AQA GCSE Biology Biology Paper 2 (Higher), June 2025: Question 8

✅ Correct Answer

Genotype: HH (homozygous dominant)

Reason: All 210 offspring have Himalayan fur. Because the male is heterozygous ( Hh ), female A must pass on a dominant allele ( H ) to every single offspring, meaning she carries no recessive allele ( h ).

🧠 Exam Technique & Insight

• Dependent Marks: In this mark scheme, the mark for the explanation is dependent on getting the genotype correct first.
• Always look at the sample size: with 210 offspring and not a single white rabbit, the female cannot have an h allele. If she were Hh , roughly 25% of offspring would have been white.

📐 Punnett Square Diagram

Male is heterozygous ( Hh ). Female B has Himalayan fur but produces white offspring ( hh ), so she must also be heterozygous ( Hh ).

Probability of white offspring: 0.25 or 1/4 or 25% or 1 in 4

❌ Common Errors to Avoid

• Missing Phenotypes: The question explicitly says: "identify the phenotype of each offspring genotype". You must write "Himalayan" next to HH / Hh and "White" next to hh to get the 3rd mark.
• Expressing Ratios Incorrectly: Writing "1 in 3", "1:4", or "3:1" for the probability of being white will be marked wrong. A ratio must be written as 1:3 (white to Himalayan), or stick to fractions/percentages ( 1/4 or 25% ).

💡 Key Knowledge: Step-by-Step Mechanism

1. Triplet change: Changing one base alters the group of 3 bases (triplet).
2. Amino acid change: This triplet now codes for a different amino acid in the polypeptide chain.
3. Active site shape altered: The protein folds differently, changing the 3D shape of the enzyme's active site.
4. Loss of fit: The substrate (tyrosine) is no longer complementary, so it cannot fit / cannot form enzyme-substrate complexes.

🧠 Top-Grade Revision Tip

• Always structure protein mutation questions sequentially: Base/Triplet → Amino Acid → Protein Folding/Active Site → Substrate Binding.
• Do not just write "the enzyme is destroyed". Specifically refer to the active site and the substrate (tyrosine).

✅ Correct Answer

• The temperature inside the mother's body / inside the nest is greater than 35 °C (mother's body temp is 39 °C) [1 mark].
• At this high temperature, the tyrosinase enzyme does not work / is inactive, so no melanin is produced [1 mark].

❌ Major Misconception

Do NOT say the enzyme is denatured!

The prompt notes that the enzyme is simply "not active above 35 °C". When the rabbits leave the nest later, the enzyme works again—meaning it was never permanently denatured. Stating that the enzyme is denatured loses the second mark.

✅ Correct Answer & Marking Points

• The outside environment is cooler / below 35 °C [1 mark].
• The rabbit loses heat from its extremities (ears, nose, feet) / the temperature of extremities drops below 35 °C [1 mark].
• At this lower temperature (15–25 °C), tyrosinase is active and produces black melanin pigment [1 mark].

💡 Environmental Effect on Phenotype

This is a classic demonstration of how phenotype is determined by an interaction between genetics (the gene for tyrosinase) and the environment (temperature affecting enzyme function).

✅ Correct Answer

• Melanin absorbs UV radiation, preventing it from causing mutations in skin cell DNA [1 mark].
• This reduces the risk of developing skin cancer (or uncontrolled cell division / tumours) [1 mark].

🧠 Exam Technique

The question reminds you that "UV radiation is a type of ionising radiation". Whenever ionising radiation is mentioned, connect it immediately to DNA mutations and cancer.