Lesson 6.07 · Spec 4.6.2.3–4.6.2.4 · Paper 2
Selective Breeding & Genetic Engineering
These two topics are routinely tested together on Paper 2 — including as 6-mark synoptic questions that ask you to chain genetic engineering (Module 6), competition (Module 7) and photosynthesis (Module 4) into one explanation of crop yield. Students who treat the topics separately get Level 1 marks; students who build the full cross-module chain hit Level 3. In the next 10 minutes, you'll know both processes and how to link them.
By the End of This Lesson…
You will be able to:
Define selective breeding and describe its 4-step process
State the four spec-named examples of selective breeding
Explain inbreeding and why it is a negative consequence of selective breeding
Define genetic engineering using exact spec wording
HT only: describe the 3 main steps of genetic engineering (enzymes → vector → recipient cells)
State examples: GM crops (disease/herbicide resistance), bacteria producing human insulin
Evaluate benefits and concerns of GM technology with balanced conclusion
Spec Refs
4.6.2.3 — Selective breeding (process, examples, inbreeding)
4.6.2.4 — Genetic engineering (definition, GM crops, examples, HT-only 3-step process)
Paper 2 · Higher Tier · ⭐ Priority topic · WS 1.3, 1.4
Selective Breeding: The Process
Spec Definition
Selective breeding (also called artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics. Humans have been doing this for thousands of years.
The spec describes a 4-step process. The last step — "repeat over many generations" — is the most commonly missed mark.
CHOOSE
Choose parents with the desired characteristic from a mixed population
BREED
Breed the chosen parents together
SELECT
From the offspring, select those that most strongly show the desired characteristic
REPEAT
Repeat over many generations until all offspring show the desired characteristic
Mark scheme note
A typical 4-mark "describe selective breeding" question awards one mark per step. Miss "many generations" and you cap at 3 — instant results are not a feature of selective breeding.
The Four Spec-Named Examples
The AQA spec lists four examples of characteristics chosen via selective breeding. Memorise all four — the spec uses these as the basis for application questions.
🌾
Disease resistance in food crops
🐄
Animals which produce more meat or milk
🐕
Domestic dogs with a gentle nature
🌸
Large or unusual flowers
Why "useful or appearance"?
The spec divides the chosen characteristic into usefulness (crops, meat, milk — agricultural value) and appearance (dog temperament, flower size — aesthetic value). Application questions may use either category.
Impact of selective breeding (spec requirement)
The spec asks you to "explain the impact of selective breeding of food plants and domesticated animals." The impact is the outcome — what selective breeding has achieved over thousands of years:
Food security — modern food crops produce far higher yields than their wild ancestors, helping feed growing populations
Disease-tolerant crops — reduced crop failures from blight or rust
Livestock productivity — modern cattle, pigs and chickens produce more meat, milk or eggs than their ancestors
Domesticated animals — dogs, horses and other species shaped to be safe and useful working partners for humans
For full marks on an "impact" question, balance the positive outcomes above with the negative consequence of inbreeding (next slide).
Spec-aligned WS skill
WS 1.3 / 1.4: students should be able to explain the benefits and risks of selective breeding, given appropriate information, and consider related ethical issues.
Inbreeding: The Negative Consequence
Definition
Inbreeding — repeatedly breeding closely related individuals together. Selective breeding inevitably involves inbreeding because the same desired alleles are repeatedly selected, leading to reduced genetic variation in the breed.
The spec states: "Selective breeding can lead to inbreeding where some breeds are particularly prone to disease or inherited defects." There are two distinct mechanisms behind this.
Mechanism 1 — Inherited defects
Reduced genetic variation means harmful recessive alleles are more likely to come together in homozygous form (e.g. ff) — increasing the chance of inherited disorders appearing in offspring.
Mechanism 2 — Disease susceptibility
Reduced genetic variation means the population has similar low resistance to pathogens. One new disease can devastate the entire breed/crop because they all share the same vulnerable alleles.
Exam phrasing
Don't write just "selective breeding has problems." The mark scheme wants the mechanism: inbreeding → reduced variation → susceptibility to inherited disorders OR new pathogens. Naming "inbreeding" + the mechanism is worth two marks.
Genetic Engineering: The Spec Definition
Spec Definition — memorise exactly
Genetic engineering — a process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic.
The key word is "another organism". This is what makes genetic engineering fundamentally different from selective breeding:
Selective breedingGenetic engineering
Source of traitExisting alleles within the same speciesA gene from another organism (different species)
TimeMany generations (slow)One generation (fast)
TechnologyNo specialist equipmentRequires enzymes, vectors, lab techniques
MechanismChoose parents → breedCut gene out → insert into vector → transfer to recipient cells
The 3-Step Process HT Only
Higher Tier students must be able to describe the main steps in the process of genetic engineering. The spec lists three steps. These appear word-for-word in mark schemes — learn them precisely.
1
Enzymes isolate the required geneEnzymes are used to "cut out" the required gene from the chromosomes of the donor organism. The gene is then inserted into a vector — usually a bacterial plasmid or a virus.
2
Vector inserts the gene into the required cellsThe vector (plasmid or virus) is used to transfer the gene into the required cells (spec wording) of the target organism. The gene is now part of the recipient's DNA.
3
Transfer at an early stage of developmentGenes are transferred to the cells of animals, plants or microorganisms at an early stage in their development — so that they develop with the desired characteristics throughout.
Spec wording — non-negotiable
The vector must be named as a bacterial plasmid or a virus. "DNA carrier" alone is not enough. The early-stage transfer point is regularly missed — without it, only some cells would carry the new gene.
Examples of Genetic Engineering
The spec names specific applications across agriculture and medicine.
🌾 GM Crops
Resistance to insect attack (e.g. genes producing insecticide proteins)
Resistance to herbicides — farmers spray weeds without harming the crop
Bigger/better fruits or higher yields
Disease resistance
🦠 Bacteria → Human Insulin
The human gene for insulin is inserted into bacteria
Bacteria express the gene and produce human insulin
Insulin is extracted and used to treat diabetes
Spec example — must be memorised
💊 Medical Research
Modern medical research is exploring genetic modification to overcome some inherited disorders
This is gene therapy
Spec context only — no detail required at GCSE
GM crops vs insect resistance
For GM crops resistant to insects, the mechanism is the plant producing its own insecticide. For GM crops resistant to herbicides, the mechanism is the plant tolerating chemicals that kill weeds — the herbicide is then sprayed on the whole field, killing weeds but not the crop. Two different mechanisms — don't confuse them.
Evaluating GM Technology
The spec requires you to "explain the potential benefits and risks of genetic engineering in agriculture and in medicine, and that some people have objections." For evaluation questions, balance both sides.
✓ Benefits
Higher yields (spec: "GM crops generally show increased yields") — more food from less land
Resistance to insects → less pesticide needed
Resistance to herbicides → easier, more effective weed control
Bacteria producing insulin → reliable, vegetarian-friendly supply, no animal extraction needed
Medical research into gene therapy could overcome inherited disorders
✗ Risks & Concerns
Effect on populations of wild flowers and insects (spec wording — non-target species affected)
Some people feel the effects of eating GM crops on human health have not been fully explored (spec wording)
Ethical objections — "playing God," interfering with natural processes
Concerns about gene transfer to wild relatives (cross-pollination)
Economic concerns — dependence on a few companies that own GM seeds
Evaluation answer structure
Give at least one benefit + one risk + a balanced conclusion. One-sided answers cap at half marks. Use the spec-listed concerns (wild flowers/insects; human health not fully explored) verbatim where possible.
Synoptic Chain: GM Crops & Yield
The most demanding GM question type asks how a GM crop increases yield. This requires a cross-module chain linking Module 6 (genetic engineering), Module 7 (competition), and Module 4 (photosynthesis). Skip any step and you cap at Level 2.
Step 1Herbicide-resistant gene inserted into crop
Step 2Farmer sprays herbicide — kills weeds, not crop
Step 3Less competition for light, water, mineral ions
Step 4Crop receives more resources
Step 5More photosynthesis → more glucose
Step 6More biomass / growth → higher yield
Why each step matters
Module 6 link: the GM technology (steps 1–2)
Module 7 link: the competition concept (steps 3–4)
Module 4 link: photosynthesis as the biomass source (steps 5–6)
Level 3 requirement
Writing "GM crops give bigger yields" is Level 1. The chain above is what scores Level 3 (5–6 marks). Cross-module linkage is the difference.
Worked Example
Selective breeding — 4 marks4 marks
A farmer wants to produce a variety of strawberry plant with larger fruit. Describe how selective breeding could be used to achieve this.
1
From a mixed population of strawberry plants, choose the plants that produce the largest fruit.
2
Breed these chosen plants together (cross-pollinate them).
3
From the offspring plants, select those that produce the largest fruit.
4
Repeat over many generations — over time, more and more offspring will produce large fruit because the alleles for large fruit become more common in each generation.
Step 1 (choose) ✓ · Step 2 (breed together) ✓ · Step 3 (select offspring) ✓ · Step 4 (many generations) ✓. Four for four. Missing "many generations" caps you at 3 — the most commonly dropped mark.
Examiner Alerts
Alert 1 — Don't confuse the two techniques
Selective breeding and GM are routinely mixed up. Selective breeding = choosing parents from the same species, no gene insertion. Genetic engineering = inserting a gene from another organism (different species). The species-boundary distinction is the cleanest test.
Alert 2 — The vector must be named (HT)
For HT genetic engineering questions, the vector must be named as a bacterial plasmid or a virus. "DNA carrier," "messenger," or vague terms do not score. Both options are listed in the spec — either is accepted.
Alert 3 — Synoptic chain integrity
For GM yield questions, all six chain steps must appear (herbicide → kills weeds → less competition → more resources → more photosynthesis → more glucose/growth). Examiner reports consistently flag students who jump straight from "spray herbicide" to "more yield" — the middle steps win the marks.
Common Errors
✗ Wrong
"Selective breeding is the same as genetic engineering"
✓ Right
"Selective breeding = choosing parents (same species, slow). GM = inserting a gene from another organism (fast, requires tech)"
✗ Wrong
"Selective breeding makes new alleles"
✓ Right
"Selective breeding selects existing alleles already present in the population — it does not create new ones"
✗ Wrong
"The plasmid becomes part of the cell"
✓ Right
"The plasmid is a vector — its job is to carry the gene into the recipient cell"
✗ Wrong
"GM crops are dangerous" (one-sided)
✓ Right
Balance benefits AND concerns AND give a conclusion — one-sided answers cap at half marks
Recap: Three Takeaways
1
Selective breeding: humans choose parents with desired characteristics, breed them, select best offspring, repeat over many generations. Spec examples = disease-resistant crops, more meat/milk animals, gentle dogs, large flowers. Negative consequence = inbreeding → reduced variation → inherited defects + disease susceptibility.
2
Genetic engineering: modifying the genome by inserting a gene from another organism to give a desired characteristic. HT 3 steps: enzymes isolate gene → gene inserted into vector (plasmid or virus) → transferred to recipient cells at an early stage of development. Spec examples = GM crops (insect/herbicide resistance), bacteria producing human insulin.
3
Evaluation: always balance benefits and concerns. Spec-listed concerns: effect on wild flowers and insects; human health effects not fully explored. The 6-step synoptic chain (GM → kills weeds → less competition → more photosynthesis → more glucose → more yield) is the Level-3 answer template.
Checkpoint
You should now be able to:
✓ Describe selective breeding in 4 steps, including "repeat over many generations"
✓ List the four spec-named examples of selective breeding
✓ Explain inbreeding using the mechanism: reduced variation → inherited defects + disease susceptibility
✓ State the genetic engineering definition word-for-word from the spec
✓ HT: describe the 3-step genetic engineering process, naming the vector as a plasmid or virus
✓ Give the spec examples: GM insect/herbicide-resistant crops, bacteria producing human insulin
✓ Build the 6-step synoptic chain linking GM, competition, and photosynthesis to yield
✓ Evaluate GM with benefits, spec-listed concerns, and a balanced conclusion
Next lesson
Lesson 6.08 — CloningBiology Only: tissue culture, cuttings, embryo transplants, and adult cell cloning (the Dolly the sheep technique).
Next: By the End of This Lesson…