AQA AS Level Biology Paper 2, June 2024: Question 7

✅ Mark Scheme Requirements

• Mark 1: Sister chromatids do not separate (in anaphase II).
• Mark 2: Non-disjunction.

💡 Key Knowledge: Meiosis I vs Meiosis II

• Meiosis I: Homologous chromosome pairs separate. Failure here = non-disjunction of homologous pairs.
• Meiosis II: Centromeres divide and sister chromatids separate. Failure here = non-disjunction of chromatids.
• Because the prompt specifies the second meiotic division, you must name chromatids!

❌ Common Errors

• Writing "homologous chromosomes do not separate" — this directly contradicts the second division condition and awards 0 marks.
• Forgetting to explicitly name the mutation: "non-disjunction".

🧠 Exam Technique

Always circle the phase given in the stem: "second meiotic division". Link phase to structure: 1st = Homologous pairs; 2nd = Chromatids.

✅ Mark Scheme Requirements

• Mark 1: The mutation / extra chromosome is present in the gamete (egg/sperm) OR in the zygote / all cells are derived from a single cell (zygote).
• Mark 2: All subsequent body cells are formed by mitosis.

💡 Key Knowledge

Fertilisation of an abnormal gamete (n + 1 = 24) with a normal gamete (n = 23) forms a zygote with 47 chromosomes (trisomy). Because mitosis produces genetically identical daughter cells, every single cell that develops from that zygote inherits the extra chromosome.

❌ Common Errors

• Mentioning meiosis instead of mitosis for the growth of body cells.
• Failing to mention that all cells originate from the fertilised egg / zygote.

✅ Mark Scheme Requirements

• Cells with an extra chromosome are produced by mitosis from cells containing the mutation.
  OR
  Cells with the normal/correct chromosome number are produced from cells without the mutation.

💡 Key Knowledge

In mosaicism, fertilisation creates a normal diploid zygote (or trisomic zygote), but an error in mitosis occurs after fertilisation in only one cell lineage during embryonic development. That mutated cell clones itself, producing a patch (mosaic) of abnormal cells alongside normal cells.

✅ Correct Answer

☑ Correlation coefficient (Box 1)

🧠 Exam Technique: Which Test When?

• Correlation coefficient (Spearman's rank): Tests for a significant relationship/association between two continuous variables (e.g., age and incidence risk).
• Student's t-test: Tests for a significant difference between two independent mean values.
• Chi-squared (χ²): Tests for differences between observed and expected frequencies in categorical data.

✅ Mark Scheme Requirements

• Mark 1: (More) blood moves/shunts from the left ventricle to the right ventricle (because the left ventricle has thicker muscular wall / contracts with higher pressure).
• Mark 2: This sends a greater volume of blood to the lungs
  OR causes higher blood pressure in the pulmonary artery.

💡 Key Knowledge: Pressure Gradients in the Heart

• The left ventricle wall has much thicker myocardium to pump blood against systemic resistance throughout the entire body.
• Therefore, left ventricular systolic pressure (~120 mmHg) is much higher than right ventricular systolic pressure (~25 mmHg).
• A hole in the intervening septum causes blood to flow down its pressure gradient from left to right, overloading pulmonary circulation.

❌ Common Errors & Trap Warnings

• Incorrect direction: Stating blood moves from right to left. Remember: left ventricle has much higher pressure!
• Vague volume statements: Saying "blood goes to lungs" without specifying greater volume or higher pressure in the pulmonary artery.
• Confusing the pulmonary artery (carries blood to lungs from RV) with the pulmonary vein.

🧠 Anatomy Checklist for 2/2

Always state cause (left has thicker muscle → higher pressure pushes blood to right) and consequence (increased volume forced into pulmonary artery / lungs).