AQA AS Level Biology Paper 2, November 2020: Question 6

8 marks · Medium difficulty · Practical Techniques & Data Analysis

Calculate the median mass of trout after 195 days using a growth graph and equation, complete a table on chromosome and DNA mass during mitosis and meiosis, explain genetic variation in meiosis, determine chromosome number in offspring from diploid eggs, and explain sterility due to an odd chromosome number.

Practise this question

Question

A multipart exam question based on trout growth and cell division. Figure 4 is a line graph showing median fish mass in mg on the y-axis against time spent feeding in days on the x-axis, with a best-fit straight line starting at (0,50). Table 2 requires completing missing values for chromosome number and mass of DNA in arbitrary units at prophase of mitosis, telophase of mitosis, and in an egg cell. Subsequent parts ask to give reasons for genetic variation in trout eggs produced by meiosis, calculate offspring chromosome numbers following fertilization with diploid eggs, and explain why offspring produced are sterile.
Question text

06 Trout is a type of fish, often produced commercially in trout farms.

A scientist investigated the growth of farmed trout. She determined the median mass

of a large population of trout at intervals. She started measuring on the day the newly

hatched fish began feeding. Her results are shown in Figure 4.

Figure 4

The best fit line shown in Figure 4 is represented using this equation.

median fish mass = (m × days feeding) + 50

where m is the gradient of the best fit line.

06.1 Use Figure 4 and the equation to calculate the median mass of fish after 195 days’

feeding.

Show your working.

[2 marks]

Answer mg

06.2 A trout body cell contains 80 chromosomes.

Table 2 shows the number of chromosomes and the mass of DNA in different nuclei.

All the nuclei are from the same trout.

Complete Table 2.

[2 marks]

Table 2

Nucleus Number of Mass of DNA /

chromosomes arbitrary units

At prophase of mitosis 80

At telophase of mitosis 25

From an egg cell

06.3 Give one reason why trout eggs produced by meiosis are genetically different.

[1 mark]

A trout body cell contains 80 chromosomes.

Farmed female trout are treated so that they produce diploid egg cells.

06.4 Give the number of chromosomes in body cells of the offspring produced from treated

farmed female trout and untreated farmed male trout.

[1 mark]

Number of chromosomes

06.5 The offspring produced from farmed trout are sterile. Suggest and explain why.

[2 marks]

Mark scheme

Show the mark scheme The mark scheme provides correct answers and marking guidance for each sub-question. For 06.1, answers between 960 to 1025 mg gain 2 marks, with 1 mark for gradient m between 4.18 and 5.00. For 06.2, a completed table showing 50 for DNA mass at prophase, 80 chromosomes and 12.5 DNA mass at telophase, and 40 chromosomes and 12.5 DNA mass in an egg cell. For 06.3, accepts independent segregation or crossing over. For 06.4, the answer is 120. For 06.5, accepts points regarding an extra set of chromosomes preventing homologous chromosome pairing during meiosis, resulting in sterility.

Question Marking Guidance Mark Comments

06.1 Correct answer of 960 to 1025 (mg) for 2 marks;; 2

Accept for 1 mark,

m = 4.18 to 5.00

06.2 Mark each column;; 2

Number of Mass of DNA /

chromosomes arbitrary units

40 12.5

06.3 1 Accept labelled

diagram

Independent segregation Accept (eggs

OR produced) have

different combinations

Crossing over; of maternal and

paternal

chromosomes

06.4 120; 1

06.5 1. Too many/extra set/three copies of 2 max 1. Accept 1 paternal,

chromosomes; 2 maternal

2. (Homologous) chromosomes do not pair

OR 2. Accept divide for

(Homologous) chromosomes do not separate separate

(evenly);

3. (So) no meiosis;

TOTAL 8

How to answer it

Trout Growth, Mitosis, Meiosis, and Ploidy

What this question tests

This question assesses data interpretation from linear graphs, calculation of gradients and extrapolation, knowledge of chromosome numbers and DNA mass changes across the cell cycle (mitosis) and gametogenesis (meiosis), sources of genetic variation, and the genetic consequences of polyploidy on fertility in commercial fish farming.

Question 06.1

Calculating Median Fish Mass via Extrapolation

Use Figure 4 and the equation to calculate the median mass of fish after 195 days' feeding.

✅ Correct Answer

Any value between 960 to 1025 mg (inclusive) for 2 marks.

1 mark is awarded if the gradient ($m$) is correctly calculated between 4.18 and 5.00, but final evaluation errors occur.

📐 Step-by-Step Calculation

  1. Find the gradient ($m$): Choose two points on the line of best fit. E.g., at day 0 mass = 50 mg, and at day 120 mass = 625 mg.
    Gradient ($m$) = (625 - 50) / 120 = 575 / 120 = 4.79
  2. Apply the equation:
    Mass = ($m$ × days feeding) + 50
  3. Substitute values:
    Mass = (4.79 × 195) + 50 = 933.75 + 50 = 983.75 mg (Falls within the 960–1025 range).

❌ Common Errors

  • Forgetting to add the y-intercept (+ 50) given in the equation text.
  • Misreading coordinates from the grid scales on Figure 4.
Question 06.2

DNA Mass and Chromosome Numbers Across the Cell Cycle

Complete Table 2 for a trout body cell containing 80 chromosomes.

Nucleus Number of chromosomes Mass of DNA / arbitrary units
At prophase of mitosis 80 50
At telophase of mitosis 80 25
From an egg cell 40 12.5

💡 Key Knowledge

  • Prophase of mitosis: Chromosomes have replicated during interphase (so DNA mass is double the normal haploid amount, i.e., 50 units), but sister chromatids are still held together at the centromere, meaning individual chromosome count remains diploid ($2n = 80$).
  • Telophase of mitosis: Chromatids have separated. Each new nucleus gets 80 chromosomes, but each chromosome now consists of a single DNA molecule, halving the DNA mass to 25 units.
  • Egg cell (Meiosed gamete): Haploid ($n$), meaning chromosome number is halved to 40, and DNA mass is halved again to 12.5 units.
Question 06.3

Sources of Genetic Variation in Meiosis

Give one reason why trout eggs produced by meiosis are genetically different. [1 mark]

✅ Correct Answer

  • • Independent segregation (of homologous chromosomes)
  • • Crossing over (between non-sister chromatids of homologous pairs)

Accept: Eggs have different combinations of maternal and paternal chromosomes.

🧠 Exam Technique

Keep terminology precise. Do not write "mutation" or "random fertilization" (fertilization happens after meiosis and involves sperm, not just eggs).

Question 06.4

Chromosome Count in Triploid Offspring

Give the number of chromosomes in body cells of the offspring produced from treated farmed female trout (diploid eggs, 2n) and untreated farmed male trout (normal haploid sperm, n). [1 mark]

✅ Correct Answer

120

Calculation: Treated female produces diploid eggs ($2n = 80$). Untreated male produces haploid sperm ($n = 40$). Fertilization yields a triploid zygote ($3n = 80 + 40 = 120$).

Question 06.5

Explaining Sterility in Triploid Organisms

The offspring produced from farmed trout are sterile. Suggest and explain why. [2 marks]

✅ Correct Answer (Max 2 marks)

  • Point 1: Triploid organisms have an odd number / three copies of chromosomes (e.g., 3n = 120).
  • Point 2: Homologous chromosomes cannot pair up evenly / undergo normal synapsis during meiosis.
  • Point 3: Therefore, no meiosis occurs / gametes cannot be formed normally.

❌ Common Errors & Examiner Guidance

  • Students often state "mitosis cannot happen" — this is incorrect. Body growth occurs normally via mitosis; meiosis is what fails during gamete production.
  • Must link the uneven chromosome set explicitly to the failure of homologous pairing or separation.

Topics

Biology · 3.2 Cells · 3.4 Genetic information, variation and relationships between organisms

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.