AQA GCSE Combined Science: Trilogy Biology Paper 2 (Foundation), June 2025: Question 5

✅ Correct Matches

• Chromosome → A whole strand of DNA found in the nucleus of a rabbit cell
• Gene → Codes for a protein that affects fur colour
• Genome → The complete genetic composition of a rabbit

💡 Key Knowledge

• Chromosome: Long, coiled structure of DNA carrying many genes.
• Gene: A small section of DNA on a chromosome coding for a particular sequence of amino acids (a specific protein).
• Genome: The entire genetic material of an organism.

❌ Common Errors & Traps

• The distractor: "A fur colour that is seen" is the phenotype. No line should connect to this box.
• Multiple lines: The mark scheme clearly states: "do not accept more than one line from a box on the left." If you draw two lines from one term, you score 0 for that term.

📐 Completed Punnett Square

Parental gametes: Bb × bb

★ Offspring identified with white fur: bb

✅ Mark Breakdown (5 Marks)

• Mark 1: Correct gametes placed around the grid ( B , b and b , b ).
• Mark 2 & 3: Correct offspring genotypes derived ( Bb , Bb , bb , bb ). (1 mark if 3 are correct).
• Mark 4: Any bb offspring clearly identified as having white fur (e.g. labelled, circled, or named).
• Mark 5: Probability = 0.5 (or 50%, ½, 1 in 2, 2 in 4, 1:1).

🧠 Exam Technique

• Don't skip the label: The question asks you to "identify any offspring that will have white fur". Simply filling in the letters is not enough for full marks; you must explicitly point out or circle the bb boxes.
• Case sensitivity: Always write capital B and lowercase b clearly to avoid ambiguity.

❌ Common Errors

• Ratio mistake: Writing 1:2 for probability loses the mark! The ratio of brown to white is 2:2, which simplifies to 1:1.
• Writing "50/50" instead of 50% or 0.5.

✅ Model Answer (2 Marks)

A high-scoring answer needs both points:

• Sperm cell: Sperm have only one allele for each gene / only one chromosome of each pair / are haploid / have half the number of chromosomes. [1 mark]
• Body cell / Division: Sperm are produced by meiosis OR body cells have pairs of chromosomes / are diploid / have two alleles for each gene. [1 mark]

💡 Key Knowledge

• Diploid (body cells): Contain two copies of each chromosome arranged in matching pairs (one inherited from each parent).
• Meiosis: Cell division that halves chromosome number, separating homologous pairs so each gamete carries only one allele of each gene.
• Haploid (gametes): Contain one chromosome from each pair (single set).

❌ Common Errors

• Stating that "sperm have half the DNA" (too vague – marks require referring to chromosomes or alleles).
• Confusing mitosis (produces identical body cells) with meiosis (produces non-identical gametes).

✅ 05.4: Chromosomes in a Skin Cell (1 Mark)

Number = 44

Also allowed: 22 pairs

❌ 05.4 Trap to Avoid

Assuming human numbers: Many students write 46 because humans have 46 chromosomes. You must use Figure 7 provided in the question!

✅ 05.5: Evidence Rabbit is Female (1 Mark)

• It has two X chromosomes (or XX).
• Also accepted: The rabbit has no Y chromosome / it is not XY / both sex chromosomes are the same size.

💡 05.5 Key Knowledge

• In mammals (both humans and rabbits), biological sex is determined by sex chromosomes:
• XX = Female (two matching, full-size X chromosomes)
• XY = Male (one X and one distinctly shorter Y chromosome)