AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2018: Question 7

✅ Full-mark focus

• Use the correct chromosome arrangement for a male oyster.
• For meiosis, include DNA/chromosomes replicate, two divisions, and four genetically different haploid cells.
• For triploids, explain that odd chromosome numbers stop pairs forming properly.
• For evaluation, give both benefits and drawbacks, then judge.

🧠 Exam technique

• One-mark questions need one precise statement.
• Three-mark “describe” questions need several linked facts.
• Six-mark evaluate questions need a balanced argument with a clear conclusion.

❌ Common errors

• Mixing up mitosis and meiosis.
• Saying “half the DNA” without explaining chromosome number or cell division.
• Thinking triploids cannot reproduce because they are “stronger” or “faster”.
• For evaluation, only giving advantages or only disadvantages.

✅ Correct answer

One X and one Y chromosome and 9 pairs of other chromosomes

💡 Key knowledge

• 20 chromosomes in a body cell = 10 pairs.
• Male animals usually have XY sex chromosomes.
• That leaves 9 pairs of non-sex chromosomes.

❌ Common errors

• Choosing XX for a male.
• Writing 8 pairs instead of 9 pairs.
• Forgetting that 20 chromosomes means 10 pairs total.

✅ Correct answer

• It is meiosis.
• DNA/chromosomes replicate first.
• The cell divides twice to form 4 cells.
• Each daughter cell has one set of chromosomes (haploid).
• The gametes are all genetically different.

💡 Key knowledge

• Meiosis makes gametes for sexual reproduction.
• It halves the chromosome number so fertilisation can restore the full number.
• In GCSE answers, “haploid” is acceptable and means one set of chromosomes.

🧠 Exam technique

• Use short, clear scientific phrases.
• Link ideas in order: replicate → divide twice → 4 cells → haploid → different.
• If you forget “meiosis”, the mark scheme allows other linked points, but the word helps.

❌ Common errors

• Saying the cells “have half the DNA” with no detail.
• Describing mitosis instead of meiosis.
• Writing that gametes are identical.
• Forgetting the final number of cells is four.

✅ Correct answer

Meiosis will not work because the chromosomes cannot form pairs properly.

💡 Key knowledge

• Triploid means three sets of chromosomes.
• Meiosis needs chromosomes to pair and separate evenly.
• If chromosomes cannot pair properly, gametes are not formed correctly.

❌ Common errors

• Saying “they have too many chromosomes” without explaining why this stops reproduction.
• Talking about mitosis instead of meiosis.
• Writing that they “don’t have DNA”.

✅ Correct answer

• They do not use energy in reproduction.
• So more energy is available to synthesise proteins / other cell components for growth.

💡 Key knowledge

• Reproduction uses energy.
• If an organism is sterile, that energy can be redirected.
• Protein synthesis is needed for making new cells and tissues.

❌ Common errors

• Saying triploids grow quickly because they have “more chromosomes” alone.
• Not mentioning energy at all.
• Writing “more food” instead of explaining what the energy is used for.

✅ Acceptable answers

• Global warming may raise water temperature, killing oysters.
• Pollutants in the water may be toxic and kill oysters.
• Increased acidity of the ocean may be killing oysters.
• New competitors may use up the normal food source.
• New pathogens may cause disease.
• New predators may eat oysters.

🧠 Exam technique

• Write one factor + one reason for each mark.
• You do not need long explanations.
• Choose two different factors for two marks.

❌ Common errors

• Just naming “pollution” with no effect.
• Repeating the same idea twice in different words.
• Writing “overfishing” — the mark scheme says ignore this.

✅ Points you could include

• Oysters are available to eat all year, so they may become more popular.
• They can be cheaper to produce, so food can be supplied for a growing population.
• They grow faster, so farmers can supply supermarkets and restaurants more quickly.
• Stocks can be replenished each year, making fishing/farming more sustainable.
• Farmers can harvest and sell all year, making income more stable.
• However, cytochalasin B is carcinogenic and may be harmful if it enters the food chain or environment.
• Triploid oysters may outcompete native oysters and upset the ecosystem.
• Farmers may need to buy new seed oysters every time, which increases cost.

💡 Key knowledge

• Triploid oysters are sterile, so they do not reproduce.
• They grow quickly because less energy is used for reproduction.
• Using a chemical in production raises safety and environmental concerns.
• Examiners rewarded answers that considered the whole system: business, consumers, environment, and safety.

🧠 How to write a high-level answer

• Start with a balanced opening: “There are benefits and drawbacks.”
• Give at least 2–3 advantages and 2–3 disadvantages.
• Explain each point, don’t just list them.
• Finish with a judgement such as “Overall, this is useful only if the chemical is proven safe and the environmental effects are controlled.”

❌ Common errors

• Only saying “it is good because they grow faster.”
• Writing vague points like “it is better for business” without saying why.
• Ignoring the risk of using a carcinogenic chemical.
• Not making a final decision.

💡 Remember these facts

• Male oyster body cell: XY + 9 pairs of other chromosomes.
• Gametes are made by meiosis.
• Meiosis makes 4 genetically different haploid cells.
• Triploid = 3 sets of chromosomes.
• Triploids are sterile because chromosomes cannot pair properly.
• No reproduction means more energy can go into growth.

❌ Final warning

• Be precise with chromosome numbers.
• Always give a reason, not just a fact.
• In long answers, include both sides and a conclusion.