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For thousands of years humans have changed other species to suit their needs — first by carefully choosing which plants and animals to breed, and now, far more directly, by moving genes from one organism into another. This lesson, the last of the content lessons in Topic B5 of OCR Gateway Science A, covers selective breeding (artificial selection) and genetic engineering: how each works, what they are used for, and the benefits, risks and ethical concerns of each — presented even-handedly. For Higher tier it also outlines the main stages of genetic engineering. It draws together the ideas of genes, alleles and selection from across the topic.
By the end of this lesson you should be able to describe selective breeding and its problems, describe genetic engineering and its uses, discuss the benefits and risks of both, and (Higher) outline the main stages of genetic engineering.
Selective breeding, also called artificial selection, is the process by which humans choose which organisms to breed from in order to produce offspring with desired characteristics. It is essentially natural selection, but with humans doing the selecting instead of the environment.
The process is carried out over many generations:
flowchart TD
A["Choose parents with the<br/>desired characteristic"] --> B["Breed them together"]
B --> C["From the offspring, choose those that<br/>best show the desired characteristic"]
C --> D["Breed those offspring together"]
D --> E["Repeat over many generations"]
E --> F["The desired characteristic becomes<br/>stronger / more common"]
Over time, the desired characteristic becomes more and more pronounced in the population. Humans have used selective breeding for a very long time, and OCR expects you to know examples:
| Type of organism | Characteristics selected for |
|---|---|
| Crops / food plants | Higher yield, resistance to disease, larger fruit, better flavour |
| Livestock (farm animals) | More meat or milk, resistance to disease |
| Domestic dogs | Particular size, temperament or appearance (giving the many different breeds) |
| Flowers / garden plants | Particular colours, larger or more attractive blooms |
Selective breeding is very useful, but it has serious drawbacks, mainly because it reduces genetic variation. By repeatedly breeding from a small number of closely related individuals with the same desired characteristic, the gene pool (the variety of alleles in the population) becomes smaller. This causes problems:
Exam Tip: The key disadvantage of selective breeding to remember is that it reduces genetic variation (shrinks the gene pool). This makes the population vulnerable to disease and can cause inbreeding problems (harmful recessive alleles showing up). Link the problem back to the loss of variation for full marks.
Genetic engineering (also called genetic modification) is a more modern and far more direct technique: it involves transferring a gene from one organism into the genome of another organism, so that the second organism produces a desired protein or shows a desired characteristic. An organism that has had its genes altered in this way is described as genetically modified (GM) or transgenic.
Unlike selective breeding, genetic engineering can move a gene between completely different species — even from an animal to a bacterium or a plant — and it produces the desired result in a single generation rather than over many.
| Example | What is done | Why it is useful |
|---|---|---|
| Bacteria producing human insulin | The human gene for insulin is inserted into bacteria, which then make human insulin as they grow | Produces large amounts of insulin to treat people with diabetes |
| GM crops with improved yield or pest resistance | Genes are added to crops to make them resistant to pests/herbicides or to increase yield | More food can be produced; less crop is lost to pests |
| Golden Rice (added vitamin) | A gene is added so the rice produces a substance the body uses to make vitamin A | Could help reduce vitamin A deficiency in regions where it is common |
| Disease-resistant or frost-resistant crops | Genes for resistance are added | Crops survive in conditions that would otherwise destroy them |
The bacteria-and-insulin example is especially important: before genetic engineering, insulin had to be extracted from animals, but GM bacteria can produce large quantities of human insulin reliably and relatively cheaply.
Genetic engineering is a powerful but controversial technology. OCR expects you to discuss the benefits, risks and ethical concerns in a balanced way, presenting them as arguments people commonly make rather than as proven facts.
| Benefits commonly cited | Risks and concerns commonly cited |
|---|---|
| Can produce useful medicines (e.g. insulin) in large amounts | The long-term effects on health are not fully known to everyone's satisfaction |
| Can increase food production and reduce crop losses, helping to feed a growing population | Transferred genes might spread to wild plants or other species |
| Can add nutrients (e.g. vitamin A) to staple crops to improve health | Some worry it could reduce biodiversity or harm other organisms (e.g. insects) |
| Can make crops resistant to disease, pests or harsh conditions | There are ethical objections to altering the genes of living organisms |
| Concerns about a few large companies controlling GM seeds and their cost to farmers |
Exam Tip: For genetic engineering, give a balanced answer with benefits and concerns, and use phrasing such as "some people are concerned that…". Do not state risks as certainties, and do not invent figures (for example about yields or health effects). The examiner is testing whether you can weigh up an issue fairly.
The production of human insulin by genetically modified bacteria is the example OCR most often expects you to be able to discuss, so it is worth understanding clearly. People with type 1 diabetes cannot make enough insulin (the hormone that controls blood glucose), and must inject it. Before genetic engineering, the insulin used had to be extracted from the pancreases of animals such as pigs and cattle. This animal insulin worked, but it was not quite identical to human insulin, supplies depended on the meat industry, and some people had concerns about using animal products.
Genetic engineering solved several of these problems at once. By inserting the human insulin gene into bacteria, scientists created bacteria that produce genuine human insulin. The bacteria can be grown in large fermenters in huge numbers, so a large, reliable supply of human insulin can be produced. This is a clear example of how genetic engineering can bring real medical benefits — which is why, even though the technology is debated, this particular use is very widely accepted.
GM crops tend to attract more debate than medical uses like insulin. Supporters point out that crops engineered for higher yield, pest resistance or added nutrients (such as the vitamin A in Golden Rice) could help feed a growing population and improve health where certain foods are scarce. Critics raise concerns that are commonly debated rather than settled: that the long-term effects on health and ecosystems are not fully known; that engineered genes might spread to wild plants; that GM crops might reduce biodiversity or affect other organisms such as insects; and that a small number of large companies could end up controlling the seeds farmers depend on. A good exam answer lays out both sides fairly, makes clear these are viewpoints, and avoids inventing data for either side.
Higher tier only: You should be able to outline the main stages of genetic engineering. Keep this at GCSE depth — you need the sequence of steps, not the detailed biochemistry.
The aim is to move a useful gene from one organism into another. The main stages are:
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