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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 your OCR Gateway Combined Science course, 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. 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, and discuss the benefits and risks of both in a balanced way.
This lesson develops AO1 (understanding how selective breeding and genetic engineering work) and AO3 (evaluating the benefits, risks and ethics of each technology).
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 you are expected to know some 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). A common misconception is that selective breeding increases variation — in fact it reduces it, which is exactly why it makes a population vulnerable to disease and prone to inbreeding problems. Link the drawback back to the loss of variation for full marks.
Selective breeding and natural selection work by the same underlying process — some individuals reproduce more than others, so their alleles become more common over generations. The difference is who does the selecting and why.
Because humans deliberately choose the same few desirable individuals again and again, selective breeding narrows the gene pool much faster than natural selection usually would, which is why the loss of variation is such a marked problem. Seeing selective breeding as "natural selection with humans as the selecting force" makes both processes easier to explain in the exam.
A farmer wants a herd of cattle that produce more milk. Describe how selective breeding could achieve this over several generations, and give one problem it could cause.
Step 1 — choose the parents: from the existing herd, select the cows (and bulls from high-milk-yielding families) that produce the most milk.
Step 2 — breed and re-select: breed these together, then from the offspring again choose those that produce the most milk.
Step 3 — repeat: continue selecting and breeding the highest-yielding animals over many generations, so that the milk yield of the herd gradually increases.
Step 4 — a problem: repeatedly breeding from a small number of related, high-yielding animals reduces genetic variation (inbreeding), which can make the herd more vulnerable to disease and can bring out harmful recessive conditions.
Answer: repeatedly choose and breed the highest-yielding animals over many generations so milk yield rises; the main problem is reduced variation (inbreeding), making the herd more prone to disease.
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).
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.
The production of human insulin by genetically modified bacteria is the example you are most likely to be asked 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.
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