Selective breeding and genetic modification — Edexcel International GCSE Science (Double Award)
How selective breeding improves plants and animals, and how genetic engineering uses enzymes and vectors to make insulin and GM crops.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Selective breeding is when humans choose parents with a desired characteristic, such as high milk yield or disease resistance, and breed them together. Offspring showing the characteristic are bred again, repeated over many generations. It gives higher yields, but breeding from a few individuals reduces genetic variation and inbreeding can make harmful inherited conditions more common.
Genetic engineering transfers a gene from one species into a different species, making a transgenic organism. A restriction enzyme cuts the gene out of DNA, leaving sticky ends. The same enzyme cuts open a vector, such as a plasmid or virus, and ligase joins the gene into it, making recombinant DNA that the vector inserts into cells.
Human insulin is made by genetically modified bacteria. The human insulin gene is cut out with a restriction enzyme, inserted into a plasmid cut with the same enzyme and joined by ligase, making a recombinant plasmid. The bacteria are grown in a fermenter, divide rapidly and make insulin, which is extracted and purified to treat diabetes.
Genetically modified plants have a gene for a useful characteristic transferred into them to improve food production. For example, a gene from the bacterium Bacillus thuringiensis codes for Bt toxin, which kills insect pests, so the crop has insect resistance. This raises yields and reduces insecticide use, but genes may spread to wild plants.
Exam questions
7 exam-style questions (20 marks), each with its mark scheme.
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Quick check questions
What is selective breeding?
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Humans choosing which organisms to breed so that desired characteristics are passed on to the next generation.
Which enzyme cuts DNA at specific sites?
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A restriction enzyme.
Which enzyme joins pieces of DNA together?
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Ligase.
What does transgenic mean?
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The transfer of genetic material from one species to a different species.
Where are GM bacteria grown to make large amounts of insulin?
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In a fermenter.
Frequently asked questions
What is selective breeding in plants?
Selective breeding in plants is when humans choose parent plants with a desired characteristic, such as disease resistance, larger grains or higher yield, and breed them together. The offspring that show the characteristic most strongly are bred again, and this is repeated over many generations until the characteristic is strongly shown in the variety.
What are the disadvantages of selective breeding?
Selective breeding reduces genetic variation, because only a few individuals are used for breeding. If a new disease or a change in the environment appears, all individuals may be affected. Breeding closely related individuals, called inbreeding, can also make harmful inherited conditions more common in domesticated animals.
What is genetic engineering in agriculture?
Genetic engineering in agriculture is transferring a gene for a useful characteristic from one species into a crop plant, making a genetically modified plant. For example, a gene from Bacillus thuringiensis makes crops produce Bt toxin, which kills insect pests. This gives higher yields and less insecticide use, though genes may spread to wild plants.
How are bacteria used to make human insulin?
Bacteria make human insulin after being genetically modified. The human insulin gene is cut out with a restriction enzyme, inserted into a plasmid cut with the same enzyme and joined by ligase. The recombinant plasmid is put into bacteria, which are grown in a fermenter, and the insulin is extracted and purified.
What is the role of restriction enzymes and ligase in genetic engineering?
Restriction enzymes cut DNA at a specific base sequence, leaving sticky ends. Using the same restriction enzyme on the gene and the vector gives matching sticky ends. Ligase is the enzyme that then joins the gene into the vector, such as a plasmid, making recombinant DNA that can be inserted into cells.
Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026