AQA A-Level Biology Paper 2, June 2025: Question 5

7 marks · Medium difficulty · Short Answer

Explain phenotype, suggest reasons why observed phenotypic ratios differ from expected, and explain autosomal linkage and crossing over from cross data.

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Question

Question 5 consists of three parts. Part 05.1 asks to explain what is meant by the term phenotype (2 marks). Part 05.2 asks to suggest two reasons why observed phenotypic ratios in offspring are often not the same as expected ratios, excluding sex-linkage or autosomal linkage (2 marks). Part 05.3 describes a cross in sweet peas between a dihybrid heterozygous purple flower, long pollen plant (FfLl) and a homozygous recessive red flower, round pollen plant (ffll). Table 2 gives percentage of total offspring: Purple flowers and long pollen (48.6%), Red flowers and round pollen (47.5%), Purple flowers and round pollen (2.1%), Red flowers and long pollen (1.8%). It asks to use autosomal linkage to explain the results, including what is meant by autosomal linkage and the relative proportions of gametes produced (3 marks).
Question text

05.1 Explain what is meant by the term phenotype.

[2 marks]

05.2 In genetic crosses, the observed phenotypic ratios obtained in the offspring are often

not the same as the expected ratios.

Suggest two reasons why.

Do not refer to sex-linkage or autosomal linkage in your answer.

[2 marks]

05.3 In sweet pea plants, the F allele for purple flowers is dominant to the f allele for red

flowers. The L allele for long pollen is dominant to the l allele for round pollen.

A scientist carried out a genetic cross between a plant with purple flowers and long

pollen (heterozygous for both genes) and a plant with red flowers and round pollen.

Table 2 shows the results of this cross.

Table 2

Phenotype of offspring Percentage of total offspring

Purple flowers and long pollen 48.6

Red flowers and round pollen 47.5

Purple flowers and round pollen 2.1

Red flowers and long pollen 1.8

Use your knowledge of autosomal linkage to explain the results in Table 2.

Your answer should include:

• an explanation of what is meant by autosomal linkage

• a comment about the relative proportions of the different genotypes of gametes

produced in this cross.

[3 marks]

Mark scheme

Show the mark scheme Mark scheme for Question 5. 05.1: 1. Expression/appearance/characteristic due to genetic constitution/genotype/alleles; 2. And due to interaction with the environment (2 marks). 05.2: Any two from small sample size, crossing over, random fusion of gametes/random fertilisation, epistasis, lethal genotypes/alleles, or epigenetics (2 marks max). 05.3: 1. Genes are on the same chromosome/autosome; 2. Crossing over or recombination occurs; 3. Fewer Fl and fL gametes OR more FL and fl gametes (3 marks).

Question Marking Guidance Mark Comments

1. Expression/appearance/characteristic/feature 1. Accept: named

/trait due to genetic constitution / genotype characteristic

/allele(s);

1. Accept:

2. (And due to interaction with the) environment; 2 homozygous/

05.1 (2 x heterozygous/genes

AO1) /DNA for ‘genotype’

1. Ignore:

chromosomes

Ignore mutation and

1. Small sample size;

independent

2. Crossing over; segregation

3. Ignore random

3. Random fusion of gametes

breeding/mating

OR

2 max

05.2 Random fertilisation; (2 x

AO2)

4. Epistasis;

5. Lethal genotypes/alleles/genes; 5. Ignore lethal

phenotypes

6. Epigenetics;

1. Genes are on the same chromosome/autosome; 1. Accept chromatid

for chromosome

2. Crossing over

1. Accept alleles of

different genes but not

OR

alleles unqualified

Recombination; 3 1. Accept alleles at

(1 x different loci on same

05.3

3. Fewer F l and f L (gametes) AO1, 2 chromosome

x AO2)

OR 2. Accept chiasma(ta)

for crossing over

More F L and f l (gametes);

3. Accept if different

letters used e.g. P

and p for F and f

How to answer it

A-Level Biology Study Guide: Genetics & Autosomal Linkage

WHAT THIS QUESTION TESTS

This question assesses key concepts from Topic 7 (Genetics, Populations, Evolution and Ecosystems):

  • Accurate definition of phenotype as the interplay between genetics and the environment.
  • Understanding biological and statistical reasons why observed offspring ratios deviate from theoretical Mendelian expectations.
  • Applying the mechanism of autosomal linkage and meiotic crossing over to explain non-Mendelian test-cross offspring distributions.
QUESTION 05.1

Explain what is meant by the term phenotype.

Definition recall • 2 Marks (AO1)

✅ Correct Answer

  1. Expression / appearance / characteristic / feature / trait due to genetic constitution / genotype / alleles; [1 mark]
  2. (And due to interaction with the) environment. [1 mark]
Mark Scheme Notes: Accept "named characteristic" or terms like "homozygous / heterozygous / genes / DNA" for genotype. Ignore "chromosomes".

💡 Key Knowledge

  • Genotype: The genetic constitution of an organism (the specific alleles it possesses).
  • Phenotype: The observable biochemical and physical characteristics produced by the expression of the genotype interacting with environmental factors:
    Phenotype = Genotype + Environment

❌ Common Errors

  • Stating only that it is "what the organism looks like" without referencing genetic makeup or genotype.
  • Forgetting to mention the environment entirely, dropping 1 of the 2 available marks.

🧠 Exam Technique

Always treat the definition of phenotype as a compulsory two-part answer: (1) genetic basis, and (2) environmental modification.

QUESTION 05.2

Reasons for Deviations from Expected Phenotypic Ratios

Understanding phenotypic variation • 2 Marks (AO2)

Note: The question states: Do not refer to sex-linkage or autosomal linkage in your answer.

✅ Correct Answer (Any Two)

  • Small sample size; [1 mark]
  • Crossing over; [1 mark]
  • Random fusion of gametes OR Random fertilisation; [1 mark]
  • Epistasis; [1 mark]
  • Lethal genotypes / alleles / genes; [1 mark]
  • Epigenetics; [1 mark]

🧠 Exam Technique: Respecting Negative Prompts

  • Examiners deliberately excluded sex-linkage and autosomal linkage to test whether you understand other causes of non-Mendelian ratios.
  • Always re-read the exclusion clause before writing. If you write "linked genes", you automatically lose that mark.

❌ Common Misconceptions & Reject Rules

  • "Random mating" / "Random breeding": Ignored. Mating behavior is not the same as the random fusion of gametes.
  • "Mutation": Ignored. Spontaneous mutation rates are far too low in standard lab crosses to skew whole-batch ratios.
  • "Independent segregation": Ignored, as independent segregation is the basis of generating the expected theoretical ratios.
  • "Lethal phenotypes": Must specify lethal alleles/genotypes.
QUESTION 05.3

Autosomal Linkage in Sweet Pea Plants

Data Interpretation & Linkage Analysis • 3 Marks (1 AO1, 2 AO2)

💡 Context & Cross Analysis

Parent Cross: Heterozygous purple & long ( FfLl ) × Red & round ( ffll )

Offspring Phenotype Percentage Classification
Purple flowers, long pollen 48.6% Parental (Non-crossover)
Red flowers, round pollen 47.5% Parental (Non-crossover)
Purple flowers, round pollen 2.1% Recombinant (Crossover product)
Red flowers, long pollen 1.8% Recombinant (Crossover product)

Expected independent assortment ratio for a test cross is 1:1:1:1 (25% each). Here, ~96% are parental and only ~4% are recombinant.

✅ Correct Answer (3 Marks Required)

  1. Genes are on the same chromosome / autosome; [1 mark]
  2. Crossing over OR Recombination (has occurred); [1 mark]
  3. Fewer Fl and fL gametes OR More FL and fl gametes; [1 mark]
Mark Scheme Guidance:
• MP1: Accept "chromatid" for chromosome. Accept "alleles of different genes on same chromosome", but reject unqualified "alleles on same chromosome".
• MP2: Accept "chiasma / chiasmata".
• MP3: Must explicitly identify the recombinant ( Fl , fL ) or parental ( FL , fl ) gamete genotypes.

📐 Step-by-Step Logic Breakdown

  1. Identify Linkage: Because the double-recessive parent only provides fl , offspring phenotypes directly mirror the gametes of the heterozygous parent.
  2. Deduce Allele Arrangement: Since Purple, Long (48.6%) and Red, Round (47.5%) dominate, the dominant alleles F and L are linked on one homologous chromosome, while f and l are linked on the other.
  3. Account for Rare Types: Recombinants ( Purple, Round and Red, Long ) only arise when crossing over occurs between gene loci during prophase I of meiosis.
  4. Evaluate Proportions: Chiasmata do not form between these loci in every meiotic division; hence, recombinant gametes ( Fl and fL ) are produced in much lower frequencies.

❌ Common Errors

  • Failing to mention gametes when discussing proportions (e.g. talking generally about offspring without identifying gamete genotypes).
  • Writing "alleles are on the same chromosome" without stating that it is the different genes that are linked.
  • Omitting crossing over as the specific cellular mechanism responsible for the small percentage of recombinants.

🧠 Examiner Insight

The question bulleted two explicit instructions: explain what is meant by autosomal linkage and comment on relative proportions of gamete genotypes. Top-grade students use these bullet points as a checklist to ensure all marking points are hit.

Topics

Biology · 3.7 Genetics, populations, evolution and ecosystems (A-level only)

Question and mark scheme from the AQA A-Level Biology examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.