BOX 2.1

Summary of Genetic Basis of Resistance Traits That Have Been Bred into Cultivated Plants Using Conventional and Transgenic Techniques

  1. Conventionally bred plants only

    1. Polygenic traits2 (controlled by several interacting genes, usually selected without knowledge of which genes are involved)

  2. Both Conventionally bred and transgenic plants

    1. Single-gene traits2 from the same species or a related species

    2. Several single-gene traits that are not genetically linked and are therefore inherited independently

    3. Several single-gene traits that are physically linked and inherited as a unit; occasionally possible with conventional breeding, as when a chromosome segment bearing more than one resistance gene is transferred to the cultivar usually accompanied by extraneous DNA; transgenic methods allow several single-gene traits to be tightly linked without extraneous DNA

    4. Single-gene traits expressed only in particular tissues or at particular developmental stages because of specific promoters; occasionally possible with conventional breeding, but more flexible and precise with transgenic methods

  3. Transgenic plants only

    1. Single-gene traits found in the same species or a related species and modified by changes in the nucleotide sequence of the structural gene or the promoter to improve the plant's phenotypic characteristics

    2. Single-gene traits obtained from unrelated organisms (such as viruses, bacteria, insects, vertebrates, and other plants); sometimes modified by a change in the nucleotide sequence of the structural gene or the promoter to improve the plant's phenotypic characteristics

    3. Single-gene traits that can be induced by a chemical spray or by specific environmental conditions (such as threshold temperature), based on the action of specific promoters; (these traits may also occur naturally in nontransgenic plants, such as those with systemic acquired resistance, but have rarely been selected intentionally by conventional breeding)

The committee recognizes that the magnitude of the risk varies on a product by product basis. The committee also agrees with points 1 and 2 in the sense that the potential hazards and risks associated with the organisms produced by conventional and transgenic methods fall into the

2  

A molecular technique known as marker-assisted selection can speed the identification of polygenic or single-gene traits in the plant 's own genome, and rapid advances in genomics are expected to speed the identification of additional single-gene resistance traits in plants and other organisms.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement