AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), June 2025: Question 1
12 marks · Low Demand difficulty · Short Answer
Answer questions on genetics in rabbits, including genetic terms, completing a Punnett square for fur colour, explaining allele differences in gametes, and interpreting a karyotype.
Practise this questionQuestion
Question text
01 This question is about genetics.
The fur colour of rabbits is controlled by two alleles.
01.1 Draw one line from each term to the meaning of the term relating to genetics
in rabbits.
[3 marks]
01.2 In rabbits:
• the allele for brown fur (B) is dominant
• the allele for white fur (b) is recessive.
A rabbit with the alleles Bb and a rabbit with the alleles bb breed together.
Determine the probability that one of the offspring will have white fur.
You should:
• complete the Punnett square diagram
• identify any offspring that will have white fur.
[5 marks]
Probability =
01.3 A male rabbit is heterozygous for fur colour.
The rabbit’s body cells have the genotype Bb.
One of the sperm cells from the rabbit has the allele b.
Explain why the alleles in the rabbit’s body cells and the allele in the sperm cell
are different.
[2 marks]
Figure 1 shows the chromosomes of a rabbit.
Figure 1
01.4 How many chromosomes would be found in a skin cell of a rabbit?
Use Figure 1.
[1 mark]
*05* Number of chromosomes =
01.5 The sex of a rabbit is controlled in the same way as the sex of a human.
Give the evidence from Figure 1 that the rabbit is female.
[1 mark]
Mark scheme
Show the mark scheme
Question 1
AO /
Question Answers Mark
Spec. Ref.
01.1
1 AO1
4.6.1.3
4.6.1.4
do not accept more than one line from a box on the left
AO /
Question Answers Extra information Mark
Spec. Ref.
01.2
gametes correct allow in either position in 1 AO2
Punnett square
offspring genotypes correctly allow correct derivation of 2 AO2
derived offspring genotypes from
incorrect gametes
allow 3 correctly derived
offspring genotypes for 1 mark
any bb offspring identified as 1 AO2
having white fur
0.5 probability must match offspring 1 AO3
genotypes if given
allow 50% or 1 in 2 or
1 4.6.1.4
1:1 or
do not accept 1:2
AO /
Spec. Ref.
allow gamete(s) or sex cells for
01.3 sperm throughout
sperm have one of each allow sperm have one set of 1 AO2
chromosome (pair) chromosomes / genes
allow sperm (only) have one
allele for each gene
allow sperm are haploid
allow sperm have half the
(number / amount of)
chromosomes (compared with a
body cell)
allow sperm have 23 / 22
8 chromosomes
ignore sperm have one allele
ignore sperm have half the DNA
/ genes
sperm are produced by meiosis 1 AO1
or
body cells have two of each allow body cells have pairs of 4.6.1.1
chromosome chromosomes 4.6.1.2
allow body cells are diploid 4.6.1.4
allow body cells have 46 / 44
chromosomes
allow B and b are (alleles) on 2
chromosomes (of the same pair)
AO /
Spec. Ref.
01.4 44 allow 22 pairs 1 AO2
4.6.1.6
AO /
Spec. Ref.
01.5 has two X (chromosomes) allow XX 1 AO1
allow the rabbit is not XY 4.6.1.6
allow the rabbit does not have a
Y (chromosome)
allow the rabbit only has X
(chromosomes)
allow both (X) sex
chromosomes are the same size
Total Question 1 12
How to answer it
Inheritance, Alleles and Chromosomes in Rabbits
What this question tests
- Key Definitions: Distinguishing between chromosome, gene, and genome.
- Genetic Crosses: Completing a Punnett square for a monohybrid cross, identifying phenotypes, and calculating probability.
- Cell Division & Gametes: Explaining allele segregation and how meiosis halves the chromosome count in sex cells.
- Karyotype Analysis: Counting total diploid chromosomes and determining biological sex from sex chromosomes (XX vs XY).
Question 01.1: Genetic Key Terms
Matching key terms to their biological definitions [3 marks]
✅ 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"
❌ Common Errors & Distractors
- Unused distractor: "A fur colour that is seen" is the definition of a phenotype.
- Multiple lines: The mark scheme states: "do not accept more than one line from a box on the left". If you draw two lines from one term, you automatically forfeit the mark for that term.
Question 01.2: Monohybrid Punnett Square
Cross between a heterozygous brown rabbit (Bb) and a white rabbit (bb) [5 marks]
✅ Completed Punnett Square
Parents: Bb × bb
| b | b | |
|---|---|---|
| B | Bb | Bb |
| b | bb | bb |
Offspring with white fur: Circle or identify both bb offspring.
Probability: 0.5 (or 50% , 1 in 2 , ½ , 1:1 )
📐 Step-by-Step Mark Breakdown
- Gametes correct (1 mark): Parent gametes B , b on one axis, and b , b on the other.
- Offspring genotypes (2 marks): Correct cross inside the grid ( Bb , Bb , bb , bb ). 1 mark if 3 are derived correctly.
- Phenotype identification (1 mark): Any bb clearly labelled or indicated as having white fur.
- Probability (1 mark): White fur occurs in 2 out of 4 squares = 0.5.
🧠 Exam Technique: Handling Allele Letters
Always make your upper-case and lower-case letters look distinctly different. Clearly show a large capital B and a smaller b so the examiner can read your genotypes without ambiguity.
❌ Common Trap: Ratio Formats
If giving your answer as a ratio of brown to white, write 1:1 (or 2:2). Do NOT write 1:2. A ratio of 1:2 implies 1 out of 3, which is incorrect!
Question 01.3: Comparing Body Cells & Gametes
Explaining why body cells have genotype Bb while sperm only has b [2 marks]
✅ Model Answer
- Point 1: Sperm cells are gametes and have only one copy of each chromosome (or only one allele for each gene / are haploid). [1 mark]
- Point 2: Sperm are produced by meiosis (or body cells are diploid / have pairs of chromosomes). [1 mark]
💡 Key Knowledge: Diploid vs Haploid
- Body cells: Diploid (2n). They contain pairs of chromosomes, meaning they carry two alleles for each gene ( Bb ).
- Gametes: Haploid (n). Formed via meiosis, where homologous pairs separate, leaving gametes with only one allele of each gene ( B or b ).
❌ Examiner Misconceptions
Vague statements such as "sperm has half the DNA" or "sperm has half the genes" are ignored by examiners. You must specifically state that gametes have half the number of chromosomes or one allele per gene.
Questions 01.4 & 01.5: Rabbit Karyotype Analysis
Extracting chromosome count and sex determination from Figure 1
Part 01.4: Number of Chromosomes in a Skin Cell [1 mark]
Correct Answer: 44 (or 22 pairs )
How to find this: Look at Figure 1. There are 21 pairs of numbered autosomes plus 1 pair of sex chromosomes (XX). That equals 22 pairs in total.
A skin cell is a normal body cell (diploid), so:
22 pairs × 2 = 44 chromosomes.
Part 01.5: Evidence that the Rabbit is Female [1 mark]
Correct Answer: It has two X chromosomes (or XX / does not have a Y chromosome).
Exam Rule: In mammals (including humans and rabbits):
- XX = Female
- XY = Male
Figure 1 explicitly labels the final pair as X X, providing direct evidence that this rabbit is female.
Topics
Biology · B6: Inheritance, Variation and Evolution
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Biology Paper 2 (Higher), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.