4.12B - GM crops
Explain how a crop carrying a gene from Bacillus thuringiensis can resist a target insect pest. Evaluate benefits and risks by tracing the consequences for crop damage, resistance, gene flow and farming costs.
4.12B — Bt insect-resistant crops
This part is Biology-only, but it is studied at both Foundation and Higher tier in GCSE Biology. It is the required named crop example. Genetic engineering deliberately changes an organism’s genome; an introduced gene must be expressed to produce its protein.
The bacterium Bacillus thuringiensis, shortened to Bt, has genes that code for proteins harmful to particular insect pests. Genetic engineering can introduce one of these genes into the genome of a crop plant. The modified crop cells express the gene and make the Bt protein.
When a target insect pest feeds on the crop, it takes in the Bt protein and is harmed or killed. Fewer target pests survive to feed on and damage the crop. Trace what is transferred and what acts:
gene from Bacillus thuringiensis -> gene introduced into crop genome -> crop makes Bt protein -> target insect eats plant tissue -> protein harms target insect -> less crop damage
The bacterium itself is not placed in every mature plant, and the gene does not directly kill the insect: the crop produces the protein encoded by the gene. The characteristic is also specific to susceptible target pests, so it is inaccurate to say that a Bt crop is resistant to every insect.
4.12B — Tracing possible advantages
An explained advantage links the biological change to a consequence. A bare word such as "yield" does not show why the yield might change.
| Change in the Bt crop system | Immediate effect | Possible advantage |
|---|---|---|
| the crop makes a protein harmful to a target insect pest | fewer target pests survive and feed on it | less crop damage can increase saleable yield and food supply |
| the crop has built-in protection against that target pest | less conventional insecticide may be needed | farmers may face lower pest-control costs and less exposure to sprayed insecticide |
| less conventional insecticide is sprayed | fewer organisms are exposed to that chemical treatment | environmental harm caused by that insecticide may be reduced |
A full explanation might read: the Bt protein harms the target pest, so less of the crop is eaten; therefore, more crop may be harvested and sold. Each arrow supplies the reasoning missing from the vague claim "GM gives more yield."
These outcomes are possible, not guaranteed. If the target pest is absent, insect resistance may not increase yield. A Bt crop may also need other pest controls, so "less conventional insecticide" must not be turned into "no pesticide is ever needed." The outcome depends on the crop, the pest pressure, the introduced gene, the receiving environment and farming practice.
4.12B — Tracing possible disadvantages
Disadvantages also need mechanisms, and a possibility should not be presented as an effect of every GM organism.
Pest resistance: insect populations contain inherited variation. If the Bt protein kills susceptible insects, an insect with inherited resistance is more likely to survive and reproduce. Its offspring may inherit the resistance allele, so the allele can become more common over generations. The Bt crop may then give less protection and other pest controls may be needed.
Gene flow: pollen from a GM crop may fertilise a closely related non-GM crop or wild relative. If the offspring inherit the introduced gene, the characteristic can enter that population. Pollen cannot pass the gene to any unrelated plant, and cross-pollination is a route for spread rather than proof of environmental damage. The consequence depends on the gene, the related plant and the environment.
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