AQA GCSE Combined Science Foundation (8464), Foundation tier · Biology › Inheritance, variation and evolution › Reproduction
Practise Sex determination. 8 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Human body cells have 23 pairs of chromosomes, and one pair decides sex: XX is female and XY is male. You need to draw a genetic cross to show how sex is inherited and explain why the chance of a boy or a girl is always 0.5.
Key facts
Human body cells: 23 pairs of chromosomes
22 pairs control characteristics only; 1 pair are the sex chromosomes
Female = XX; male = XY
Eggs: all X. Sperm: half X, half Y
XX × XY → XX, XX, XY, XY → 1 : 1 female : male
Probability of a boy (or a girl) = 0.5 for every child
Notes
Chromosomes and sex
Ordinary human body cells contain 23 pairs of chromosomes (46 in total).
22 pairs control characteristics only.
One pair carries the genes that determine sex. These are the sex chromosomes.
In females the sex chromosomes are the same: XX.
In males the sex chromosomes are different: XY.
Gametes
A gamete has one chromosome from each pair, so it has one sex chromosome.
All egg cells contain an X chromosome.
Half of all sperm cells contain an X chromosome and half contain a Y chromosome.
The genetic cross
diagram
Put the mother's gametes (X and X) along one side of a Punnett square and the father's gametes (X and Y) along the other.
The offspring are XX, XX, XY and XY: 2 female and 2 male, a ratio of 1 : 1.
The father's sperm (X or Y) decides the sex of the baby.
So the probability of a boy is 0.5 (50%), and the probability of a girl is also 0.5.
The probability is 0.5 for every child, however many boys or girls a family already has.
The sex of the baby depends on whether the egg is fertilised by a sperm carrying an X or a Y chromosome.
How to answer each type of question
Complete a genetic cross to show sex inheritance
2 to 3 marksGrade 5
Mother's gametes: X and X. Father's gametes: X and Y.
Fill in the four boxes.
Label XX as female and XY as male.
Example. Complete a Punnett square to show how sex is inherited in humans. Label each offspring as male or female. [3 marks]
Show the model answerHide the model answer
Correct gametes: X and X from the mother, X and Y from the father (1). Offspring XX, XX, XY, XY (1). XX labelled female and XY labelled male (1).
Don’t lose marks
Saying human body cells have 23 chromosomes (that is a gamete) or 46 pairs. Body cells have 23 pairs (46 chromosomes).
Putting a Y in the mother's gametes. Only males have a Y chromosome.
Thinking a family with several girls is 'due' a boy. The probability is still 0.5.
Saying only males have sex chromosomes. Both sexes have a pair; they are XX or XY.
More tips
Memory tricks
Y is for guY: the Y chromosome makes a male.
Mum can only give an X; Dad gives an X or a Y, so the sperm decides.
The sex cross always gives 1 : 1, so any 'boy or girl' probability is 0.5.
Exam technique
Draw the Punnett square neatly and label the gametes; each part carries marks.
In 'explain' questions, link the chromosomes to the gametes (eggs X; sperm X or Y), not just 'it is random'.
Give probabilities as 0.5, 50%, ½ or '1 in 2', not as a ratio.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State the number of chromosome pairsOrdinary human body cells contain 23 pairs of chromosomes.
Grade 4
Give the sex chromosomes of each sexFemales are XX; males are XY.
Grade 5
State what the other 22 pairs do22 pairs control characteristics only; one pair carries the genes that determine sex.
Grade 5
Draw a genetic cross for sexXX × XY in a Punnett square gives XX, XX, XY, XY.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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