GENETICS

the study of heredity in general and of genes in particular. Although the influence of heredity has been recognized since prehistoric times, scientific understanding of inheritance is a fairly recent event. Modern genetics began with the work of Gregor Mendel, an Austrian monk whose breeding experiments with garden peas led him to formulate the basic laws of heredity. Mendel concluded that his plants inherited two factors (one from each parent) for each of the hereditary traits he studied. He further deduced that these factors do not mix in the offspring, that some factors are dominant over others, and that a parent plant randomly transmits one factor from each pair to an offspring. Mendel published his findings in 1866, but his discoveries were not appreciated by the scientists of his day. By the turn of the century, however, the intellectual climate had changed; in 1900 a number of researchers independently rediscovered Mendel's work and grasped its significance. The infant science of genetics flowered rapidly. By 1902 Walter Sutton of the United States had proposed that chromosomesmajor components of the cell nucleuswere the site of Mendel's hereditary factors. The Hardy-Weinberg law, which established the mathematical basis for studying heredity in populations, was independently formulated by the English mathematician Godfrey H. Hardy and the German physician Wilhelm Weinberg in 1908. In 1910 the American geneticist Thomas Hunt Morgan began his studies with the fruit fly, Drosophila melanogaster. Morgan provided evidence not only that genes (as Mendel's factors had come to be called) occur on chromosomes but that those genes lying close together on the same chromosome form linkage groups that tend to be inherited together. During the 1940s George W. Beadle and Edward L. Tatum of the United States demonstrated that genes exert their influence by directing the production of enzymes, proteins that facilitate chemical reactions in the cell. By 1944 Oswald T. Avery had shown that deoxyribonucleic acid (DNA) was the chromosome component that carried genetic information. The molecular structure of DNA, however, was not deduced until 1953 by James D. Watson of the United States and Francis H.C. Crick of Great Britain. By 1961 the French geneticists Franois Jacob and Jacques Monod had developed a model for the process by which DNA directs protein synthesis in bacterial cells. These developments led to the deciphering of the genetic code of the DNA molecule, which in turn made possible the recombinant DNA techniques that hold immense potential for genetic engineering (q.v.). Modern genetics studies include population genetics (the study of genetic patterns within populations), classical genetics (how traits are transmitted and expressed), cytogenetics (the mechanics of heredity within the cell), microbial genetics (the heredity of microorganisms), and molecular genetics (the molecular study of genes and related structures). To some extent, these divisions are artificial; every field overlaps with other genetic fields, and all have implications for the other biological sciences. Genetics has been applied to the diagnosis, prevention, and treatment of hereditary diseases; to the breeding of plants and animals; and to the development of industrial processes that utilize microorganisms. study of heredity in general and of genes in particular. Since prehistoric times, man has recognized the influence of heredity and has applied its principles to the improvement of cultivated crops and domestic animals. A Babylonian tablet more than 6,000 years old, for example, shows pedigrees of horses and indicates possible inherited characteristics; other old carvings show cross-pollination of date palm trees. Most of the mechanisms of heredity, however, remained a mystery until the 20th century, when scientifically supported information became available. Genetics may be defined as the study of the way in which genes operate and the way in which they are transmitted from parents to offspring. Modern genetics involves study of the mechanism of gene actionthe way in which the genetic material (deoxyribonucleic acid, or DNA) affects physiological reactions within the cell. Although genes determine the features an individual may develop, the features that actually develop depend upon the complex interaction between genes and their environment. Normal green plants, for example, have genes containing the information necessary to synthesize the chlorophyll that gives them their green colour, and chlorophyll is synthesized in an environment containing light; i.e., the gene for chlorophyll is expressed. If the plant is placed in a dark environment, chlorophyll synthesis stops; i.e., the gene is no longer expressed. Genetics overlaps many different branches of biology and many other sciences; e.g., chemistry, physics, mathematics, sociology, psychology, and medicine. Microbiologists who study inheritance in microorganisms are called microbial geneticists; cytologists who study the genetics of cells are called cytogeneticists. Biochemical, or molecular, geneticists investigate the chemical nature of the gene and its methods of action. Some physicists have applied their techniques to molecular genetics, and mathematicians may specialize in population genetics. Behavioral scientists also look to genetics to solve certain problems of human and animal behaviour. Specialists in medical genetics or genetic counselling act on the knowledge that many of man's afflictions are hereditary. Additional reading David A. Micklos and Greg A. Freyer, DNA Science: A First Course in Recombinant DNA Technology (1990), provides an introductory guide that covers well the history, concepts, and applications of molecular biology. Paul Berg and Maxine Singer, Dealing with Genes: The Language of Heredity (1992), is a well-illustrated overview of molecular genetics and its relationship with developmental biology, medicine, and biochemistry. Anthony J.F. Griffiths et al., An Introduction to Genetic Analysis, 5th ed. (1993); and Benjamin Lewin, Genes V (1994), emphasize the molecular aspects of genetics, although the former work deals more extensively with population genetics, quantitative genetics, and evolution. A complementary work is Roger L.P. Adams, DNA Replication (1991). R.W. Old and S.B. Primrose, Principles of Gene Manipulation: An Introduction to Genetic Engineering, 5th ed. (1994), is one of the best texts dealing exclusively with genetic engineering. James D. Watson et al., Recombinant DNA, 2nd ed. (1992), provides a relatively short but extremely useful treatment of the methods and applications of recombinant DNA technology and other current methods in molecular genetics; to some extent it updates an earlier work, James D. Watson et al., Molecular Biology of the Gene, 4th ed., 2 vol. (1987), which still retains utility despite having been overtaken by the sheer momentum of theory and technology in this subject.Gunther S. Stent and Richard Calendar, Molecular Genetics: An Introductory Narrative, 2nd ed. (1978), although out of date in many parts, nonetheless provides a history of genetics beginning with medieval observations on inheritance and traces progress to the late 1970s. S. Brenner (compiler), Molecular Biology: A Selection of Papers (1989), collects a series of the most important research publications on molecular genetics appearing in the Journal of Molecular Biology from the 1950s onward. The best historical surveys of molecular biology are Horace Freeland Judson, The Eighth Day of Creation: Makers of the Revolution in Biology (1979), which concentrates on research after World War II; and Robert Olby, The Path to the Double Helix (1974, reissued 1994), which focuses on research from the 1920s to the 1950s. For the same period, Robert E. Kohler, Lords of the Fly: Drosophila Genetics and the Experimental Life (1994), studies the research and culture of early Drosophila geneticists; while Lily E. Kay, The Molecular Vision of Life (1993), examines the funding and politics behind the rise of molecular biology. Nina Fedoroff and David Botstein (eds.), The Dynamic Genome: Barbara McClintock's Ideas in the Century of Genetics (1992), treats this scientist's work and the history of modern genetics. McClintock's research is also ably described in Evelyn Fox Keller, A Feeling for the Organism (1983, reissued 1993). Zhores A. Medvedev, The Rise and Fall of T.D. Lysenko (1969), documents a particularly negative chapter in the history of Soviet genetic study, the promulgation of the idea of the inheritance of acquired characters. James D. Watson, The Double Helix: A Personal Account of the Discovery of the Structure of DNA (1968), available also in a critical edition ed. by Gunther S. Stent (1980), is written by one of the discoverers. Anne Sayre, Rosalind Franklin and DNA (1975); and Francis Crick, What Mad Pursuit (1988), give somewhat different accounts of the discovery of DNA structure. Works delineating the science and sociology of the discovery are John Cairns, Gunther S. Stent, and James D. Watson (eds.), Phage and the Origins of Molecular Biology, expanded ed. (1992); and Maclyn McCarty, The Transforming Principle: Discovering That Genes Are Made of DNA (1985). Paul Rabinow, Making PCR: A Story of Biotechnology (1996), is an ethnographic account of the discovery of one of the most important tools in contemporary genetics and molecular biology, the polymerase chain reaction.Bernadette Modell and Michael Modell, Towards a Healthy Baby: Congenital Disorders and the New Genetics in Primary Care (1992), summarizes knowledge about many common genetic disorders in humans and the health approaches and policies regarding their diagnosis and treatment. Richard Dawkins, The Selfish Gene, rev. ed. (1989), is an easily read book espousing the author's views on evolution.The rise of contemporary molecular genetics, which now extends to all known cell types, had its inception in studies on bacterial genetics and, in particular, on viruses that infect bacteria. Those foundational studies have made bacteria the best-known organisms from a genetic point of view. Arnold J. Levine, Viruses (1992), is an excellent treatment. Fundamental ideas about bacterial structure and function from a molecular genetic viewpoint are presented in Thomas D. Brock, The Emergence of Bacterial Genetics (1990), a detailed perspective; T.A. Brown (ed.), Essential Molecular Biology: A Practical Approach, 2 vol. (1991), much of which is concerned with the genetics of bacteria; Stephen Cooper, Bacterial Growth and Division: Biochemistry and Regulation of Prokaryotic and Eukaryotic Division Cycles (1991), an important contribution; John L. Ingraham, Ole Maale, and Frederick C. Neidhardt, Growth of the Bacterial Cell (1983); and Frederick C. Neidhardt, John L. Ingraham, and Moselio Schechter, Physiology of the Bacterial Cell: A Molecular Approach (1990). The Editors of the Encyclopdia Britannica