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6.1.8Sex determination

AQA GCSE Biology (8461), Higher tier · Inheritance, variation and evolution › Reproduction

Practise Sex determination. 13 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.

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Quick recall

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How many pairs of chromosomes are there in an ordinary human body cell?
23
What percentage of the sperm made by a man carry a Y chromosome?
50%

Sample questions

Written for this site in the style of AQA exam questions. They are not taken from real past papers.

Question 1Easy5 marks
Ordinary human body cells contain pairs of chromosomes. One pair carries the genes that determine sex.
(a) How many pairs of chromosomes are there in an ordinary human body cell?[1]
(b) Which sex chromosomes does a human female have?
Tick (✓) one box.[1]
  • XX
  • XY
  • YY
  • X only
(c) Which sex chromosome is found in every human egg cell?
Tick (✓) one box.[1]
  • X
  • Y
  • XX
  • XY
(d) What do the other 22 pairs of chromosomes control?[1]
(e) Which parent's gamete determines the sex of a child?[1]
Show the answer and mark scheme
(a) Answer: 23
  • 23
(b) Answer: XX
(c) Answer: X
(d) Answer: Characteristics only (not sex)
  • (other) characteristics only / not sex
(e) Answer: The father's (the sperm)
  • the father's / the sperm
Question 2Medium5 marks
In humans, females have the sex chromosomes XX and males have the sex chromosomes XY.
(a) Explain why it is the sperm, not the egg, that determines the sex of a baby.[2]
(b) What percentage of the sperm made by a man carry a Y chromosome?[1]
(c) Explain why a human sperm cell contains 23 chromosomes.[1]
(d) Describe how the sex chromosomes of a male are different from the other 22 pairs of chromosomes in his cells.[1]
Show the answer and mark scheme
(a) Answer: All eggs carry an X chromosome, but half of sperm carry X and half carry Y; if a Y sperm fertilises the egg the baby is a boy (XY)
  • all eggs contain an X chromosome
  • sperm contain either an X or a Y chromosome, so the sperm decides whether the baby is XX or XY
(b) Answer: 50%
  • 50 (%)
(c) Answer: It is made by meiosis, so it contains one chromosome from each of the 23 pairs
  • made by meiosis / contains one chromosome from each pair / a single set of chromosomes
(d) Answer: The X and Y chromosomes are different from each other, whereas the chromosomes in each of the other pairs match
  • the X and Y are different (from each other) / do not match, but the other pairs match
Question 3Hard7 marks
In one year, 2562 boys and 2438 girls were born in a large hospital.
(a) Calculate the ratio of boys to girls.
Give your answer in the form x : 1, to 3 significant figures.[2]
(b) Suggest one reason why the numbers of boys and girls born were not exactly equal.[1]
(c) Calculate the probability that a family with four children has four girls.
Give your answer as a fraction.[2]
(d) A family has three children.
Calculate the probability that exactly two of the children are girls. Show your working.[2]
Show the answer and mark scheme
(a) Answer: 1.05 : 1
  • 2562 ÷ 2438
  • 1.05 (: 1)
(b) Answer: Fertilisation is random, so the actual numbers vary by chance
  • fertilisation is random / it is a matter of chance which sperm fertilises the egg / slightly more male embryos may survive
(c) Answer: \(\frac{1}{16}\)
  • ½ × ½ × ½ × ½
  • 1/16 (0.0625)
(d) Answer: \(\frac{3}{8}\)
  • there are 8 equally likely outcomes, and 3 have exactly two girls (GGB, GBG, BGG)
  • 3/8 (0.375)

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