Selective Breeding and Genetic Engineering

This lesson covers how humans use their understanding of genetics to modify organisms for useful purposes. You need to understand selective breeding and genetic engineering, including their benefits, risks and ethical implications. These are key topics in the AQA GCSE Combined Science Trilogy specification (8464).

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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 frequency of the desired characteristic increases in the population.

The Process

1. Choose parents with the desired characteristic from the existing population
2. Breed these selected individuals together
3. Select the best offspring — those that show the desired characteristic most strongly
4. Repeat the process over many generations until the desired trait is consistently present

graph TD
    A["Identify desired<br/>characteristic"] --> B["Select parents with<br/>that characteristic"]
    B --> C["Breed selected<br/>parents together"]
    C --> D["Select best offspring<br/>showing desired trait"]
    D --> E["Breed selected<br/>offspring together"]
    E --> F["Repeat over many<br/>generations"]
    F --> G["Population consistently<br/>shows desired trait"]
    style A fill:#bbdefb,stroke:#1565c0
    style B fill:#c8e6c9,stroke:#2e7d32
    style C fill:#fff9c4,stroke:#f9a825
    style D fill:#ffccbc,stroke:#d84315
    style E fill:#e1bee7,stroke:#6a1b9a
    style F fill:#b2dfdb,stroke:#00796b
    style G fill:#c8e6c9,stroke:#2e7d32

Examples of Selective Breeding

Organism	Desired Characteristic	Result
Cows	Higher milk yield	Modern dairy cows produce far more milk than wild cattle
Wheat	Disease resistance, higher grain yield	Modern wheat varieties produce more food per hectare
Dogs	Specific traits (size, temperament, appearance)	Hundreds of dog breeds from the wolf ancestor
Chickens	Larger eggs, more eggs per year	Hens lay far more eggs than wild jungle fowl
Roses	Colour, scent, disease resistance	Thousands of rose varieties
Cattle	Large muscle mass for beef production	Breeds like Belgian Blue have exceptional muscle growth

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Problems with Selective Breeding

Problem	Explanation
Reduced genetic variation	Breeding from a small number of selected individuals reduces the gene pool (inbreeding)
Inbreeding depression	Can lead to health problems — e.g. hip dysplasia in German Shepherds, breathing problems in Bulldogs
Vulnerability to disease	If all individuals are genetically similar, a new disease could wipe out the entire population
Loss of other useful traits	Selecting for one trait may reduce other characteristics (e.g. breeding for fast growth may reduce disease resistance)

Exam Tip: AQA (8464) often asks you to explain the disadvantages of selective breeding. The key point is that it reduces genetic variation, which makes the population vulnerable to environmental changes and disease.

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Genetic Engineering

Genetic engineering is the process of modifying the genome of an organism by introducing a gene from a different organism. The organism that receives the new gene is called a genetically modified (GM) organism or transgenic organism.

The Process of Genetic Engineering

1. Identify the useful gene in one organism (e.g. the gene for human insulin in human cells)
2. Cut the gene out of the DNA using restriction enzymes (biological "scissors")
3. Cut the DNA of the target organism (e.g. a bacterial plasmid) with the same restriction enzyme
4. Insert the useful gene into the target DNA using ligase enzymes (biological "glue")
5. The modified organism now has the new gene and can produce the desired protein

Examples of Genetic Engineering

Application	What Is Done	Benefit
Human insulin	The human insulin gene is inserted into bacteria; bacteria produce human insulin	Treats diabetes; more effective and cheaper than animal insulin
Golden Rice	A gene from daffodils is inserted into rice to produce beta-carotene (vitamin A)	Could reduce vitamin A deficiency in developing countries
Bt crops	A gene from the bacterium Bacillus thuringiensis is inserted into crop plants	Crops produce a natural insecticide, reducing need for chemical pesticides
Disease-resistant crops	Genes for disease resistance are inserted into crop plants	Higher yields; less crop loss

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Arguments For and Against Genetic Engineering

Arguments For	Arguments Against
Can produce medicines like insulin more cheaply	Long-term effects on health are not fully understood
Can increase crop yields to feed growing populations	GM crops could cross-pollinate with wild plants, spreading modified genes
Can make crops resistant to disease, reducing pesticide use	Raises ethical concerns about "playing God" or modifying nature
Could be used to treat genetic disorders in humans	Corporate control of GM seeds could harm small-scale farmers
Reduces reliance on animal insulin for diabetics	Some people have concerns about eating GM food

Exam Tip: AQA (8464) may ask you to "evaluate" genetic engineering or GM crops. You must discuss both advantages and disadvantages, and it helps to consider economic, social and ethical arguments.

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Comparing Selective Breeding and Genetic Engineering

Feature	Selective Breeding	Genetic Engineering
How it works	Breeding chosen organisms over many generations	Directly inserting a specific gene into an organism's DNA
Speed	Slow — takes many generations	Fast — one generation
Precision	Imprecise — many genes are transferred	Precise — a specific gene is targeted
Source of genes	Same species (or closely related)	Any species (even across kingdoms)
New alleles created?	No — only existing alleles are selected	Yes — new combinations are possible
Examples	Dog breeds, high-yield crops	GM insulin, Golden Rice

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Cloning

Although not genetic engineering, cloning is another technique that uses knowledge of genetics:

Plant Cloning: Tissue Culture

1. A small group of cells (explant) is taken from the parent plant
2. The cells are placed on a nutrient medium containing plant hormones
3. Cells divide by mitosis and grow into a new plant
4. The new plant is genetically identical to the parent