Selective Breeding and Genetic Engineering

Selective breeding is the process of choosing organisms with desired characteristics to breed together, producing offspring that inherit those desirable traits.

The Process

1. Choose parents with the desired characteristic (e.g., cows that produce the most milk)
2. Breed these parents together
3. Select the best offspring — those that show the desired characteristic most strongly
4. Repeat over many generations
5. Over time, the characteristic becomes more prominent in the population

Examples

Advantages

• Can improve crop yields and food production
• Produces animals with economically desirable traits
• No complex technology needed
• Has been practised for thousands of years with proven results

Disadvantages

• Reduces genetic variation (gene pool becomes smaller) — called inbreeding
• Inbred populations are vulnerable to disease (if one is susceptible, most will be)
• Can cause health problems in animals (e.g., breathing issues in flat-faced dogs like pugs)
• Undesirable traits may be accidentally selected alongside desirable ones
• Takes many generations — slow process

Genetic engineering (also called genetic modification or GM) is the direct manipulation of an organism's DNA to introduce a gene from another organism.

The Process

1. Identify the desired gene in one organism (e.g., a gene for insect resistance in a bacterium)
2. Cut the gene out of the DNA using restriction enzymes (molecular scissors that cut at specific base sequences)
3. Cut the DNA of the target organism (e.g., a crop plant) using the same restriction enzyme
4. Insert the desired gene into the target organism's DNA using ligase enzymes (molecular glue)
5. Often, a vector is used to transfer the gene — this could be a plasmid (small ring of DNA in bacteria) or a virus
6. The target organism now has the new gene and can produce the protein it codes for
7. The organism is a genetically modified organism (GMO) or transgenic organism

Examples of Genetic Engineering

Advantages

• Can produce exactly the desired outcome in one generation (much faster than selective breeding)
• Can transfer genes between species that could never naturally breed
• Can produce life-saving medicines (e.g., insulin)
• Can improve crop yields and nutritional value
• Can develop crops that grow in harsh conditions (drought-resistant, salt-tolerant)

Disadvantages and Concerns

• Ethical concerns — "playing God" / interfering with nature
• GM organisms might spread into wild populations and disrupt ecosystems
• Potential unknown long-term health effects of eating GM food (though no evidence of harm has been found)
• Cross-pollination — GM crops could pollinate wild plants, creating herbicide-resistant "superweeds"
• Reduces biodiversity if GM crops replace traditional varieties
• Corporate control — GM seeds may be patented, making farmers dependent on biotechnology companies

Cloning produces genetically identical copies of an organism.

Plant Cloning — Tissue Culture

1. Take a small piece of plant tissue (often from a meristem/growing tip)
2. Place it on agar growth medium containing nutrients and hormones
3. Keep in sterile conditions to prevent contamination
4. The tissue grows into a small plant (plantlet)
5. Transfer to soil — the plant is a clone of the original

Advantages: Produces many identical plants quickly; preserves desirable characteristics; works with rare plants.

Plant Cloning — Cuttings

• Cut a section of stem with a leaf node
• Dip in rooting hormone powder
• Plant in moist compost
• The cutting develops roots and grows into a new plant — genetically identical to the parent

Animal Cloning — Somatic Cell Nuclear Transfer (Dolly the Sheep)

1. Remove the nucleus from an egg cell (enucleation)
2. Take the nucleus from a body (somatic) cell of the animal to be cloned
3. Insert the somatic cell nucleus into the enucleated egg cell
4. Stimulate the cell with an electric shock to start cell division
5. The embryo develops and is implanted into a surrogate mother
6. The resulting offspring is a genetic clone of the animal that donated the body cell

Dolly the Sheep (1996) was the first mammal cloned from an adult cell.

Advantages of Cloning

• Preserves desirable genetic characteristics
• Can save endangered species
• Produces identical organisms for research
• Agricultural benefits (identical high-yield crops)

Disadvantages of Cloning

• Reduces genetic variation — vulnerable to disease
• Cloned animals may have health problems and shorter lifespans (Dolly developed arthritis and lung disease)
• Low success rate — many embryos fail
• Ethical concerns — is cloning animals morally acceptable? What about cloning humans?

Question: Describe how selective breeding could be used to produce wheat plants that are resistant to a fungal disease. (4 marks)

Solution:

From an existing population of wheat plants, the farmer would select individual plants that show the most resistance to the fungal disease (e.g., those least affected by the fungus). These disease-resistant plants would be bred together. From the offspring, the plants showing the greatest resistance would be selected again and bred together. This process would be repeated over many generations. Over time, the proportion of disease-resistant plants would increase, eventually producing a variety of wheat that is largely resistant to the fungal disease.

• Selective breeding = same species, many generations, natural variation. Genetic engineering = different species possible, one generation, DNA manipulation.
• For genetic engineering, always mention: restriction enzymes (cut), ligase (join), vector (transfer).
• In ethics questions, give both sides — advantages AND disadvantages. Be balanced.
• Don't confuse cloning with genetic engineering — cloning produces identical copies; genetic engineering changes the DNA.

• Selective breeding: choose desirable individuals → breed → select best offspring → repeat over many generations.
• Genetic engineering: cut a gene from one organism → insert into another → produces GMOs.
• Key tools: restriction enzymes (cut DNA), ligase (join DNA), vectors (transfer DNA, e.g., plasmids).
• Cloning: produces genetically identical organisms (tissue culture for plants, nuclear transfer for animals).
• All techniques raise ethical concerns and have implications for genetic variation and biodiversity.