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F Genetic Engineering: or thousands of years, humans have modified the organisms around them to meet practical needs. The development of agriculture included the selection and Past and Present breeding of plants, animals, and microbes that provide greater yields of food and fiber or have other desirable traits. Such selective breeding was repeated many times to produce strains with strong expression of the desired traits; examples include com with high oil content and dairy cattle with high milk yields. Artificial selection has been applied to thousands of traits in a vast array of organisms, ranging from the yeasts used in baking and wine making to the livestock and plants that constitute a major pan of our diet Although the mechanisms of heredity were unknown to early breeders, their procedures for selective breeding were a form of genetic engineering. Some agriculturally important traits, such as yield and most forms of disease and insect resistance in plants, are determined by many genes, each with a small effect; others, such as a few forms of disease resistance in plants, are governed by just one gene or at most a few, each with a large effect Breeders introduce desirable genes into crop plants by appropriate genetic crosses, followed by many generations of funher crossing and selection to produce improved marketable strains. Such traditional types of genetic manipulation are lim ited to organisms that can crossbreed and are therefore quite closely related to each othet: The accumulated experience in plant and animal breeding allows some generalizations. Although a breeder's genetically modified organism is useful in the managed ecosystem for
11 which it was created, such as a fanner's fertilized and weed methods are much less precise and controlled. A mutation controlled field, it is usually changed in such a way that it is not made by traditional techniques may be accompanied by many as fit as its natural progenitor to survive in "the wild"-its unknown mutations, which often have deleterious effects on original, nonmanaged environment For example, some plants, the organism. Furthermore, when genes are moved by tradi like com, have lost their ability to disseminate their seeds; tional sexual crosses, unwanted genes may go along; thus, other plant varieties have a high requirement for fertilizers; and many cycles of selection are necessary to obtain the desired domesticated animals are often dependent on people for feed. traits. The power of R-ONA techniques lies in their ability to Moreovet; the genes of an organism do not function indepen make extremely precise alterations in an organism rapidly and dently. but rather constitute a system of interacting compo to overcome the barriers of sexual incompatibility that have nents. Organisms that carry genes introduced from other hitheno stymied breeders' efforts to move genes. It is precisely species tend to be at a competitive disadvantage. With a few these features of genetic engineering with R-ONA techniques exceptions to the general pattern (such as the establishment of that have caused concern. feral pigs and dogs), the conventional genetic manipulations done by human beings to increase an organism's utility are detrimental to the organism's survival outside the special envi ronments provided. The R-ONA technology developed over the last 15 years has permitted a new and more precise kind of genetic manipula tion. These techniques make it possible to isolate genes, to change the genes and how they are expressed, and, together with other techniques, to insen the genes into whole organ isms. R-ONA techniques are unique because they permit genes isolated from almost any organism to be modified to function and be introduced into almost any other organism, regardless of the sexual compatibility of the organisms or the distance of their evolutionary relationship. Breeders who use traditional techniques change (or mutate) genes and move them, but they cannot change or move just one gene or a few at a time. Their
