Genetically Modified Organisms and Transgenics

A genetically modified organism (GMO) is an organism whose genetic material has been altered by genetic engineering, and a transgenic organism is a GMO that carries a gene (a transgene) transferred from a different species. In brief: examples include Bt maize, which makes an insect-killing protein from the bacterium Bacillus thuringiensis, golden rice, which makes beta-carotene (a precursor of vitamin A), and bacteria that make human insulin. Gene cloning means making many identical copies of a gene, either inside a host cell or in a test tube by PCR.

GMOs are used in agriculture, medicine and industry, and they raise scientific, ethical and economic questions. This guide explains how transgenic plants, animals and microbes are made, gives examples, describes gene cloning, discusses benefits and concerns in a balanced way, compares examples in a table, works through an economic example of a Bt crop and corrects common mistakes.

What is a GMO?

A GMO has DNA that has been modified in a way that does not occur naturally by mating or natural recombination. The modification may be:

• Transgenic: a gene from another species is inserted.
• Cisgenic: a gene from a sexually compatible species of the same kind is inserted.
• Gene edited: the organism’s own gene is changed, without adding foreign DNA, with tools such as CRISPR (described in a later article). Regulations on edited organisms differ among countries.

The term “transgenic” is used for organisms that contain DNA from another species. Not every modified organism is a GMO in the legal sense: selective breeding, and the mutation breeding that uses radiation or chemicals to induce mutations, are not counted as GMOs in most regulations, although they also change the genome.

What is gene cloning?

Gene cloning is the production of many identical copies of a particular gene. There are two main methods:

• In vivo cloning (in cells): the gene is inserted into a plasmid vector, which is introduced into bacteria. When the bacteria divide, each daughter cell receives copies of the plasmid, so the gene is amplified. This is described in the article on recombinant DNA.
• In vitro cloning (in a tube): the polymerase chain reaction (PCR) copies a chosen DNA sequence millions of times in a few hours, as explained in a later article.

The term “cloning” is also used for the production of genetically identical organisms or cells (reproductive and therapeutic cloning), which is a different process described in another article.

How are transgenic plants made?

1. Gene selection: choose a gene for the desired trait and combine it with a promoter that switches it on in plant cells, and a marker gene.
2. Delivery: the gene is inserted into plant cells by Agrobacterium tumefaciens, a soil bacterium that naturally transfers a part of its Ti plasmid into plant DNA (the transfer DNA, T-DNA is replaced with the transgene), or by a gene gun, which fires microscopic gold or tungsten particles coated with DNA into plant cells.
3. Selection: cells that have the gene are identified using the marker, such as resistance to an antibiotic or herbicide.
4. Regeneration: individual transformed cells are grown on culture medium with plant hormones (auxin and cytokinin) into whole plants by tissue culture.
5. Testing: plants are tested in the laboratory, greenhouse and field trials for effectiveness, stability over generations and safety. Regulatory approval takes many years.

How are transgenic animals and microbes made?

In animals, the gene can be injected into the nucleus of a fertilized egg, or inserted into embryonic stem cells, which are then used to make an embryo. Only a small fraction of the animals integrate the gene. Examples:

• Mice with human genes are used as models of disease, and “knockout mice” with a gene disabled help to study its function.
• Goats and sheep that make human proteins in their milk (e.g. antithrombin), which are then purified.
• AquAdvantage salmon with a growth hormone gene, approved for sale in the US and Canada, reaches market size faster.
• Mosquitoes modified to reduce disease transmission or population size.
• Zebrafish with a fluorescent protein gene (GloFish), mainly as pets and for research.

Microbes are the simplest to engineer: bacteria and yeast make insulin, growth hormone, vaccines, enzymes (rennet for cheese, detergent enzymes) and biofuels.

Examples of GMOs

What are the benefits and concerns?

Potential benefits:

• Higher yields and lower losses from pests, diseases and weeds, and less spraying of insecticides in the case of Bt crops.
• Improved nutrition (golden rice) and longer shelf life.
• Medical products made more cheaply and safely.
• Crops that tolerate drought, salt or cold.

Concerns that are raised:

• Environmental: possible gene flow to wild relatives; the evolution of resistance in pests and weeds (which has occurred); effects on non-target organisms; reduced biodiversity from large monocultures.
• Health: allergenicity of new proteins and possible toxicity. Major scientific bodies such as the US National Academies and the World Health Organization have concluded that approved GM foods are as safe to eat as their conventional counterparts, and each new product is assessed case by case.
• Economic and social: corporate patents and control of seeds, costs for small farmers, and labeling and consumer choice.
• Ethical: moving genes between species, the welfare of transgenic animals and the “playing god” argument.

Opinions differ, and both scientific evidence and values play a role. A balanced view looks at the evidence for each product and its specific context.

Worked example: planning a Bt crop

A farmer’s maize field of 100 hectares loses 12 percent of its yield to the European corn borer. The conventional yield without borer damage would be 10 tonnes per hectare. Bt maize costs an additional 25 per hectare in seed (currency units), and reduces the loss from 12 percent to 2 percent. Maize sells for 200 per tonne.

1. Yield of conventional maize with damage: 10 × (1 − 0.12) = 8.8 t/ha.
2. Yield of Bt maize: 10 × (1 − 0.02) = 9.8 t/ha.
3. Gain: 9.8 − 8.8 = 1.0 t/ha. At 200 per tonne: 200 per hectare.
4. Net gain after the extra seed cost: 200 − 25 = 175 per hectare.
5. For 100 hectares: 175 × 100 = 17 500.

The calculation shows a potential economic benefit, but real results depend on pest pressure, which varies from year to year, so the benefit is not guaranteed. Resistance management is also needed: farmers are advised to plant “refuges” of non-Bt maize so that susceptible insects survive and mate with any resistant ones, which slows the development of resistance. Total costs, such as the value of the technology fee and seed prices, also affect the decision.

How did GMOs develop?

The first genetically modified bacteria were made in 1973 by Herbert Boyer and Stanley Cohen. The first transgenic mice were made in 1974 (virus DNA) and 1980s. The first genetically modified crop for sale was the Flavr Savr tomato in 1994, designed to ripen more slowly. Bt cotton and soybeans tolerant of herbicides were commercially grown in the US from 1996. Ingo Potrykus and Peter Beyer developed golden rice in the late 1990s. By the 2020s, GM crops were grown on roughly 190 million hectares worldwide, mainly soybean, maize, cotton and canola, in about 30 countries.

Which mistakes are common?

Mistake 1: Confusing GMOs with selective breeding

Selective breeding uses natural variation. Genetic engineering directly changes DNA, and can move genes between species.

Mistake 2: Thinking that all GM foods are the same

Each GM product has a different gene and trait, and is assessed separately.

Mistake 3: Mixing up gene cloning and organism cloning

Gene cloning copies a DNA sequence. Cloning an organism makes a genetically identical individual.

Mistake 4: Saying that GM crops contain “genes” while ordinary food does not

All foods contain genes and DNA. The question is which genes have been altered.