What you'll learn
- How humans use selective breeding to produce useful food plants and domesticated animals.
- How tissue culture can produce many identical plants or cells.
- What genetic engineering is, and how genes can be moved between organisms.
- How to evaluate benefits, risks, practical issues and ethical concerns.
Starting point: genes, genomes and variation
A characteristic is a feature of an organism, such as flower colour, milk yield, disease resistance or growth rate.
A gene is a section of DNA that codes for a particular protein or affects a characteristic. Different versions of the same gene are called alleles.
Genome
The genome of an organism is all of its genetic material — all the DNA in its cells.
Organisms in the same species are not all identical. This is called variation. Variation can be caused by:
- genetic differences, such as different alleles
- environmental differences, such as diet, temperature, light or disease
For selective breeding to work, the useful characteristic must be at least partly inherited — passed from parents to offspring through genes.
Breeding needs inherited variation
Selective breeding can only “build up” a characteristic over generations if there is genetic variation for that characteristic in the population.
Deciding whether a characteristic can be bred for
A farmer wants to breed tomato plants that produce large fruit. Some plants have larger tomatoes because they were given more fertiliser.
- Compare whether the large fruit size is caused mainly by genes or environment. If only the fertilised plants have large fruit, fertiliser may be the main cause.
- Grow seeds from large-fruited plants and small-fruited plants in the same conditions. This makes the environment fairer.
- If offspring from the large-fruited parents still tend to produce larger fruit, the characteristic is likely to have a genetic basis, so selective breeding could increase it over generations.
Selective breeding
Selective breeding
Selective breeding is when humans choose organisms with desired characteristics and breed them together, then repeat this over many generations.
It is also called artificial selection because humans, not the environment, decide which organisms reproduce.
Selective breeding is used in:
- food plants — for higher yield, larger fruits, disease resistance, better taste or drought tolerance
- domesticated animals — for more milk, meat or eggs, faster growth, calm behaviour or disease resistance
The basic pattern is: choose parents with the wanted characteristic, breed them, choose the best offspring, then repeat.
The stages of selective breeding
- Identify variation in the population.
- Select individuals with the desired characteristic.
- Breed these selected individuals together.
- Select offspring that show the characteristic most strongly.
- Repeat for many generations until the characteristic becomes common.
Breeding wheat for disease resistance
A plant breeder wants wheat that survives a fungal disease.
- Start with a mixed population of wheat plants and expose them to the disease under controlled conditions.
- Select the plants that survive best and still produce good grain yield.
- Breed those selected plants together and grow their seeds.
- Test the offspring again against the disease, then select the best survivors.
- Repeat this over generations, so alleles linked to disease resistance become more common in the wheat population.
Impacts of selective breeding
Selective breeding can be very useful in agriculture. It can increase food production and make farming more reliable.
But there are risks. Repeatedly breeding closely related organisms can reduce the gene pool — the total variety of alleles in a population. This can cause inbreeding, where harmful recessive alleles are more likely to be inherited together.
Forgetting the downside
Selective breeding is not always “safe because it is natural”. It can reduce genetic variation and may increase inherited disorders or animal welfare problems.
Examples of possible problems include:
- pedigree dogs with breathing or joint problems
- dairy cattle selected for high milk yield but with increased strain on the body
- crops that are genetically similar, so one disease could affect many plants
Tissue culture
In Separate Biology, you also need to know about tissue culture.
Tissue culture
Tissue culture is a technique used to grow cells, tissues or whole plants from a small sample in sterile conditions.
In plants, tissue culture is often used to produce clones. A clone is an organism or cell that is genetically identical to another.
A small piece of plant tissue is called an explant. It is sterilised, then grown on a nutrient medium containing mineral ions, sugars and plant growth hormones. The cells divide to form a mass of unspecialised cells called a callus. Shoots and roots then grow, forming small plantlets.
Advantages of tissue culture
Tissue culture is useful because it can:
- produce many plants quickly from one parent plant
- make plants with a desired characteristic, such as disease resistance
- produce plants that are genetically identical, which is useful in experiments
- help conserve rare plant species
- produce disease-free plants if healthy tissue is used
In medical research, tissue culture can be used to grow cells for testing medicines or studying diseases. This helps scientists control variables more carefully because the cells are genetically identical.
Clones are useful for fair tests
If all plants in an experiment are clones, genetic differences are reduced. That makes it easier to see the effect of the factor being tested, such as fertiliser concentration or light intensity.
Genetic engineering
Genetic engineering
Genetic engineering is modifying the genome of an organism by introducing, removing or changing genes to give a desired characteristic.
An organism produced using genetic engineering is often called a genetically modified organism, or GM organism.
Selective breeding works by choosing from existing variation over generations. Genetic engineering can introduce a specific gene more directly, sometimes from a different species.
Selective breeding versus genetic engineering
Selective breeding changes which organisms reproduce over many generations. Genetic engineering changes the DNA of cells directly by modifying the genome.
Main stages of genetic engineering
At Higher Tier, you should be able to describe the main stages using the correct enzyme and DNA vocabulary.
Important terms:
- A restriction enzyme cuts DNA at a specific base sequence.
- Sticky ends are short, unpaired sections of DNA bases left after some restriction enzymes cut DNA.
- Ligase is an enzyme that joins pieces of DNA together.
- A vector carries the gene into another cell. A bacterial plasmid, a small circular piece of DNA, is a common vector.
- Recombinant DNA is DNA made by joining DNA from different sources.
The process
- Identify the gene for the desired characteristic.
- Use a restriction enzyme to cut out the gene.
- Use the same restriction enzyme to cut open a vector, such as a plasmid.
- The gene and plasmid have matching sticky ends.
- Use ligase to join the gene into the plasmid.
- Insert the recombinant plasmid into a host cell.
- The host cell expresses the gene, producing the desired characteristic or protein.
Explaining why the same restriction enzyme is used
A scientist wants to insert a human gene into a bacterial plasmid.
- The human DNA and plasmid are cut using the same restriction enzyme, so both pieces are cut at the same base sequence.
- This produces complementary sticky ends on the human gene and the opened plasmid.
- The matching sticky ends can base-pair, lining up the human gene with the plasmid.
- Ligase then joins the sugar-phosphate backbones, forming recombinant DNA that can be placed into a bacterial cell.
GM crops, including Bt crops
In Separate Biology, you also consider examples of GM crops in more detail.
One example is adding a gene from the bacterium Bacillus thuringiensis into crop plants. This gene makes a protein that is toxic to certain insect pests. These are often called Bt crops.
Advantages can include:
- less crop damage by insects
- higher yield
- less need for chemical insecticides
- lower costs for spraying and labour
- reduced environmental impact from insecticide use
Disadvantages and risks can include:
- insects may evolve resistance over time
- possible effects on non-target insects
- GM genes could spread to wild relatives
- seeds may be expensive or controlled by companies
- some people have ethical or safety concerns about GM food
Comparing crop yield data
A non-GM crop produces 7.5 tonnes per hectare. A GM insect-resistant crop produces 9.0 tonnes per hectare.
- Find the increase in yield: 9.0 tonnes per hectare minus 7.5 tonnes per hectare = 1.5 tonnes per hectare.
- Calculate the percentage increase using the original yield as the denominator:
- The GM crop produced 20% more yield. To evaluate whether it is better overall, also consider seed cost, environmental effects and whether insects may become resistant.
Farming for a growing human population
As the human population grows, agriculture must produce more food. In Separate Biology, you also need to evaluate solutions such as fertilisers and biological control.
Fertilisers
A fertiliser adds mineral ions to soil, such as nitrates for making proteins and phosphates for growth. This can increase crop yield.
Benefits include faster plant growth and higher food production. However, fertilisers can be expensive and may be washed into rivers and lakes, causing eutrophication. This can lead to algal blooms, reduced light, death of aquatic plants, and low oxygen levels as decomposers respire.
Biological control
Biological control
Biological control is using another living organism, such as a predator, parasite or pathogen, to reduce the population of a pest.
For example, ladybirds can be used to reduce aphid numbers.
Benefits include fewer chemical residues and a more targeted method than broad insecticide spraying. But the control organism may not work quickly, may not remove all pests, or could become invasive if introduced into a new ecosystem.
Biological control must be checked carefully
Introducing a new species can have unexpected effects on food webs. Once released, it may be difficult to remove.
Evaluating selective breeding and genetic engineering
In questions, “evaluate” means you must consider both sides and make a judgement using evidence.
Benefits
Selective breeding and genetic engineering can help by:
- increasing food production
- producing crops resistant to pests, disease or drought
- improving nutritional content
- producing medicines, such as human insulin made by genetically engineered bacteria
- reducing pesticide use in some cases
- making research more controlled using cloned cells or tissues
Risks and concerns
You should also consider:
- reduced genetic variation from selective breeding
- animal welfare issues
- possible spread of GM genes into wild populations
- evolution of resistant pests
- unknown long-term ecological effects
- cost and access, especially for poorer farmers
- ethical concerns about changing genomes or patenting organisms
Practical versus ethical
A practical issue is about whether something works, how much it costs, or what effects it has. An ethical issue is about whether people think it is morally acceptable.
In the exam
- For selective breeding, always include selection, breeding, offspring, and repetition over generations.
- For genetic engineering, use precise terms where needed: restriction enzyme, sticky ends, ligase and vector.
- In evaluation questions, give both benefits and risks, then make a balanced judgement linked to the context.
Check yourself
- How is selective breeding different from genetic engineering?
- Why can selective breeding reduce genetic variation?
- What roles do restriction enzymes, ligase and vectors play in genetic engineering?
