Selective Breeding and Genetic Engineering

This lesson covers selective breeding (artificial selection) and genetic engineering as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how humans manipulate the characteristics of organisms, the benefits and risks of each technique, and the ethical issues involved.

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Selective Breeding (Artificial Selection)

Selective breeding is the process by which humans choose organisms with desirable characteristics to breed together. Over many generations, the desired trait becomes more common in the population.

The Process

1. Choose individuals with the desired characteristic from the existing population.
2. Allow these individuals to breed together.
3. From the offspring, select those that best show the desired trait.
4. Allow these selected offspring to breed together.
5. Repeat the process over many generations.

graph TD
    A["Choose parents with desirable traits"] --> B["Breed selected individuals"]
    B --> C["Select best offspring"]
    C --> D["Breed selected offspring together"]
    D --> E["Repeat over many generations"]
    E --> F["Desired trait becomes more common"]

Examples of Selective Breeding

Organism	Desirable trait	Outcome
Cows	High milk yield	Dairy cows that produce more milk per day
Wheat	Disease resistance	Crop varieties that resist fungal infections
Dogs	Specific appearance/temperament	Different breeds from wolves (e.g. poodles, bulldogs)
Chickens	Large body size, fast growth	Broiler chickens for meat production
Roses	Colour, scent, petal number	Ornamental rose varieties
Tomatoes	Large fruit, sweet taste	Varieties for supermarkets

Exam Tip: Selective breeding is the same as artificial selection. Use both terms in your answers to show the examiner you understand the terminology.

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Advantages and Disadvantages of Selective Breeding

Advantages	Disadvantages
Produces organisms with desired characteristics	Reduces genetic variation (gene pool narrows)
Can increase food production (larger yields, faster growth)	Can lead to inbreeding — related individuals are bred together
Has been practised for thousands of years — well understood	Inbreeding increases the risk of genetic diseases
No expensive technology required	Some breeds have health problems (e.g. bulldogs with breathing difficulties)
Can improve disease resistance in crops	Takes many generations — slow process

The Problem of Reduced Genetic Variation

When selective breeding is practised intensively:

• The gene pool (the total variety of alleles in a population) becomes smaller.
• If a new disease appears, the population may lack the genetic variation to survive it.
• Example: The Great Famine in Ireland (1845–1852) — potatoes had been selectively bred for high yield but lacked genetic variation, making them all vulnerable to potato blight (a fungal disease).

Exam Tip: A key disadvantage of selective breeding is reduced genetic variation, making populations vulnerable to disease. This is the most important disadvantage to mention in exam answers.

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Genetic Engineering

Genetic engineering (also called genetic modification or GM) is the process of altering the genome of an organism by inserting a gene from a different organism. The organism that receives the new gene is called a genetically modified organism (GMO).

The Process

1. Identify the gene that codes for the desired characteristic in the donor organism.
2. Cut the gene out of the donor's DNA using restriction enzymes (biological scissors).
3. Cut open the DNA of the recipient organism (e.g. a plasmid in a bacterium) using the same restriction enzyme.
4. Insert the desired gene into the recipient's DNA using ligase enzymes (biological glue).
5. The recipient organism now has the new gene and can produce the desired protein.

graph TD
    A["Identify desired gene in donor"] --> B["Cut gene using restriction enzymes"]
    B --> C["Cut open recipient DNA (e.g. plasmid)"]
    C --> D["Insert gene using ligase enzyme"]
    D --> E["Recipient organism produces new protein"]
    E --> F["Genetically Modified Organism (GMO)"]

Examples of Genetic Engineering

Application	Description
Insulin production	Human insulin gene inserted into bacteria; bacteria produce insulin for diabetics
Golden Rice	Rice modified to produce beta-carotene (vitamin A precursor) to combat vitamin A deficiency
Herbicide-resistant crops	Crops modified to survive weedkiller, making farming more efficient
Pest-resistant crops	Crops modified to produce a toxin that kills insect pests (e.g. Bt crops)
Gene therapy	Replacing faulty genes in human cells to treat genetic disorders (experimental)

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Benefits and Risks of Genetic Engineering

Benefits	Risks / Concerns
Can solve specific problems quickly (e.g. insulin production)	Unknown long-term effects on health and environment
GM crops can increase food production in developing countries	GM genes could spread to wild populations (gene flow)
Can reduce pesticide use (pest-resistant crops)	Could reduce biodiversity if GM crops dominate
Can produce medical proteins cheaply (e.g. insulin, growth hormone)	Ethical concerns about "playing God" and modifying life
Golden Rice can combat vitamin A deficiency	Some people refuse to eat GM food due to safety concerns
Faster than selective breeding — takes one generation	Corporate control of seed supplies may harm small farmers

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Ethical Issues

Genetic engineering and selective breeding raise several ethical questions:

Selective Breeding Ethics

• Is it right to breed animals for extreme traits that cause health problems (e.g. brachycephalic dog breeds)?
• Should we reduce genetic variation in food crops when it makes them vulnerable to disease?
• Does selective breeding cause animal suffering (e.g. chickens too heavy to walk)?

Genetic Engineering Ethics

• Is it right to modify the genes of living organisms?
• Should we allow human gene therapy — altering human DNA to treat disease?
• Who should control access to GM technology?
• Could GM organisms escape and affect natural ecosystems?
• Is it fair that some countries benefit from GM crops while others cannot afford the technology?

Exam Tip: When answering ethics questions, always give both sides of the argument. Use specific examples (e.g. Golden Rice, insulin production) rather than vague statements.

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Comparing Selective Breeding and Genetic Engineering

Feature	Selective Breeding	Genetic Engineering
Speed	Slow (many generations)	Fast (one generation)
Precision	Low — selects whole organisms	High — targets specific genes
Genetic variation	Reduces it over time	Can introduce entirely new genes
Technology needed	None (traditional farming)	Advanced laboratory equipment
Species barrier	Same species only	Can transfer genes between species
Used for	Agriculture, pet breeding	Medicine, agriculture, research

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Summary