspects of the new science of genetics, Johannsen coined the terms phenotype and genotype, which are now used to indicate the appearance of the individual and its actual genetic makeup, respectively.
By the turn of the century, some scientists had begun to suspect that there was a relationship between the classical factors tabulated in breeding experiments and the behavior of the chromosomes in mitosis and meiosis. It was known that chromosomes occurred in homologous pairs that conjugated and then separated during the formation of germ cells. By 1910, some biologists argued that the behavior of the chromosomes during gamete (egg and sperm) formation and the process of fertilization indicated that the paired factors could be on the paired chromosomes contributed by egg and sperm. This cytological approach was supported by the work of Walter S. Sutton, Theodor Boveri, Nettie M. Stevens, and Edmund B. Wilson. The work of Boveri, Sutton, Stevens, and others suggested that the chromosomes were individuals in the morphological and the functional sense. Moreover, cytology and Mendelian breeding studies complemented each other in supporting the hypothesis that the heredity Mendelian factors were actually constituents of the chromosomes.
The chromosome theory, also known as the theory of the gene, was established by Thomas Hunt Morgan. Working with mutants of the fruit fly, Drosophila melanogaster,Morgan and his coworkers proved that genes are located on the chromosomes in a specific linear sequence. Morgan's experiments demonstrated linkage between genes on the same chromosome and recombination (due to crossing over) between genes on paired paternal and maternal chromosomes. F. A. Janssens's (1863-1924) chiasmatype hypothesis provided a cytological basis for the recombination of genetic factors.
Research in Morgan's laboratory led to the construction of the first chromosome map, which was based on the relationship between the strength of linkage of genes and their linear sequence on the chromosome. By 1915 Morgan's group had described four groups of linked factors which corresponded to the four pairs of Drosophilachromosomes. Morgan assigned five principles to the gene: segregation, independent assortment, crossing over, linear order, and linkage groups.
By 1927 Hermann Joseph Muller had demonstrated that it was possible to induce mutations in Drosophilaby means of radiation. Using X-rays, Muller produced several hundred mutants in a short time. Most of these induced mutations were stable over many generations and behaved like typical Mendelian factors when subjected to breeding tests.
Insights provided by classical genetics during the first half of the twentieth century made it possible for scientists to ask questions about the chemical nature of the gene and its mechanism of action. Classical genetics could answer questions about how genes were transmitted, but it could not answer questions about how genes work. Since the 1950s, studies of the molecular biology of the gene have provided new answers to the fundamental questions about the mechanism of inheritance and the relationship between genes and gene products.
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