Genetic engineering is responsible for the so-called "second green revolution." Genes that encode herbicide resistance, insect resistance, drought tolerance, frost tolerance, and other traits have been added to many plants of commercial importance. In 2003, 167 million acres of farmland worldwide were planted in genetically modified (GM) crops equal to one fourth of total land under cultivation. The most widely planted GM crops are soybeans, corn, cotton, canola, and papaya. Two important transgenes have been widely introduced into crop plants. The Bt gene, from Bacillus thuringiensis, produces a toxin that protects against caterpillars, reducing applications of insecticides and increasing yields. The glyphosate resistance gene protects food plants against the broad-spectrum herbicide Roundup, which efficiently kills invasive weeds in the field. The major advantages of the "Roundup Ready®" system include better weed control, reduction of crop injury, higher yield, and lower environmental impact than traditional herbicide systems. Most Americans would probably be surprised to learn that more than 60% of fresh vegetables and processed foods sold in supermarkets today are genetically modified by gene transfer. In 2004, approximately 85% of soy and 45% of corn grown in the U.S. were grown from Roundup Ready® seed.
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Throughout the first half of the 20th century, geneticists assumed that a stable genome was a prerequisite for faithfully transmitting genes from one generation to the next. Working at Cold Spring Harbor Laboratory in the post-WWII era, Barbara McClintock found quite a different story in maize (corn). She observed numerous “dissociations” – broken and ring-shaped chromosomes – and traced the source of these mutations to the short arm of chromosome 9. There she identified two related loci, “dissociator” (Ds) and “activator” (Ac).
Since Alfred Sturtevant constructed the first genetic map of a Drosophilachromosome in 1913, new mutations have been mapped using his method of linkage analysis. Determining the map position of a new mutation -- and its corresponding gene -- consists of testing for linkage with a number of previously mapped genes or DNA markers. Linkage is the principle that the closer two genes or markers are located to one another on a chromosome, the greater the chance that they will be inherited together as a unit (linked). Conversely, locations farther apart on the chromosome are more likely to be separated by chromosome recombination during meiosis. Thus, the frequency of recombination with previously mapped genes or markers allows one to determine the map position of a gene of interest. The increasing availability of whole genome sequences and sophisticated computer software has made it possible to map genes using bioinformatic approaches. However, traditional mapping techniques are still used to map genes for which no sequence information is available -- for example, mutant phenotypes produced by chemical mutagenesis. Although early gene maps relied on genes and mutations with observable phenotypes, modern gene maps are populated with DNA polymorphisms that are detected by molecular methods. In Arabidopsis, molecular markers exploit the natural differences between distinct ecotypes, such as the widely used Landsberg (Ler) and Columbia (Col), which differ about 1% at the DNA level.