AQA GCSE Combined Science: Trilogy Biology Paper 2 (Foundation), 2024: Question 1

💡 Key knowledge

• The cell membrane controls movement of substances into and out of cells.
• A dominant allele is shown by a capital letter, such as B .
• Bb is heterozygous because the alleles are different.
• A mutation is a change in DNA.

🧠 Exam technique

• For multiple choice, pick the biology term that matches the definition exactly.
• For Punnett squares, make sure each parent's gametes are written correctly before filling the boxes.
• When asked for chance, read the completed square and convert to a percentage.

❌ Common errors

• Choosing cytoplasm instead of cell membrane in 01.1.
• Thinking B is a gene rather than a dominant allele.
• Writing phenotype when the question asks for heterozygous.
• Forgetting that cystic fibrosis is caused by the recessive allele b .

✅ Correct answer

Cell membrane

💡 Key knowledge

The cell membrane is selectively permeable, so it controls what enters and leaves the cell.

❌ Common errors

Cytoplasm is the jelly-like material inside the cell, and mitochondria release energy. Neither controls movement in and out.

✅ Correct answer

Dominant

💡 Key knowledge

In GCSE genetics, a capital letter usually shows the dominant allele and a lowercase letter shows the recessive allele.

🧠 Exam technique

If the question gives you one allele as recessive, the opposite one in the pair is usually the dominant allele.

✅ Correct answer

Heterozygous

💡 Key knowledge

• Heterozygous = two different alleles.
• Homozygous = two same alleles.
• Genotype = the alleles an organism has.
• Phenotype = the observable characteristic.

❌ Common errors

Students often chose phenotype because they mix up the word for the genes with the word for the feature shown.

✅ Correct answer

• Each parent produces gametes B and b.
• Completed square gives: BB, Bb, Bb, bb.

📐 Step-by-step

1. Write the gametes from Parent 1: B and b.
2. Write the gametes from Parent 2: B and b.
3. Combine letters in each box:
   • B × B = BB
   • B × b = Bb
   • b × B = Bb
   • b × b = bb

🧠 Exam technique

The top-mark responses showed all three offspring genotypes correctly derived from the gametes. Write the alleles clearly and keep the letters in the correct order.

❌ Common errors

• Writing b instead of B for one parent gamete.
• Forgetting to include both Bb outcomes.
• Mixing up the gametes and offspring boxes.

✅ Correct answer

25%

📐 Step-by-step

1. From the Punnett square in 01.4, the possible genotypes are BB, Bb, Bb, and bb.
2. Cystic fibrosis is caused by the recessive genotype, so only bb is affected.
3. There is 1 affected outcome out of 4.
4. Convert to a percentage: 1/4 = 25%.

🧠 Exam technique

The mark scheme says the answer must match the Punnett square in 01.4. If your square is wrong, the chance may also be wrong. The final answer must be a percentage.

❌ Common errors

• Choosing 50% because there are two Bb boxes.
• Forgetting that only bb has cystic fibrosis.
• Not linking the probability to the completed diagram.

✅ Correct answer

Screening

💡 Key knowledge

Screening means testing to find out whether a person or embryo has a particular condition or allele.

❌ Common errors

Genetic engineering changes DNA, and selective breeding is choosing parents with desired characteristics. Neither means testing an embryo.

✅ Correct answer

Mutation

💡 Key knowledge

A mutation is a change in the sequence of DNA. Mutations can be inherited if they occur in reproductive cells.

🧠 Exam technique

Look for the exact term the exam wants. Genome means all the genetic material, and helix describes DNA shape, not a change in DNA.

✅ Correct answers

Any one of:

• diet
• behaviour
• infection
• sunlight
• air pollution

💡 Key knowledge

Environmental factors affect how a characteristic develops, even if the genes stay the same.

Examples accepted by the mark scheme included smoking, tattoos, piercings, hair or nail colouring, and disease/infection.

❌ Common errors

• Giving an inherited cause instead of an environmental one.
• Writing “climate” without a clearer explanation, which may be too vague.
• Talking about “changes in DNA” when the question says not to.

🧠 What full-mark answers did well

• Used the exact genetics terms: cell membrane, dominant, heterozygous, screening, mutation.
• Completed the Punnett square accurately from the parent genotypes.
• Linked the final probability to the completed square.

❌ Biggest traps

• Confusing genotype and phenotype.
• Using the wrong process name for embryo testing.
• Forgetting that cystic fibrosis is recessive.