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.
Characteristics produced by the segments of DNA may be expressed when these seg-
ments 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 develop-
ment 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.