The words "genetically modified" constitute a serious misnomer. Humans have bred plants and animals for thousands of years. In selective breeding, we have built into animals and plants gene combinations that are not normally found in nature and that probably would not survive without human intervention. For example, Chihuahuas and Great Danes did not appear spontaneously, nor can these or other dog breeds be maintained without human intervention. We also know that both breeds are quite different from "primeval" dogs domesticated thousands of years ago. The latter looked like wolves.
Similarly, several types of food plants look quite different from their progenitors. Corn, for example, derives from plants that produced very small purple kernels, not large, yellow ones. Progenitors of modern corn, however, apparently appeared spontaneously from wild parents and were propagated and bred to enhance the characteristics that we enjoy in modern corn. On the other hand, plant breeders mutated the genes of the progenitors of modern barley with chemicals or radiation and selected for mutants with desirable characteristics to produce the high-yielding, short-stemmed strains of barley cultivated today. Thus, we see that humans have modified the genes of the creatures all around us, starting a long time ago in some cases, either by mixing genes from different parents, by selecting interesting variants, or by directly acting on genes through chemical or physical agents. All these plants and animals can legitimately be called "genetically modified organisms" relative to their progenitors.
When the term "genetically modified organism" is used, however, it does not refer to the examples mentioned above. Rather, the term "genetically modified" (GM) is currently applied to plants and animals that result from adding genes, in particular genes from completely unrelated organisms, to preexisting plants or animals. In other words, GMOs are regarded as life forms that could not have appeared naturally. This is indeed true for the types of genetically engineered plants that we will discuss in this chapter. Genetic modification of animals, including humans, is discussed in chapter 13.
Genetically modified plants are commonly grown today in the United States (see table 6.1). Most of the corn (including corn products found in breakfast cereals) and soybean- and canola-derived products sold in this country are the result of plants engineered with recombinant genes. The most common genetic modifications are those that confer resistance to certain insect pests and those conferring resistance to certain herbicides. Let us first see how genetic modification of plants is achieved. Two techniques are used to genetically engineer plants: Agrobacterium-mediated gene transfer and biolistics.
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