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MAJOR UNDERSTANDINGS 2.2a For thousands of years new varieties of cultivated plants and domestic animals have resulted from selective breeding for particular traits. 2.2b In recent years new varieties of farm plants and animals have been engineered by manipulating their genetic instructions to produce new characteristics. 2.2c Different enzymes can be used to cut, copy, and move segments of DNA. Characteristic produced by the segments of DNA may be expressed when these segments are inserted into new organisms, such as bacteria. 2.2d Inserting, deleting, or substituting DNA segments can alter genes. An altered gene may be passed on to every cell that develops from it. 2.2e Knowledge of genetics is making possible new fields of health care; for example, finding genes which may have mutations that can cause disease will aid in the development of preventive measures to fight disease. Substances, such as hormones and enzymes, from genetically engineered organisms may reduce the cost and side effects of replacing missing body chemicals.