单项选择题

The Debate over Genetically Modified Foods --Rice with built-in Vitamin A that can help prevent blindness in 100 million children suffering from Vitamin A deficiency --A tomato (hat softens more slowly, allowing it to develop longer on the vine and keep longer on the shelf --Potatoes that absorb less fat when fried, changing the ever-popular French fries from junk food into a more nutritional food --Strawberry crops that can survive frost These are some of the benefits promised by biotechnology. The debate over its benefits and safety, however, continues. Do we really need to fear mutant (突变体) weeds, killer tomatoes, and giant corn and will the benefits be delivered Conventional Breeding Versus Genetically Modified (GM) Crops For thousands of years farmers have used a process of selection and cross breeding to continually improve the quality of crops. Even in nature, plants and animals selectively breed, thus ensuring the optimum gene pool for future generations. Traditional breeding methods are slow, requiring intensive labor: While trying to get a desirable trait in a bred species, undesirable traits will appear and breeders must continue the process over and over again until all the undesirables are bred out. In contrast, organisms acquire one specific gene or a few genes together through genetic modification, without other traits included and within a single generation. However, this technology too is inherently unpredictable and some scientists believe it can produce potentially dangerous results unless better testing methods are developed. Traditional breeding is based on sexual reproduction between, like organisms. The transferred genes are similar to genes in the cell they join. They are conveyed in complete groups and in a fixed sequence that harmonizes with the sequence of genes in the partner cell. In contrast, bioengineers isolate a gene from one type of organism and splice (接合) it haphazardly into the DNA of a dissimilar species, disrupting its natural sequence. Further, because the transplanted gene is foreign to its new surroundings, it cannot adequately function without a big artificial boost. One of the main differences between conventional and genetically modified crops is that the former involves crosses either within species or between very closely related species. GM crops can have genes either from closely related species or from distant species, even bacteria and viruses. Benefits: One Side of the Debate --Economical benefits GM supporters tell farmers that they stand to reap enormous profits from growing GM crops. Initially, the cost is expensive but money is saved on pesticides. To produce the GM crops, modern biotechnology is used which requires highly skilled people and sophisticated and expensive equipment. Large companies need considerable investments in laboratories, equipment and human resources, hence the reason why GM crops are more expensive for farmers than traditional crops. GM crops, farmers are told, are a far better option. It takes a shorter time to produce the desired product. It is precise and there are no unwanted genes. --Herbicide-Resistant Crops So what other advantages do GM crops hold for farmers GM crops can be produced to be herbicide-resistant. This means that farmers could spray these crops with herbicide and kill the weeds, without affecting the crops. In effect, the amount of herbicide used in one season would be reduced, with a subsequent reduction in costs for farmers and consumers. For Ingard cotton, pest-resistance was built into the cotton, hence reducing and even removing the use of pesticides, which are not only expensive but, more importantly, harmful to the environment. Biotechnology companies are even experimenting with crops that can be genetically modified to be drought- and salt-tolerant, or less reliant on fertilizer, opening up new areas to be farmed and leading to increased productivity. However, the claims of less herbicide usage with GM crops have till now not been independently supported by facts. --Better Quality Foods Even animals can be genetically modified to be leaner, grow faster, and consume less food. They could be modified to have special characteristics, such as greater milk production in cows. These modifications again lead to improved productivity for farmers and ultimately lower costs for the consumer. Modified crops could perhaps prevent outbreaks such as foot and mouth disease, which has devastated many farmers and local economies. No such products have been released to date; however, some are under consideration for release. For example, GM salmon, capable of growing almost 30 times faster than natural salmon, may soon be approved by the FDA (Food and Drug Administration) in the U.S. for release into open waters without a single study on the impact on human health or the environment. Risks: the Other Side of the Debate The major concerns of those who oppose GM foods center on the following: --Environmental Damage The problem with GM crops is that there is little known about what effect they will have in, say, 20 years time. The genetic structure of any living organism is complex and GM crop tests focus on short-term effects. Not all the effects of introducing a foreign gene into the intricate genetic structure of an organism are tested. Will the pests that a crop was created to resist eventually become resistant to this crop Then there is always the possibility that we may not be able to destroy GM crops once they spread into the environment. --Risk to Food Web A further complication is that the pesticide produced in the crop may unintentionally harm creatures. GM crops may also pose a health risk to native animals that eat them. The animals may be poisoned by the built-in pesticides. Tests in the U.S. showed that 44% of caterpillars of the monarch butterfly died when fed large amounts of pollen (花粉) from GM corn. --Cross-Pollination(授粉) Cross-pollination is a concern for both GM crops and conventional breeding, especially with the more serious weeds that are closely related to the crops. With careful management this may be avoided. Genetic modification to herbicide-resistant crops could insert the gene that prevents the problem. The number of herbicide-tolerant weeds has increased over the years from a single report in 1978 to the 188 herbicide-tolerant weed types in 42 countries reported in 1997. They are an ever-increasing problem and genetic engineering promises to stop it. But will genes from GM plants spread to other plants, creating superweeds and superbugs we won’’t be able to control --GM Mix-Ups Humans can inadvertently eat foods that contain GM products meant as animal feed, i.e., crops modified for increased productivity in animals. This happened in the U.S., where traces of a StarLink GM crop, restricted to use only in feed, were found in taco shells. Apparently no one became ill, but other such occurrences may lead to health problems. --Disease Another concern is disease. Since some crops are modified using the DNA from viruses and bacteria, will we see new diseases emerge What about the GM crops that have antibiotic-resistant marker genes Marker genes are used by scientists to determine whether their genetic modification of a plant was successful. Will these antibiotic-resistant genes be transferred to microorganisms that cause disease We already have a problem with ineffective antibiotics. How can we develop new drugs to fight these new bugs Conclusion Proponents of GM crops claim that advantages may be many, such as: --improved storage and nutritional quality --pest- and disease-resistance --selective herbicide-tolerance --tolerance of water, temperature and saline extremes --improved animal welfare --higher yields and quality However, until further studies can show that GM foods and crops do not pose serious threats to human health or the world’’s ecosystems, the debate over their release will continue. Living organisms are complex and tampering with their genes may have unintended effects. It is in our common interest to support scientists and organizations concerned, such as Friends of the Earth who demand "mandatory labeling of these food products, independent testing for safety and environmental impacts, and liability for harm to be assumed by biotech companies". Conventional breeding is a slow, predictable process.