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The Terms Genes , Genetics, and Genetic Engineering all refer to the molecular units of heredity and variability in living organisms. Gene comes from the German word Pangen, which is derived from the Greek pan (all) and genos (kind, offspring). Genes are the units of heredity that are recombined and passed through reproduction; they express characteristics in living organisms and contribute to biological variability. Genes are found in the germplasm, specifically the chromosomes, and are made of a sequence of amino acids found in an organism’s DNA. In the modern discourse about genes and genetics, life is shaped by sequences of information carried by genes. This information codes the production of specified proteins or enzymes.
The Study of Genetics
Genetics is the study of inheritance and variation. It seeks to understand the units of genetic action, heredity, mutation, and recombination. Since Darwin’s work on the evolution of pigeons, finches, and earthworms, natural historians had been looking for casual mechanisms to explain evolution. The theory of natural selection proposed by both Darwin and independently by Alfred Russell Wallace posited that populations that can adapt to their environment are more likely to pass on their traits to future generations or progeny.
By 1900, natural historians had a prime candidate for the casual mechanisms of evolution when Hugo de Vries, Erich von Tschermak, and Carl Correns rediscovered the basic laws of inheritance. Gregor Mendel, an Austrian monk studying in the 19th century, developed a theory of inheritance while studying the reproduction of peas from 1857-63. According to Mendel, the variation of an organism’s characteristics is the outcome of combinations and expressions of genes. For each characteristic of an organism, each individual inherits genes called alleles from each parent. Each characteristic is an expression of an alleles’ molecular synthesis. If the alleles differ, some alleles will be dominant with others being recessive. The presence of a dominant
allele will express that particular trait, but the recessive allele will still be in the hereditary material of the organism. For a recessive allele to be expressed all the units of heredity must be the same.
Biologists distinguish between phenotype and genotype. The phenotype of an organism is the expression of the organism’s traits like eye color or leaf size. The phenotype of an organism can also be affected by its environment, such as when some plants are stunted by exposure to excess light. The genotype is all the hereditary material carried by an organism including the recessive traits not expressed in the organism. The genotype is the coded information found in almost all living cells that are passed along through heredity. The genotype codes for the expression of phenotype, and the phenotype can subsequently altered by its environment.
Soon after the rediscovery of Mendel’s work, William Bateson coined the term genetics in 1905. Thomas Hunt Morgan incorporated much of Mendel’s work into his own chromosomal theory of inheritance where chromosomes carry hereditary materials. Morgan’s work suggested a sex-linked model of inheritance based on the white eye mutation in fruit flies (spp. Drosophila), which has become the model insect for geneticists.
Applications and Discoveries
One early application of genetics was in plant breeding, particularly corn or maize. Here the hybridization of maize was studied at public breeding stations until it became commercially profitable for private enterprise. By the 1940s, genetics shifted from statistical breeding techniques to techniques based on molecular biology and biochemistry. Griffith’s notion of the transformation principle and Avery’s research showing the ability of Deoxyribonucleic acid (DNA) to transform cells pointed to the centrality of DNA and its constitutive amino acids in inheritance. They resolved a contemporary debate regarding whether proteins or amino acids contained the hereditary information.
In 1953, Crick and Watson made their famous discovery of the double helix through X-ray diffraction. The race to solve the structure of DNA was quite competitive, with all parties agreeing that the timing of the discovery was immanent. Wilkins and Franklin took the first X-ray photographs of DNA at King’s College in London and shortly thereafter passed the images on Watson and Crick, helping them solve the puzzle.
The discovery of the double helix through X-ray diffraction was a critical juncture in the marriage of genetics and physics. It was another demonstration of the superiority of reductionist model of science. It soon became common to describe the process of heredity as the passage of information, messages, and code. Life itself was simply characterized as a computer program built on a cybernetic feedback system. These mechanisms oriented molecular biologists, as the profession soon began to inquire about the ways that DNA was encoded in the cell nucleus.
Genetics as a discipline emerged in elite universities such as Caltech, MIT, Harvard, and the University of California at Berkeley; research centers like Cold Spring Harbor Labs, New York; and new centers of the information economy developing in places like Santa Fe, New Mexico, California’s Bay Area, and Cambridge, MA. These places boomed as private and Federal research dollars were directed to the pursuit of new molecular sciences: immunology, virology, cell biology, biochemistry and microbiology.
DNA Ancestry
The tools of genetics were soon extended into areas of archaeology and anthropology. Attempts to understand historic human migrations out of Africa and into the Americas often rely on studies of mitochondrial DNA. All human have two sets of DNA. The first is found in the chromosomes found in the cell nucleus, while the second freely floats in the cell in the mitochondria. Chromosomal or nuclear DNA is inherited from both parents while mitochondrial DNA is only inherited through the female lines of inheritance. This means that every parent has contributed to an individual’s nuclear DNA, but mitochondrial DNA is traced only through the mothers mitochondria. Those who share the same mitochondrial DNA are a member of the same haplo-group. This provides clues to ancestry, as certain haplo-groups will always have similar ancestors.
Polymerase Chain Reaction
The polymerase chain reaction (PCR) is a significant tool for geneticists working on a wide variety of problems including those in ecology and anthropology. PCR is a chemical reaction used to copy fragments of DNA. PCR has been used to map the genomes of many species including humans. The mapping of the human genome has raised considerable controversy as questions about what to do about genetic “defects” and alterations of the gene pool are constantly raised. Other controversies involve how genetics might be used as a tool for discrimination, genetic profiling, and the role that genetics might play in determining a genetic basis for human behavior.
Genetic Engineering
Genetic engineering is the term usually reserved for these molecular modifications that use recombinant techniques. With genetic engineering, scientists argue they can more precisely manipulate the units of heredity at the molecular level. Novel assemblages of genes can made by moving genes across the species barrier, bypassing the condition of sexual compatibility previously required for genetic recombination.
Genetic engineering introduces foreign DNA into the host organism in several different ways. The most popular way is to introduce the DNA into host with a viral or bacterium invasion into the host’s nucleus. This virus or bacteria is known as the promoter. Transferring genes using a bacterium involves combining the desired gene with a plasmid, which is then carried by an agrobacterium. The agrobacterium inserts itself through the cell wall depositing the desired gene in the host organism. After gene transfer both the promoter and the desired gene remain in the plasmid. These plasmids are then cultured, and in the case of plants, moved to a greenhouse where it is determined whether or not the desired gene is expressed in the plant’s phenotype.
Often this is done with a marker gene, which when expressed, makes it easy to identify which plants contain the desired gene. The most common marker genes are those for antibiotic resistance so that the determination can be done early. Antibiotics kill cells without the new genes. This has raised many food safety concerns about genetically engineered foods because it is unclear whether or not the antibiotic resistance affects human health or promotes resistance. Newer marker genes include traits of phosphorescence from jellyfish, where the desired trait can be ascertained from the organism’s exposure to a black light. Other genetic engineering techniques use non-viral promoters. The particle gun technique uses gold or tungsten covered pellets coated with bits of DNA that penetrate the cell wall and randomly insert themselves into the hosts DNA.
Genetic engineering is used synonymously with the term genetic modi(ication. The term is used politically to denote the precision of r-DNA techniques. Scientists often argue that much of plant breeding, for example, is a form of genetic modification. By proclaiming the practice as genetic engineering, scientists invoke a sense of control that was previously unattainable in molecular biology, medicine, and plant breeding.
Controversies and Social Concerns
Genetic engineering has been extensively incorporated into medical practice. DNA techniques are used to diagnose genetic diseases and to develop medicines such as human-made insulin for diabetics, promising treatments for breast cancer, and medicines to help kidney transplant patients avoid rejecting the new organ. However, these technologies have not been without social concerns. Concerns about social justice emerge with human engineering and the way that the identification of genetic “defects” will affect insurability of some social groups.
Genetic engineering is far more controversial in agricultural biotechnology where it has become embroiled in controversies in places as ideologically distant as Geneva and Mendocino County, California. Subsequent to the containment issues raised with the early r-DNA experiments, the deliberate introduction of genetically engineered organisms [GEOs; also known as genetically modified organisms (GMOs) or transgenic organisms] into the environment set off a host of new controversies.
In 1983, the deliberate release of the “ice minus” bacterium developed by University of California biologist Steven Lindow set off a new round of local reactions in the Bay Area cities of San Francisco, Berkeley, and Oakland. Lindow planned to spray potatoes in the Tule Lake area of Northern California with an “ice-nucleation active” bacterium that would inhibit the formation of frost on the plants. These field tests were approved by the NIH’s RAC. Activist Jeremy Rifkin of the Foundation on Economic Trends obtained a court injunction to stop the release, arguing before the court that the experiment posed an environmental hazard.
In 1985, Congress decided that new regulatory agencies were not necessary, and that the existing regulatory system was appropriate for handling the classes of concerns raised by ecologists and activists. Many activists and ecologists simply saw this as an effort to manage GEO introduction instead of regulate them. The Food and Drug Administration would evaluate food safety concerns; the Environmental Protection Agency (EPA) would oversee concerns about toxicity; and the Department of Agriculture’s Animal and Plant Health Inspection Service would regulate problems related to increased weediness and biological invasion.
Also in 1985, the Ecological Society of America released a position statement noting the potential ecological and environmental hazards associated with introducing GEOs into the environment. They noted that the products of r-DNA technologies, genetic engineering, posed no new classes of ecological hazards. But the novelty of the new technology warranted regulatory oversight, because there is the potential for more extreme and uncertain ecological hazards. Ecological and environmental problems may follow from intrinsic qualities of the plant itself, its interaction with its environment, and/or the practices associated with its cultivation. These hazards in many cases are the extreme versions of conventional analogs, but the novelty of some traits makes ecological effects even more likely. If the organism became endowed through gene flow with traits that improved its fitness, the organism could act as invasive plants do.
Gene Flow
Gene flow is the movement of genes from one place to another as when seed is transported or pollen drifts and subsequently hybridizes. If gene flow leads to introgression, the subsequent backcrossing of two hybrids, the transgene may remain in the wild or weedy population potentially increasing weediness or invasiveness if the transgene confers fitness advantages.
Gene flow also poses consequences to genetic diversity as outbreeding depression or genetic swamping could result in the extinction of wild relatives in the Vavilov centers of genetic diversity. The potential for outbreeding depression would follow if short-term fitness advantages favor the increased presence of the transgene in the population but with long-term fitness consequences over time (e.g., reduced fecundity, increased disease susceptibility). Genetic swamping would occur where the receiving plants are relatively rare and exposed to high rates of hybridization. These concerns are paramount in the case of transgenic maize in Mexico, as there are instrumental as well as intrinsic values of biodiversity at play; small farmers as well as international research institutions depend upon the diversity of wild relatives and landraces for plant breeding.
Gene Flow and Transgenic Hazards
Other hazards associated with the adoption of GEOs include those to agroecosystems. Widespread use of herbicide-tolerant (HT) RoundUp Ready™ and Liberty Link® crops could lead to the rapid evolution of resistance to herbicides like glyphosate and glufosinate in weeds, either as a result of increased exposure to the herbicide, or as a result of the horizontal transfer of the HT trait to weedy relatives of crops. HT crops could change the mix of herbicides used as some become ineffective, which could result in greater levels of overall environmental harm. Since herbicides differ in acute toxicity and persistence, loss of some herbicides may be detrimental to the environment overall.
The introduction of transgenic crops also raises concerns about insect resistance. The naturally occurring microorganism bacillus thuringiensis (Bt) has been used as a pesticide for several decades, as it crystallizes and blocks the passage of food into the stomach of many species of Lepidoptera, effectively killing them. Its rapid degradation when exposed to UV light keeps it outside of the EPA’s oversight, allowing it to be widely used in powdered form by organic farmers. However, many studies have shown that Bt resistance can evolve rapidly in agroecosystems. Incorporating the genes that produce Bt’s endotoxin into plants and subsequently planting them on such a large scale could, unless properly managed, hasten the evolution of resistance, with implications for both organic and conventional farmers. Currently, industry argues that high dose-refuge model will suppress the evolution of resistance in Lepidoptera. They argue that the high dose of Bt will kill most of the pests and that the alleles that develop resistance will be “diluted” by the presence of a non-Bt refuge harboring Bt-susceptible Lepidoptera. However, this argument rests on two assumptions. The first is that Bt resistance is a recessive trait; the second is that farmers actually plant the refuge.
The impacts of transgenic crops on biodiversity from changes in farming practices may be to the detriment of the biodiversity near and in farms. In October 2003, the Royal Society of the United Kingdom published its findings from farm scale evaluations. Two out of the three crops studied demonstrated an association between transgenic crops and practices harmful to wildlife as well as a tendency to decrease biodiversity. The report attributed the impacts to changing in spray regimes of herbicides, finding that wildlife adjacent to GE crops were subject to increased exposure to agrochemicals such as atrazine, pointing to a significant difference in agronomic practices associated with GE and conventional varieties.
Nontarget effects of GE crops could threaten both biodiversity and agronomic practices such as biological control. Plants engineered to produce toxins in mobile tissue parts such as pollen pose threats not only susceptible species that enter into areas where the crop is grown, but also to the adjacent field margins where the pollen may drift as in the monarch butterfly controversy. Researchers suggested that Bt, bacillus thuringiensis, which drifted onto milkweed growing in adjacent to fields of Bt corn, increased the mortality rates of monarch larva. Toxic mobile plant tissues may impact soil biota as well. Bt has been shown to accumulate in the soil through the root exudates of transgenic plants. The impact of dosing the rhizosphere with the Bt endotoxin has not been evaluated for consequences to nontarget soil organisms or to soil health. Beneficial insects used in the biological control of pests are also subject to nontarget effects. One study suggests that the green lacewing, an insect beneficial to farmers because it predates the same pest that Bt is used against, suffers greater mortality rates after consuming Bt-fed prey.
Transgenic crops conditioned to produce viral-resistance potentially can create new or more virulent viruses through two mechanisms: recombination and transcapsidation. The former can occur between the plant-produced viral genes and closely related genes of incoming viruses; the latter occurs when nucleic acids from one virus are incorporated into the protein structure of plants. Both can result in viruses that infect a wider range of hosts, demonstrate increased virulence or lead to a biological resistance “arms race.” Further, some viruses play an ecological role in plant community dynamics. For example, barley yellow dwarf virus resistance has been engineered into cultivated oats to prevent yield losses. It has also been shown to suppress invasive wild oats. The transfer of viral resistance in this case may increase the invisibility of wild oats in natural communities as it alters plant competitive interactions.
Biosafety
GE animals and insects pose other questions about biosafety. Transgenic salmon engineered with genes from an ocean pout grow at rates six to ten times faster, because growth hormone production, which seasonally shuts down in salmon found in the environment, does not shut off. Because of the advantages of fast growth, these fish may out-compete native fish if they are released into the ocean. Researchers at the University of Purdue developed the Trojan gene hypothesis: Under this scenario, the fish out-compete naturally occurring fish, but suffer long term deficiencies associated with the growth hormone staying turned on. The critical question in the regulation of transgenic salmon is whether salmon grown in open sea aquaculture pens will be required to be sterile, or whether growing transgenic salmon will only be permitted on land.
Biosafety ecologists agree that ecological impacts are greatly unknown. Ecological risk assessment suggests that some organisms pose threats to the environment, while others will suffer greater fitness consequences from having their phenotypic expression altered. A more modest approach to evaluating the risks of biotechnology recognizes uncertainty, complexity, and incomplete knowledge while emphasizing the precautionary principle from post-release monitoring to designing rigorous ecological risk assessment.
Activists urge that assessments of genetic engineering be accompanied by analyses of the social consequences of these novel technologies. The history of technology adoption is littered with inequality and disproportionate burdens of impacts. To this end, many activists have been successful in using biosafety as a surrogate for getting at questions about access, control, and development of new technology.
Bibliography:
- Peter J. Bowler, The Mendelian Revolution: The Emergence of Hereditarian Concepts in Modern Science and Society (John Hopkins University Press, 1989);
- Lily Kay, Who Wrote the Book of Life, A History of the Genetic Code (Stanford University Press, 2000);
- Sheldon Krimsky, Genetic Alchemy: The Social History of the Recombinant DNA Controversy (MIT Press, 1982);
- National Research Council, Environmental Effects of Transgenic Plants (National Academy of Sciences, 2002);
- Paul Rabinow, Making PCR: A Story of Biotechnology (University of Chicago Press, 1996);
- Alison Snow et al., “GEOs and the Environment: Current Status and Recommendations,” Ecological Applications 2004.
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