A couple has four daughters. The father argues that it is the mother's genetics that keeps 'producing' girls. Is he correct? Justify your answer using the mechanism of sex determination in humans.
Generated by claude-sonnet-4-6 · 2026-06-26 01:04 · grounding rag
Model Answer
No, the father is not correct.
In human beings, sex is determined by the father's contribution, not the mother's.
- All eggs produced by the mother (XX) carry only the X chromosome.
- The father (XY) produces two types of sperms — 50% carry X and 50% carry Y.
- If a sperm carrying X fertilises the egg → girl (XX)
- If a sperm carrying Y fertilises the egg → boy (XY)
Since the mother can only contribute an X chromosome, the sex of the child depends entirely on which type of sperm from the father fertilises the egg. The father is responsible, not the mother.
Source: Chapter 8, Section 8.2.4 — Sex Determination
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Explanation
What examiners look for:
- Clear statement that the father is wrong (1 mark).
- Explanation that mother is XX → only X-bearing eggs (1 mark).
- Father is XY → produces X and Y sperms; Y sperm gives a boy, X sperm gives a girl — sex determined by father's sperm (1 mark).
Key tip: Always state the conclusion first, then justify. Don't forget to mention both types of sperms from the father — that's the heart of the answer.