Practise 39 exam-style questions on Sex determination for AQA GCSE Biology. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
39 questions
2–4 marks each
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20 questions, marked as you go
Sample questions with model answers
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
Question 1
3 marks
Explain how a Punnett square is used to show the possible sex genotypes of children when a mother (XX) and father (XY) reproduce. Include the possible genotypes and the ratio.
Model answer
Gametes: mother produces X, X; father produces X, Y. Punnett square shows children: XX and XY in a 1:1 ratio (50% XX, 50% XY).
Explanation
Set up a 2x2 Punnett square with mother's gametes (X and X) on one side and father's (X and Y) on the other. The four boxes show XX and XY outcomes equally, giving a 1:1 ratio or 50% chance for female (XX) and 50% chance for male (XY).
Mark scheme
1 mark
Mother gametes = X, X; Father gametes = X, Y
1 mark
Punnett square gives genotypes XX and XY
1 mark
Ratio 1:1 or 50% XX and 50% XY
Question 2
3 marks
A man with a dominant allele on the Y chromosome (Y*) will pass that allele to which children, sons or daughters? Explain.
Model answer
Y-chromosome genes are passed only to sons (Y from father to son). Daughters receive father's X, so they do not inherit Y-linked traits.
Explanation
Y-linked alleles are inherited only by male offspring because males inherit the Y chromosome from their father, while daughters inherit the father's X chromosome.
Mark scheme
1 mark
Only to sons
1 mark
Y is passed from father to son (daughters get X from father)
1 mark
Y-linked traits cannot go to daughters
Question 3
4 marks
A father is affected by an X-linked recessive condition (XrY) and the mother is not a carrier (XX). What are the expected genotypes and phenotypes of their daughters and sons? Show the ratio.
Model answer
All daughters will be carriers (XrX) but not affected; all sons will be normal (XY). Ratio: daughters 100% carriers (not affected); sons 100% unaffected.
Explanation
The father gives his Xr to every daughter, making them carriers (XrX). He gives Y to every son, so sons receive the mother's X (normal) and are not affected.
Mark scheme
1 mark
Daughters genotypes XrX (carriers), not affected
1 mark
Sons genotypes XY (unaffected)
1 mark
State ratios: 100% daughters carriers; 100% sons unaffected
1 mark
Explanation that father passes Xr to all daughters and Y to sons
Question 4
4 marks
Describe how to perform a basic Mendelian cross on paper to predict the sex distribution of 12 offspring from parents with genotypes XX (female) and XY (male). Show working and expected numbers.
Model answer
Gametes: female X,X ; male X,Y. Punnett square yields XX and XY equally. Probability female = 1/2, male = 1/2. For 12 offspring expect 6 females and 6 males (12 × 1/2 = 6).
Explanation
Set up a cross with gametes X and X from mother, X and Y from father to get equal XX and XY outcomes. Multiply probabilities by 12 to get expected counts of 6 and 6.
Mark scheme
1 mark
State gametes: female X,X ; male X,Y
1 mark
State probabilities 1/2 female, 1/2 male
1 mark
Calculation for 12: 12 × 1/2 = 6 females and 6 males
1 mark
Conclude expected numbers: 6 of each
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