Selective Breeding and Genetic Engineering

This lesson covers two ways humans intentionally alter the genetics of organisms: selective breeding (artificial selection) and genetic engineering. For AQA GCSE Biology, you need to understand how each process works, their advantages and disadvantages, and the ethical issues surrounding genetic engineering.

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Selective Breeding (Artificial Selection)

Selective breeding (also called artificial selection) is the process by which humans choose organisms with desirable characteristics to breed together. Over many generations, the frequency of the desired trait increases in the population.

How selective breeding works:

1. Choose individuals with the desired characteristic from the population
2. Breed these individuals together
3. Select the offspring that show the desired characteristic most strongly
4. Repeat over many generations until the trait is consistently expressed

graph TD
    A[Identify desired characteristic] --> B[Select individuals showing the trait]
    B --> C[Breed selected individuals together]
    C --> D[Select best offspring]
    D --> E[Breed selected offspring together]
    E -->|Repeat over many generations| D
    D --> F[Desired trait becomes common in population]

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Examples of Selective Breeding

Selective breeding has been used by humans for thousands of years:

In Plants

Purpose	Example	Desired characteristic
Higher crop yields	Wheat, rice, maize	Larger grain size, more grains per plant
Disease resistance	Wheat varieties resistant to fungal infections	Ability to resist specific pathogens
Improved flavour or appearance	Tomatoes, strawberries	Sweeter taste, larger size, appealing colour
Drought tolerance	Drought-resistant maize for arid regions	Ability to grow with less water
Faster growth	Modern wheat varieties	Shorter growing season

In Animals

Purpose	Example	Desired characteristic
Higher milk yield	Dairy cattle (e.g. Holstein-Friesian)	Produces more litres of milk per day
More meat	Beef cattle (e.g. Aberdeen Angus), broiler chickens	Faster growth, larger muscle mass
Better temperament	Dogs bred for companionship	Friendly, calm behaviour
Disease resistance	Livestock resistant to specific diseases	Reduced mortality, less need for antibiotics
Speed or strength	Racehorses, greyhounds	Athletic performance

Exam Tip: When describing selective breeding, always use the four-step process: choose, breed, select, repeat. This structured approach will secure marks. Also, be ready to give specific examples — examiners like to see that you understand real applications.

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Problems with Selective Breeding

While selective breeding is useful, it has significant drawbacks:

Problem	Explanation
Reduced genetic variation	By selecting only certain individuals to breed, other alleles are lost from the population. This is called a gene pool reduction
Inbreeding	Breeding closely related individuals increases the chance of offspring being homozygous for harmful recessive alleles, leading to health problems
Inbreeding depression	Reduced fertility, increased susceptibility to disease, and other health problems caused by inbreeding
Vulnerability to disease	A genetically uniform population can be wiped out by a single disease because all individuals are equally susceptible
Loss of natural behaviours	Selectively bred animals may lose natural instincts or behaviours
Ethical concerns	Some breeds suffer health problems (e.g. bulldogs with breathing difficulties, dairy cows with udder problems)

Exam Tip: The key disadvantage of selective breeding is reduced genetic variation. This makes the population vulnerable to new diseases or environmental changes because there is less variation for natural selection to act upon.

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Genetic Engineering

Genetic engineering (also called genetic modification or GM) is the process of transferring a gene from one organism to another in order to produce a desired characteristic. Unlike selective breeding, genetic engineering can transfer genes between different species.

How genetic engineering works:

1. Identify the gene responsible for the desired characteristic in the donor organism
2. Cut the gene out of the donor's DNA using restriction enzymes (biological "scissors" that cut DNA at specific base sequences)
3. Cut open the DNA of the recipient organism (or a vector, such as a bacterial plasmid) using the same restriction enzymes
4. Insert the desired gene into the recipient's DNA using ligase enzymes (biological "glue" that joins DNA strands together)
5. The recipient organism now has the new gene and can produce the desired protein

graph TD
    A[Identify desired gene in donor organism] --> B[Cut gene using restriction enzymes]
    B --> C[Cut open vector e.g. plasmid using same restriction enzymes]
    C --> D[Insert gene into vector using ligase enzyme]
    D --> E[Insert vector into recipient organism]
    E --> F[Recipient expresses new gene]
    F --> G[Desired protein is produced]

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Examples of Genetic Engineering

Application	Details
Insulin production	The human insulin gene is inserted into bacteria (E. coli). The bacteria reproduce rapidly and produce large quantities of human insulin for treating diabetes
Golden Rice	Rice genetically modified to produce beta-carotene (a precursor to vitamin A), helping to prevent vitamin A deficiency in developing countries
Bt crops	Crops modified with a gene from the bacterium Bacillus thuringiensis that produces a toxin lethal to insect pests, reducing the need for pesticides
Disease-resistant crops	Crops modified to resist specific plant diseases, increasing yields
Gene therapy [H]	Inserting functional genes into human cells to treat genetic disorders (e.g. cystic fibrosis) — still largely experimental

Insulin Production: A Key Example

Before genetic engineering, insulin for diabetics was extracted from pig or cow pancreases. This had several problems:

• Some patients had allergic reactions to animal insulin
• Supply was limited and expensive
• Religious or ethical objections to using animal products

Genetically engineered human insulin solved all of these problems. It is identical to natural human insulin, can be produced in unlimited quantities, and does not involve animals.

Exam Tip: The production of human insulin by genetically modified bacteria is the most commonly examined example of genetic engineering. Make sure you know the steps: identify gene, cut with restriction enzymes, insert into plasmid, place in bacterium, bacterium produces insulin.

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Advantages and Disadvantages of Genetic Engineering

Advantages

Advantage	Explanation
Can transfer genes between species	Not limited to closely related organisms, unlike selective breeding
Faster than selective breeding	A new characteristic can be introduced in one generation
Can solve specific problems	E.g. vitamin A deficiency (Golden Rice), insulin shortage
Reduces pesticide use	Pest-resistant crops need fewer chemical pesticides
Can improve food security	Higher-yielding, drought-resistant crops help feed growing populations

Disadvantages