Crop Improvement by Conventional Breeding or Genetic Engineering: How Different Are They?

Methods compared Let’s look at methods first. When it comes to the “nuts and bolts” of crop improvement by conventional means versus genetic engineering, are we talking about differ­ ent things? The short answer is, “Yes, they are different.” Most conventional breeding can be reduced to two fundamen­ tal steps. The first step is to generate a breeding popula­ tion that is highly variable for traits that are agricultur­ ally interesting. This is accomplished by identifying par­ ents having traits that complement each other, the strengths of one parent having the capacity to augment the shortcomings of the other, and then cross-pollinating the parents to initiate sexual recombination. The genetic mechanisms that drive sexual recombination operate during gamete (egg and pollen) formation via meiosis, and include Gregor Mendel’s famous discovery of inde­ pendent assortment of genes and T.H. Morgan’s discov­ ery of crossing-over of homologous chromosomes. The key feature of sexual reproduction is that it allows and assures that all of the traits that differ between the par­ ents are free to reassociate (segregate) in new and poten­ tially better combinations in the offspring. The second fundamental step involves selection among the segregating progeny for individuals that com­ bine the most useful traits of the parents with the fewest of their failings. Thus, conventional breeding is essen­ tially the normal mating process, but it is manipulated through human choice of the parents and selection of their offspring so that evolution is directed toward pro­ duction of crops and animals with characteristics closely suited to human needs. Such selection over thousands of years has changed marginally useful wild plants into the specialized crops one sees in the produce depart­ ments of grocery stores today. Most of these are fully domesticated, having diverged from their wild ances­ tors to the extent that they can no longer survive outside of an agricultural environment. Genetic engineering, on the other hand, employs a very different method to produce improved crops and animals. Instead of relying on sexual recombination to thoroughly stir the parental genes, genetic engineering preserves the integrity of the parental genotype, insert­ ing only a small additional piece of information that controls a specific trait. This is done by splicing a well­ characterized chunk of foreign DNA containing a known gene into a chromosome of the host species using “re­ striction” enzymes. Restriction enzymes cut the long DNA strand that makes up a chromosome at very spe­ cific places and in a very repeatable way, so that foreign DNA fragments, cut out with the same restriction en­ zyme, can be inserted and integrated into the host chro­ mosome at the restriction site. There are many different restriction enzymes in use today, each recognizing spe­ cific, but different, sites in DNA molecules, providing great versatility in snipping out and inserting specific genes. Restriction enzymes are also employed in the sophisticated biochemical procedures that “engineer” the foreign gene, enabling the host organism to recognize the new information and use it at the proper time, in the proper cellular location, and to the proper extent.