A student argues: 'Since sex in humans is genetically determined, and each chromosome pair provides one copy from each parent, a mother must be responsible for determining the sex of her child.' Identify the flaw in this argument and explain the correct mechanism.
Generated by claude-sonnet-4-6 · 2026-06-26 01:04 · grounding rag
Model Answer
Flaw: The argument is incorrect. The mother cannot determine the sex of the child.
Correct Mechanism: All women are XX, so a mother can only pass an X chromosome to her child. Men are XY, so a father can pass either an X or a Y chromosome. If the child inherits X from the father → girl (XX); if the child inherits Y from the father → boy (XY). Since the father's contribution determines whether X or Y is passed, it is the father, not the mother, who determines the sex of the child.
Source: Chapter 8, Section 8.2.4 – Sex Determination
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Explanation
- The key examiner expectation is: (1) identify the flaw clearly — mother only has X chromosomes to give, so she cannot determine sex; (2) state the correct mechanism — father has both X and Y, making him the deciding factor.
- Use the terms XX (female) and XY (male) — examiners specifically look for these.
- A common mistake is writing a vague answer without explaining why the mother cannot determine sex. Always link it back to chromosome composition.