AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2023: Question 4

Salmon Classification and Selective Breeding

💡 Key knowledge

• In a binomial name, the first word is the genus.
• Animals are in the domain Eukaryota (or eukaryote accepted).
• Selective breeding means choosing organisms with desired characteristics and breeding them over many generations.
• A disadvantage of selective breeding is reduced genetic variation.

🧠 Exam technique

• For 1-mark questions, give just the exact term.
• For 3-mark explain/describe questions, make three separate points.
• Use linking words like so , therefore , because to show the chain of reasoning.
• Do not add extra incorrect information, especially on classification.

✅ Correct answer

Oncorhynchus

💡 Key knowledge

In the name Oncorhynchus keta , the first word is the genus.

❌ Common errors

• Writing Oncorhynchus keta — the mark scheme says do not accept the full binomial name here.
• Writing only keta — that is the species part, not the genus.

✅ Correct answer

Eukaryota

The mark scheme also allows eukaryote.

💡 Key knowledge

Sea lice are animals, and animals have cells with a nucleus, so they are in the domain Eukaryota.

❌ Common errors

• Giving Animalia — that is a kingdom, not a domain.
• Mixing up domain with species or genus.

✅ Correct answer

1. Select salmon that have the least / less / no sea lice attached.
2. Breed these selected salmon together, then select offspring that also have the least sea lice and breed those together.
3. Repeat over many generations until offspring have no sea lice attached.

💡 Key knowledge

Selective breeding is a process:

• choose parents with the desired characteristic
• breed them
• choose the best offspring
• repeat for many generations

🧠 Exam technique

To get all 3 marks, your answer must show selection of parents, selection of offspring, and repetition over generations.

A strong exam answer would be:

Select salmon with the fewest or no sea lice attached and breed them together. Then select the offspring with the fewest sea lice and breed these together. Repeat this over many generations until salmon are produced with no sea lice attached.

❌ Common errors

• Saying just “breed fish with desired genes” — the mark scheme says this is ignored unless the actual selective breeding steps are given.
• Saying “repeat with the same parents” — this is not accepted.
• Confusing selective breeding with genetic engineering.

✅ Correct answers

• Sea lice will not feed on the salmon's skin or blood.
• So the salmon can grow larger / faster.
• So the salmon can be sold for more money.

💡 Key knowledge

Accepted linked ideas from the mark scheme include:

• skin is less likely to be damaged or infected
• salmon lose less energy
• less need for pesticides / insecticides
• salmon are more appealing to buyers
• more likely to be sold / more profit

🧠 Exam technique

This is an explain question, so link the ideas in order:

No sea lice feeding → less damage / less energy loss → fish grow better → farmers make more money

Top answers clearly showed the benefit to the farmers, not just to the fish.

❌ Common errors

• Only saying “the fish are healthier” with no further explanation.
• Forgetting to link the advantage back to the farmer, for example increased sales or profit.
• Giving just one idea when 3 marks need a chain of points.

✅ Correct answer

Selective breeding can make the fish genetically similar.

Therefore they may all be vulnerable to the same inherited disorder / defect, or all be susceptible to the same disease, or have a lack of variation to survive future environmental change.

💡 Key knowledge

The main disadvantage is reduced genetic variation. If the environment changes or a disease appears, a genetically similar population is at greater risk.

🧠 Exam technique

Because the question says do not refer to cost or time, avoid those completely.

A full-mark answer needs both parts:

1. state that the fish become genetically similar
2. state the consequence of that reduced variation

❌ Common errors

• Saying the fish are clones — this is not accepted.
• Saying the fish are genetically identical — also not accepted.
• Using unqualified “inbreeding” without explaining the consequence.
• Talking about cost or time even though the question rules this out.

🧠 What distinguished stronger answers

• Using the exact classification term needed: Oncorhynchus and Eukaryota .
• Showing the full process of selective breeding, not just mentioning “desired genes”.
• Explaining advantages with a logical chain that ends in a farmer benefit such as more sales or profit.
• For the disadvantage, clearly linking genetic similarity to disease risk or reduced ability to survive change.

❌ Where students lose marks

• Writing the full species name instead of just the genus in 04.1.
• Confusing domain with kingdom in 04.2.
• Missing out the idea of selecting offspring and repeating for many generations in 04.3.
• Not linking the answer to the farmer in 04.4.
• Saying “clones” or “genetically identical” in 04.5.

✅ 04.1

Oncorhynchus

✅ 04.2

Eukaryota

✅ 04.3

Select salmon with the fewest or no sea lice attached and breed them together. Then select offspring with the fewest sea lice and breed these together. Repeat this over many generations until salmon are produced with no sea lice attached.

✅ 04.4

Sea lice will not feed on the salmon’s skin or blood, so the salmon lose less energy and can grow larger or faster. This means farmers can sell them for more money.

✅ 04.5

The fish become genetically similar, so they may all be affected by the same disease or inherited disorder, and there is less variation to survive environmental change.