GENETIC DISEASE, HUMAN

any of the diseases and disorders that are caused by mutations in one or more genes. With the increasing ability to control infectious and nutritional diseases in the so-called developed countries, there has come the realization that genetic diseases are a major cause of disability, death, and human tragedy. Rare, indeed, is the family that is entirely free of genetic disorder. Almost 3,500 autosomal dominant, autosomal recessive, and sex-linked disorders have been cataloged. Epidemiological surveys suggest that approximately 1 percent of all newborns have a single gene defect and that 0.5 percent have gross chromosomal anomalies severe enough to produce serious physical defects and mental retardation. Of the 3 to 4 percent of newborns with birth defects, surveys indicate that at least half suffer a major genetic contribution. A minimum of 5 percent of all conceptions that evidence themselves have gross chromosomal anomalies, and 40 to 50 percent of spontaneous abortions involve chromosomally abnormal embryos. About 40 percent of all infant mortality is due to genetic disease; 30 percent of pediatric and 10 percent of adult patients require hospital admission because of genetic disorders. Medical investigators estimate that genetic defectsalbeit often minorare present in 10 percent of all adults. The hopeful aspect of these grim figures is that medical genetics can do much to prevent these conditions or to ameliorate them by early diagnosis and treatment. A congenital defect is any biochemical, functional, or structural abnormality that originates prior to or shortly after birth. About 20 percent of all stillbirths and infant deaths are associated with severe anomalies, and about 7 percent of all births show some mental or physical defect. It must be emphasized that birth defects do not all have the same basis, and it is even possible for apparently identical defects in different individuals to be due to different causes. Though the basis for most defects is still uncertain, it is evident that they are due to a combination of genetic and environmental factors. Despite the absurdity of the old belief that a pregnant woman frightened by a hare will bear a child with a cleft lip (once colloquially known as a harelip), it is quite possible that severe emotional shock early in embryogenesis might disturb normal development. As mentioned above, many defects are the result of single-gene mutations, but the same defect can be produced by different genes (genocopies) or by nongenetic factors (phenocopies). Moreover, a single-gene defect can produce symptoms involving many different organs (pleiotropy), and frequently the primary gene product, which is a defective protein, is unknown. Arthur Robinson Additional reading David W. Smith and Kenneth Lyons Jones, Recognizable Patterns of Human Malformation, 3rd ed. (1982), deals with birth defects and growth disorders. Vincent M. Riccardi, The Genetic Approach to Human Disease (1977), is concise, clear, and very readable. Assembly of Life Sciences (U.S.), The Effects on Populations of Exposure to Low Levels of Ionizing Radiation, 1980 (1980), is a comprehensive document. See also such symposium and conference proceedings as Theodore T. Puck, The Mapping of the Human Genes and Its Implications in Developmental Disease, in Topics in Environmental Pathology, vol. 11 (1981); and G.R. Burgio et al. (eds.), Trisomy 21 (1981), on Down's syndrome. Arthur Robinson