biologic kingdoms. In addition, detailed knowledge of the trait being introduced (such as a DNA sequence or cellular function) can lead to less variability in the offspring and eliminate some of the uncertainty about linked traits. The site of insertion of a gene can affect its expression, but generally plants with the appropriate level of expression can be selected if a number of transgenic plants are produced. After a trait is introduced by transgenic methods, the resulting plant can be sexually hybridized with useful varieties developed by conventional breeding.

Recombinant DNA methods emerged in the early 1970s after the discovery of restriction enzymes (Linn and Arber 1968; Meselson and Yuan 1968), DNA-sequencing methods (Sanger and Coulson 1975; Maxam and Gilbert 1977), and plasmid and viral vectors for engineering organisms (Jackson et al. 1972;Cohen et al. 1973). The methods have been used ever since to manipulate DNA fragments that contain genes of interest.

With the advent of this technology, concerns about the safety of experiments that use rDNA methods developed. The National Academy of Sciences (NAS) in 1974 convened a committee to assess the safety concerns associated with rDNA research. The committee recommended that rDNA experiments be postponed until further evaluation of the risks (Berg et al. 1974). Soon after, the International Conference on Recombinant DNA Molecules, better known as the Asilomar Conference, was held (Berg et al. 1975). An outline of guiding principles and restrictions for rDNA research was generated at this conference. In 1976, the principles were reviewed by the National Institutes of Health (NIH), which implemented official guidelines to be administered by the NIH Recombinant DNA Advisory Committee (RAC) (NIH 1976). The guidelines focused on laboratory containment of rDNA microorganisms. As more experiments were conducted and more data on the risks were generated, less restrictive guidelines were put into place (NIH 1978). In recent years, the guidelines have been expanded to include other rDNA applications, such as gene therapy, and have been adopted not only by institutions receiving federal funding, but also by industry and state institutions.

About a decade after the emergence of rDNA technology, genes from the bacterium Agrobacterium tumefaciens were used to carry foreign genes into plants. Agrobacterium inserts portions of its tumor-inducing (T_i) plasmid-encoded genes into plant chromosomes as part of its natural, parasitic life cycle (Nester et al. 1983). When researchers add foreign DNA (such as a gene for pest-protection) in between T_i plasmid-encoded insertion sequences, the foreign DNA sequences are also inserted into the plant's chromosome. The first transgenic plants were developed with