AQA GCSE Biology Biology Paper 2 (Higher), June 2023: Question 3

✅ Correct Answers

03.1 (Genetically different - 1 mark):

• If a disease or pathogen appears, not all plants will be susceptible / killed.
• Produces a variation in desirable characteristics (e.g. different flavours, colours, or sizes suited to different customer preferences).
• Maintains or increases the gene pool.

03.2 (Genetically identical - 1 mark):

• All crops share the same desirable trait (e.g. all high yield, sweet flavour, uniform size).
• They grow at the same rate and are ready to harvest at the same time.

❌ Common Errors

• Vague statements like "they look better" or "they sell more" without naming a specific biological or commercial trait.
• Claiming identical plants "cannot catch diseases" (in reality, if one catches it, all catch it).
• Confusing identical plants with genetically modified (GM) crops.

✅ Correct Answers

03.3 Definition of Tissue Culture [1 mark]:

A group of cells (or small pieces of plant tissue) are grown into a new plant / whole organism (using a nutrient medium with plant hormones under sterile conditions).

03.4 Reason for Different Heights [1 mark]:

Differences in environmental conditions (named factor), such as:

• Different availability of light
• Different amounts of water / rain
• Differences in mineral ions (e.g. magnesium, nitrates)
• Differences in soil pH, temperature, or spacing / competition

🧠 Exam Technique: Phenotype = Genotype + Environment

• In 03.3: Simply writing "making clones" receives 0 marks. You must state what is done: growing cells/tissue into new plants.
• In 03.4: Since the plants are genetically identical, any variation in height must be caused entirely by the environment.
• Avoid unqualified words like "food" or "nutrients" for plants. Use mineral ions, nitrates, or light.

📐 Completing the Punnett Square [3 marks]

💡 03.6 Explaining Expected Ratio [1 mark]

Question: Why expect 3 male and 3 female from 6 offspring?

Model Answer:

• There is an equal (50% / 1:1) chance of an embryo being male ( XY ) or female ( XX ).
• Alternative wording accepted: Half the offspring in the Punnett square are XX and half are XY (equal probability of an X-sperm or Y-sperm fertilising the egg).

✅ 03.7 Two Control Variables [2 marks]

Any two of the following (1 mark each):

• Food: Type / nutritional content or amount of food provided
• Water: Volume / availability of water
• Temperature: Keeping both groups in the same ambient temperature
• Housing: Whether kept indoors or outdoors / stocking density / space to move
• Age: Age / starting mass of chickens at the start of the 5 weeks
• Light: Lighting hours per day (photoperiod affects egg laying)

🧠 03.8 Why Use 500 Chickens? [1 mark]

Correct reasoning (any one):

• To reduce the effect of anomalies / random variation.
• To calculate a more representative / valid / accurate mean.

❌ Common Errors in 4-Mark Selective Breeding Questions

• Missing the repetition: Stating "repeat the experiment" instead of "repeat over many generations".
• Breeding the parents again: Do NOT say "breed the original parents together again". You must breed the selected offspring.
• Forgetting to specify traits: Stating just "breed the best chickens" without defining that "best" means both fast growth and high egg production.

🧠 Examiner Checklist for Full Marks

• Did you name varieties A and B in your first cross?
• Did you state selection of offspring with both traits?
• Did you say breed the offspring together?
• Did you use the phrase "over many generations"?