HEREDITY

sum of all biological processes by which particular characteristics are transmitted from parents to their offspring. Among organisms that reproduce sexually, progeny are not exact duplicates of their parents but usually vary in many traits. Heredity and variation, two sides of the same coin, are the subject matter of the science of genetics. Genetics may be defined as the study of the way in which genes-the functional units of heritable material-operate and are transmitted from parents to offspring. Modern genetics also involves study of the mechanism of gene action; that is, the way in which the genetic material affects physiological reactions within the cell. In many languages the same words are used for both the inheritance of biological traits and the inheritance of property. Biological and legal inheritances are, however, very different processes. Inherited objects are actually transferred from one owner to another. Inherited traits are not. Offspring inherit a genetic constitution from their parents. This hereditary endowment, the sum total of the genes that the individual has received from both parents, is called the genotype. The genotype must be contrasted to the phenotype, which is the organism's outward appearance: its bodily structures, physiological processes, behaviour, etc. Although the genotype determines the broad limits of the features an organism may develop, the features that actually develop-i.e., the phenotype-depend upon complex interactions between genes and their environment. Since the environment, both internal and external, of an individual changes continuously, so does the phenotype. Thus the same individual shows different phenotypes in childhood, in adulthood, and in old age. The genotype, on the other hand, does not change during an individual's lifetime. In conducting genetic studies it is crucial to discover the degree to which the observable trait (the phenotype) is attributable to the pattern of genes in the cells (the genotype) and to what extent it arises from environmental influence. The essence of heredity is the reproduction of the carriers of genetic information, the genes. As a result, biological organisms, including human beings, reproduce organisms resembling themselves; human children are always recognizably human and have phenotypes similar to those of their parents. On the other hand, since the offspring of sexually reproducing organisms receive varying combinations of genetic material from both parents, no two offspring (except for identical twins) have exactly the same genotype. This genetic diversity is always modified by an equally diverse environment, so the resulting phenotype is never exactly the same, even among identical twins. Genetics is often called the core science of biology. This does not necessarily mean that genetics is the most fundamental among the biological disciplines. It implies only that genetics impinges upon almost every kind of study of life. Anthropology, medicine, biochemistry, physiology, psychology, ecology, systematics, comparative morphology, and paleontology all have intersections with genetics. Like so many basic, or "theoretical," sciences, genetics has many actual and potential practical applications. The understanding and control of hereditary disorders and the breeding of improved crops and livestock are just two such applications. Knowledge of heredity dates to prehistoric times and has been applied to the breeding of plants and animals for centuries. Most of the mechanisms of heredity, however, remained a mystery until the 20th century. The pioneering work in elucidating the mechanisms of gene action took place even more recently, and the science of genetics is considered as yet in its infancy. This article examines the discoveries that led to an understanding of heredity and discusses in detail the structure and function of the gene and mutation and other processes by which genetic information is altered. the transmission of physical and mental characteristics, or traits, from parents to offspring through basic units called genes. Until the late 19th century, it was widely believed that hereditary traits were the result of a mixing of parental characteristics. According to this line of thought, a short woman and tall man would have children of medium height. In 1866, however, Gregor Mendel, an Austrian monk, published the results of his cross-breeding experiments with pea plants. From his studies, Mendel derived certain basic laws of heredity: hereditary factors (now called genes) do not mix but remain segregate; some factors are dominant, while others are recessive; each member of the parental generation transmits only half of its hereditary factors to each offspring; and different offspring of the same parents receive different sets of genes. Mendel's work went unnoticed until 1900, years after his death, when it became the foundation for the modern science of genetics. To fully appreciate the role of heredity, one must make the important distinction between an organism's genotype and its phenotype. The genotype is an individual's total hereditary makeup. It may include recessive genes whose traits are not expressed but which can be passed to future generations; moreover, except for mutations, genotype does not change during an individual's life. By contrast, the phenotype is the visible characteristics of an organism. It is the product of complex interactions between the individual's genetic makeup and the environment. For example, a tree might have a genetic endowment for tallness but grow as a twisted shrub on a cold, windswept mountain slope. Unlike genotype, phenotype is highly changeable. A person may inherit genes for pale or dark skin, for example, but the individual's skin colour at a particular time depends heavily on exposure to the sun. The translation of genetic information to phenotypic traits is a complex process. Genes exert their influence at the cellular level by directing the production of enzymes. These, in turn, regulate the metabolic activities of the cell. Many characteristics are polygenic-i.e., influenced by more than one gene. In addition, many genes exist in numerous variations (alleles) throughout a population. Given the polygenic and multiple allelic nature of many traits, it is easy to see the vast potential for variability among hereditary characteristics. Additional reading Classical genetics Theodosius Dobzhansky, Heredity and the Nature of Men (1964); Theodosius Dobzhansky et al., Evolution (1977); and I. Michael Lerner and William J. Libby, Heredity, Evolution, and Society, 2nd ed. (1976), are excellent discussions of classical genetics and its social and cultural implications. Curt Stern, Genetic Mosaic, and Other Essays (1968), is a group of historical essays by a leading authority who discusses the development of knowledge on hermaphrodites and the relation of general to human genetics. James A. Peters (ed.), Classic Papers in Genetics (1959), is a collection of papers extending from 1865 (Mendel) to 1966 (Benzer) that form the cornerstone of classical Mendelian genetics. Archibald E. Garrod, Inborn Errors of Metabolism, ed. by Harry Harris (1963), is a newer edition of this classic work. Genetic texts George P. Rdei, Genetics (1982); and Laura Livingston Mays, Genetics: A Molecular Approach (1981), provide good, up-to-date overview treatments for students. Theodore T. Puck, The Mammalian Cell as a Microorganism: Genetic and Biochemical Studies in Vitro (1982), presents a lucid introduction to the field of somatic cell genetics. James D. Watson, Molecular Biology of the Gene, 3rd ed. (1976), gives a thorough introduction to molecular biology. Richard L. Davidson and Felix F. De La Cruz (eds.), Somatic Cell Hybridization (1974), is a collection of papers presented at a symposium on somatic cell genetics, interspecies hybrids, and gene localization; see also Richard L. Davidson (ed.), Somatic Cell Genetics (1984); Robert T. Schimke (ed.), Gene Amplification (1982); and Thomas J. Silhavy, Michael L. Berman, and Lynn W. Enquist, Experiments with Gene Fusions (1984). H. Hugh Fudenberg et al., Basic Immunogenetics, 3rd ed. (1984), is a comprehensive technical discussion.The following articles in the monthly Scientific American are readable and well illustrated: F.H.C. Crick, "The Genetic Code," 207(4):66-74 (October 1962), and "The Genetic Code III," 215(4):55-62 (October 1966); Pierre Chambon, "Split Genes," 244(5):60-71 (May 1981); W. French Anderson and Elaine G. Diacumakos, "Genetic Engineering in Mammalian Cells," 245(1):106-121 (July 1981); Philip Leder, "The Genetics of Antibody Diversity," 246(5):102-115 (May 1982); J. Michael Bishop, "Oncogenes," 246(3):80-92 (March 1982); Arthur C. Upton, "The Biological Effects of Low-Level Ionizing Radiation," 246(2):41-49 (February 1982); Richard E. Dickerson, "The DNA Helix and How It Is Read," 249(6):94-111 (December 1983); James E. Darnell, Jr., "The Processing of RNA," 249(4):90-100 (October 1983); Tony Hunter, "The Proteins of Oncogenes," 251(2):70-79 (August 1984); Corey Goodman and Michael J. Bastiani, "How Embryonic Nerve Cells Recognize One Another," 251(6):58-66 (December 1984); Michael S. Brown and Joseph L. Goldstein, "How LDL Receptors Influence Cholesterol and Atherosclerosis," 251(5):58-66 (November 1984); Nina V. Fedoroff, "Transposable Genetic Elements in Maize," 250(6):84-98 (June 1984); and Richard H. Scheller and Richard Axel, "How Genes Control an Innate Behavior," 250(3):54-62 (March 1984). Arthur Robinson