4.3.1 Selective breeding and its impact

Selective breeding concentrates chosen alleles

1. Selective breeding is the repeated breeding of organisms chosen for an inherited characteristic so that alleles for that characteristic become more common.
2. Choose parents that show the desired phenotype, such as high milk yield, disease resistance or large fruit.
3. Breed the selected parents and assess the offspring under the same conditions.
4. Select offspring that show the characteristic most strongly and breed them together.
5. Repeat the process over many generations until the characteristic is common and stable in the population.
6. Selective breeding uses existing variation and sexual reproduction, so it is slower and less precise than transferring a known gene.

Benefits can reduce genetic diversity

1. Food plants can be bred for higher yield, improved nutritional value, resistance to disease, uniform ripening or tolerance of drought.
2. Domesticated animals can be bred for greater meat, milk or egg production, a calm temperament or resistance to disease.
3. Repeatedly using closely related individuals narrows the gene pool and increases inbreeding.
4. A smaller gene pool raises the chance that harmful recessive alleles meet in offspring and reduces the population's ability to respond to a new disease or environmental change.

4.3.2 Tissue culture

Tissue culture makes genetically identical cells

1. Tissue culture is the growth of cells or small pieces of tissue on a sterile nutrient medium so that many genetically identical cells or organisms are produced.
2. For plants, remove a small piece of tissue called an explant from a parent with the desired characteristics.
3. Sterilise the explant and equipment to prevent bacteria or fungi from outcompeting the plant cells.
4. Place the explant on sterile agar containing minerals, sugar and plant growth regulators.
5. Keep the culture under controlled light and temperature until cells divide by mitosis and form a mass of cells.
6. Transfer small groups of cells to media with suitable hormone balances so shoots and roots develop, then move plantlets to compost and acclimatise them gradually.

Clones give consistency and rapid production

1. A clone is an organism or cell that is genetically identical to another because it was produced from one parent by mitosis.
2. A small sample can produce many plants quickly, at any time of year and in little space.
3. Rare plants can be conserved and crop varieties with high yield or disease resistance can be copied without losing their allele combination.
4. Genetically identical cells allow medical researchers to test treatments while reducing genetic variation between samples.
5. All clones may share the same susceptibility to a pathogen, and mutation or contamination can affect many cultures.

4.3.3 Genetic engineering overview

Genetic engineering changes an organism's genome

1. Genetic engineering is the deliberate modification of an organism's genome by inserting, removing or changing DNA to produce a desired inherited characteristic.
2. A gene contains the base sequence for a protein, so transferring a gene can give the recipient cell instructions to make a new protein.
3. The inserted gene becomes part of the recipient's genetic material and can be copied when the cell divides.
4. An organism containing DNA introduced from another species is described as transgenic.
5. Genetic engineering targets a known gene, while selective breeding mixes many alleles from two parents.

Desired features depend on gene expression

1. The transferred gene must enter suitable cells and be switched on for its protein to be made.
2. A regulatory DNA sequence may be needed so the host cell transcribes the inserted gene.
3. Successfully modified cells must be identified and copied to produce enough cells or a complete organism.
4. The new phenotype depends on the protein's action, such as producing an insecticidal protein or a medically useful hormone.

4.3.4 Main stages of genetic engineering

Enzymes cut and join the required gene

1. A restriction enzyme cuts DNA at a specific recognition sequence.
2. Use a restriction enzyme to cut the required gene from donor DNA.
3. Cut a plasmid with the same restriction enzyme so the gene and plasmid have complementary sticky ends.
4. Sticky ends are short exposed single-stranded DNA sequences that pair by complementary base pairing.
5. Mix the gene and opened plasmid so matching sticky ends join.
6. DNA ligase seals the sugar-phosphate backbone, producing a recombinant plasmid.

A vector carries the gene into cells

1. A vector is a carrier used to transfer genetic material into a cell.
2. Plasmids are small circular DNA molecules that can be removed from bacteria, modified and returned to bacterial cells.
3. Viruses can also act as vectors because they naturally insert genetic material into cells.
4. Introduce the recombinant vector into recipient cells, then use a marker to identify cells that received it.
5. Grow the selected cells by mitosis so the inserted gene is copied and its protein can be produced on a large scale.

The whole process forms a causal chain

1. The restriction site determines where DNA is cut and whether compatible sticky ends form.
2. Complementary sticky ends hold the two DNA pieces together before ligase makes the join permanent.
3. The vector protects and transports the gene, while the host cell supplies the machinery for gene expression.
4. Selection separates successfully modified cells from cells that did not take up the vector.

4.3.5 Advantages and disadvantages of GM organisms

GM crops can gain useful characteristics

1. A transgenic organism contains genetic material introduced from another species by genetic engineering.
2. A gene from the bacterium Bacillus thuringiensis can be inserted into a crop plant so its cells produce the Bt protein.
3. The protein is toxic to particular insect pests that feed on the crop, so less crop tissue is damaged.
4. Reduced pest damage can increase yield and reduce the amount of broad-spectrum insecticide sprayed.
5. Other modifications can improve nutritional content, delay ripening, produce medicine or increase tolerance of drought, salt or disease.

Benefits must be weighed against risks

1. Biodiversity is the variety of different species living in an area.
2. Pollen can transfer an inserted allele to wild relatives, where the characteristic may spread beyond the crop field.
3. Heavy use of one insecticidal trait creates selection pressure that can increase resistant pest populations.
4. A toxin may affect non-target organisms, food webs or biodiversity, so effects must be monitored.
5. Seeds may be expensive or controlled by a small number of companies, which can make farmers dependent on suppliers.
6. Potential health effects are tested before approval, but uncertainty about long-term ecological effects remains part of an evaluation.

4.3.6 Agricultural solutions for a growing population

Fertilisers raise yield but can pollute water

1. Fertilisers supply mineral ions that may limit plant growth, including nitrate ions for amino acids and magnesium ions for chlorophyll.
2. Correct application can increase growth and crop yield, helping more food to be produced from the same area of land.
3. Excess soluble fertiliser can be washed from soil into rivers and lakes by rain.
4. The extra nitrate stimulates rapid algal growth, forming an algal bloom that blocks light.
5. Aquatic plants die, decomposers multiply and their respiration removes dissolved oxygen.
6. Low oxygen can kill fish and other aerobic organisms, a process called eutrophication.

Biological control uses natural enemies

1. Biological control reduces a pest population by introducing or encouraging a predator, parasite or pathogen of that pest.
2. It can reduce chemical pesticide use, leave fewer chemical residues and provide longer-term control if the control organism reproduces.
3. The control organism may not remove the pest quickly enough and usually does not eradicate it.
4. An introduced organism may attack non-target species, spread beyond the intended area or disrupt food webs.
5. The control species should be host-specific, tested in containment and monitored after release.

Agriculture needs a balanced judgement

1. A growing population increases demand for reliable food production, but higher yield can create environmental costs.
2. Fertilisers give rapid, predictable mineral supply, while biological control can reduce repeated pesticide use.
3. The best method depends on crop, climate, pest, cost, speed, environmental risk and the evidence available.
4. Integrated pest management combines monitoring, targeted chemicals, resistant varieties and biological controls to reduce harm while maintaining yield.

4.3.7 Evaluating genetic engineering and breeding

The methods change populations in different ways

1. Genetic engineering directly changes DNA by inserting, removing or altering a gene, so a chosen characteristic can be introduced in one generation.
2. It can cross species boundaries and target a known protein, but the inserted gene may have unexpected effects or spread into other populations.
3. The method requires specialist equipment, testing and regulation, which can make development expensive.

Selective breeding uses existing variation

1. Selective breeding chooses parents with desirable inherited characteristics and repeats selection over many generations.
2. It is widely used and does not require direct DNA manipulation, but it is slower and transfers many alleles together.
3. Inbreeding can reduce genetic diversity and increase the frequency of harmful recessive disorders.

Agricultural benefits have linked risks

1. Higher yield, improved nutrition, disease resistance and drought tolerance can strengthen food security.
2. Reduced pesticide use can lower costs and harm to non-target organisms.
3. Gene flow, resistant pests, reduced biodiversity and dependence on patented seed can create environmental or economic costs.
4. Animal breeding for extreme production may reduce welfare if rapid growth or body shape causes pain or disease.

Medical uses require ethical safeguards

1. Genetically modified microorganisms can produce human proteins such as insulin in large, consistent quantities.
2. Genetic technologies may help treat disease, but changes to body cells and inherited changes to embryos have different consequences.
3. Ethical concerns include consent, fairness of access, animal welfare, ownership of genetic material and effects on future generations.
4. Risk is reduced by testing, containment, monitoring, regulation and comparing the method with available alternatives.