1. Overview
A genetically modified organism (GMO) is one whose genome has been deliberately changed by genetic engineering, usually by inserting a gene from another source so the organism gains a useful new characteristic. When the inserted gene comes from a different species, the GMO is also described as transgenic.
In food production, the aim is to increase productivity (more food per unit of land, time or input) and improve quality. You need to know three established examples:
- GM salmon — faster growth
- Herbicide-resistant soybean — easier weed control
- Insect-resistant Bt cotton — built-in pest protection
Alongside these potential benefits, using GMOs in food raises ethical, social and environmental concerns that must be weighed in any balanced discussion.
Key Definitions
- Genetically modified organism (GMO): an organism whose genome has been deliberately altered by inserting, removing or changing one or more genes using genetic engineering.
- Transgenic organism: an organism that contains one or more genes transferred from a different species.
- Herbicide resistance: a characteristic that allows a crop plant to survive being sprayed with a weedkiller that would normally kill it, so weeds are killed but the crop is not.
- Insect resistance: a characteristic that protects a crop from insect pests, often by the plant producing a toxin that harms insects that feed on it.
- Bt toxin: an insecticidal protein originally from the bacterium Bacillus thuringiensis, produced by GM crops such as Bt cotton to kill feeding insect larvae.
- GM salmon: a genetically modified Atlantic salmon containing an inserted growth-hormone gene and a promoter that keeps it active, so the fish grows to market size faster.
- Productivity: the amount of useful product, such as food, obtained from a given amount of land, time or resources.
- Ethical implication: a concern about whether an action is morally right, for example whether it is acceptable to alter the genes of living organisms used for food.
Content
Why use GM in food production
The human population keeps growing, but the area of farmland and freshwater available is limited. Genetic engineering offers a way to raise output without simply clearing more land. It has two clear advantages over traditional selective breeding:
- The new characteristic appears in a single generation, rather than over many years of breeding.
- The useful gene can come from an unrelated species, which conventional breeding cannot achieve.
GM salmon (faster-growing farmed fish)
GM Atlantic salmon are engineered to grow continuously instead of only in warm seasons. Two pieces of DNA are inserted:
- A growth-hormone gene taken from a larger species, the Chinook salmon.
- A promoter from a different fish, the ocean pout, which keeps that gene switched on all year round (the salmon's own version is only active seasonally).
Because the gene is now active continuously, the fish produce growth hormone all year and reach market size much faster, using less feed per kilogram of fish produced. This raises productivity in fish farming and can reduce pressure on wild fish stocks.
A key principle to note: the inserted DNA changes a regulatory outcome (continuous hormone production), not the food value of the fish itself.
Herbicide resistance in soybean
A gene that codes for a modified enzyme unaffected by a specific weedkiller (a broad-spectrum herbicide such as glyphosate) is inserted into the soybean. In a normal plant the herbicide blocks an enzyme needed to make essential amino acids; the GM plant's enzyme is not inhibited, so the crop survives.
Farmers can then spray the whole field: the weeds are killed but the soybean is not. This makes weed control simpler and can reduce the number of separate herbicide applications and the amount of ploughing needed — and less ploughing (no-till farming) conserves soil structure and reduces soil erosion, a concrete productivity benefit beyond simply being "easier". The trade-off is that over-reliance on one herbicide can lead to herbicide-resistant "superweeds" evolving through natural selection.
Insect resistance in Bt cotton
The bacterium Bacillus thuringiensis naturally makes an insecticidal protein, the Bt toxin. The gene coding for this toxin is inserted into cotton, so the GM cotton plant makes the toxin in its own tissues. When insect larvae (such as bollworms) feed on the plant, the toxin damages their gut and kills them, while the crop is protected from inside.
Benefits include:
- Higher yields of undamaged cotton.
- Less spraying of chemical insecticides, which lowers cost and exposure of farm workers.
The main biological risk is that, because the toxin is always present, insect pests can evolve resistance to Bt by natural selection, so pests that were once controlled may return.
Improving quality as well as productivity
Genetic modification is also used to improve the quality of food, not just the amount produced — for example, by raising the content of a useful nutrient or vitamin in a staple crop, or by improving how long a product keeps. The same general method applies: a chosen gene is inserted so the organism gains a specific, useful characteristic.
Ethical and social implications of GMOs in food
A good discussion presents arguments on both sides rather than only listing benefits or only listing fears.
Arguments in favour include:
- GM crops can give higher yields and so help feed a growing population.
- They can reduce chemical use (for example, less insecticide on Bt cotton).
- They can improve nutritional quality to tackle deficiency diseases.
- They can let crops grow in poor conditions such as drought or salty soil.
Concerns are best grouped by category:
| Category | Concern |
|---|---|
| Environmental | GM genes could spread to wild relatives or non-GM crops by cross-pollination; toxins such as Bt might harm non-target insects (e.g. pollinators); pests or weeds may evolve resistance, reducing biodiversity. |
| Health | Worries about unknown long-term effects of eating GM food, or new allergens being introduced — although approved GM foods are tested for safety. |
| Social / economic | GM seeds are often patented and sold by large companies, so farmers may have to buy new seed each year rather than saving their own, which can disadvantage poorer farmers. |
| Ethical | Some object on principle to altering the genes of living organisms or moving genes between species; others argue consumers have a right to clear labelling and informed choice. |
There is no single "correct" answer: the role of a scientist is to set out the evidence and the values involved so that society can make an informed decision.
Worked example
Exam-style question: A company develops a variety of GM maize that produces the Bt toxin in its leaves and stems. Explain how this modification protects the crop, and give one reason why a farmer might still be concerned about growing it. [3]
Model answer:
- The maize contains an inserted gene (from Bacillus thuringiensis) so the plant makes the Bt toxin in its own tissues.
- When insect larvae feed on the maize they take in the toxin, which kills the pest, so the crop is protected and the yield is higher / less insecticide is needed.
- A valid concern (any one): the pest population could evolve resistance to the toxin by natural selection; the toxin may harm non-target / beneficial insects; or the GM gene could spread to other plants by cross-pollination.
Worked example
Exam-style question: A region grew Bt cotton for several years. The table shows the mean cotton yield and the proportion of the local bollworm population that survived feeding on Bt plants in a controlled test.
| Year | Mean yield / tonnes ha⁻¹ | Bollworms surviving on Bt plants / % |
|---|---|---|
| 1 | 4.2 | 2 |
| 3 | 4.1 | 9 |
| 5 | 3.6 | 28 |
| 7 | 2.9 | 55 |
Using the data, describe the trend over the seven years and explain it in terms of natural selection. [4]
Mark budget: this command is doing two jobs, so spend the marks accordingly — about 1 mark for describing the data (and "using the data" means you must quote at least one start/end figure pair) and about 3 marks for the natural-selection explanation.
Model answer:
- Describe the trend: mean yield falls (from 4.2 to 2.9 tonnes ha⁻¹) while the percentage of bollworms surviving the toxin rises sharply (from 2% to 55%); the two changes are linked / negatively correlated.
- Variation: within the bollworm population some individuals already carry an allele giving resistance to the Bt toxin.
- Selection: the toxin is a selection pressure that kills non-resistant larvae, so resistant individuals survive and reproduce, passing the resistance allele to offspring.
- Outcome: the frequency of the resistance allele increases over generations, so more pests survive to damage the crop and the yield falls.
Key Equations
This is a qualitative topic, so no equations are required; marks come from precise biological explanation and balanced discussion.
Common Mistakes to Avoid
- Describing GM benefits in vague, general terms. Always tie your answer to the named example — for instance, say the soybean's modified enzyme is not inhibited by the herbicide, or that Bt cotton makes the toxin in its own tissues — rather than just writing "it is better" or "it grows more".
- Saying GM salmon were "given more genes to grow bigger" without detail. State precisely that a growth-hormone gene plus an always-on promoter are inserted so hormone is produced continuously, making the fish grow faster to market size.
- Confusing herbicide resistance with insect resistance. Herbicide resistance lets the crop survive a weedkiller (killing weeds); insect resistance means the plant produces a toxin that kills feeding pests. Match the right mechanism to the right example.
- Talking loosely about "changing the genes" of an organism. Be precise: genetic modification changes the DNA / nucleotide sequence by inserting a gene, which then leads to a new protein and a new characteristic — genes are not made of amino acids.
- Writing a one-sided answer for "discuss" questions. A discussion of ethical and social implications needs both benefits and concerns; listing only advantages (or only dangers) limits the marks.
- Treating "ethical", "environmental" and "social" as the same point. Separate them: ethical = whether it is morally right; environmental = effects on wildlife/biodiversity; social/economic = effects on people and farmers (for example, patented seed).
- Claiming GM food is "proven dangerous" or "completely safe". State concerns as possible risks (unknown long-term effects, new allergens) that are weighed against tested benefits, rather than as established fact.
Exam Tips
- Learn the three set examples thoroughly — GM salmon, herbicide-resistant soybean, insect-resistant Bt cotton — and be ready to explain the specific mechanism of each, as questions often name one of them.
- For "explain how" questions, follow the chain: gene inserted → protein/enzyme made → new characteristic → benefit. Make the link to the benefit explicit.
- For "discuss" or "evaluate" questions, structure your answer as balanced points: give arguments for, then arguments against. Note the difference in command word: "discuss" asks you to set out both sides, whereas "evaluate" also expects you to weigh them and reach a justified conclusion / judgement — for an "evaluate" question, dropping that final reasoned verdict usually costs the last mark.
- When data is given, quote figures from the table or graph and, where pest resistance is involved, explain it using the language of natural selection (variation → selection pressure → survival of resistant individuals → rise in allele frequency).
- Use precise command-style vocabulary: an organism is genetically modified / transgenic, a gene is inserted, and the result is resistance to a herbicide or insect, not "immunity".
- When discussing risks, name the category (environmental, health, social/economic or ethical) to show clearly that you are covering the full range of implications.