sequent generations. His revolutionary experiments paved the way for modern agriculture by showing that through controlled pollination crosses, characteristics are inherited in a logical and predictable manner. In 1905, Roland Biffen, of England, built on Mendel's experiments by illustrating that the ability of wheat (Triticum aestivum) to resist a rust fungus could be passed to later generations (NAS 1998).
Since then, many plants have been bred to include desirable traits, such as pest resistance. Blight resistance traits from a Mexican potato species (Solanum demissum) have been introduced into over 50% of all potato cultivars (NRC 1989). Blight-resistant corn (Zea mays), rust-resistant wheat (T. aestivum), and aphid-resistant alfalfa (Medicago sativa) are other notable examples of conventional plant breeding. Major gains in crop yields have been attributed partially to advances in classical plant breeding and plants developed for pest resistance. Corn yields have increased from 5 metric tons per hectare in 1967 to 8 metric tons per hectare in 1997, cereal harvests have been increasing at an average rate of 1.3% per year (Mann 1999), world food production has doubled since 1960, and agricultural productivity from land and water use has tripled (NSTC 1995).
Conventional breeding will likely continue to play an essential role in the improvement of agricultural crops. However, many believe that traditional breeding methods will not be sufficient to meet increasing demands in developing countries for staple crops, such as wheat ( T. aestivum), rice (Oryza sativa) and corn (Zea mays) (Mann 1999). Classical methods are time-consuming (that is they take approximately 10 years to develop a variety) and labor-intensive (only one line of thousands becomes a useful variety). In addition, beneficial traits can be linked to or lead to undesirable traits, such as disease susceptibility. For example, when male-sterile corn was extensively grown in the 1960-1970s to promote hybrid-corn production, a new race of southern corn leaf blight fungus (Helminthosporium maydis) evolved which successfully attacked this type of corn and significantly decreased US corn yields (Dewey et al. 1988). Some have proposed transgenic methods to augment the advances in conventional breeding.
In the past two decades, scientists have focused on expanding genetic modification methods to include the use of recombinant DNA (rDNA) techniques. New varieties generally can be produced faster by rDNA than by conventional breeding methods. rDNA methods allow the introduction of genes from distantly related species or even from different