Sex Determination
Master Sex Determination for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. In humans the sex chromosomes are XX in females and XY in males, and a Punnett square shows the expected 50:50 ratio.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Sex is determined by one pair of the 23 pairs of chromosomes. In females the pair is XX, and in males it is XY.
Because a female has two X chromosomes, every egg carries an X. A male has one X and one Y, so half his sperm carry an X and half carry a Y. It is therefore the sperm that determines the sex of the offspring.
A Punnett square with XX and XY gives XX, XX, XY and XY, so the probability of a male is 50 per cent and of a female 50 per cent. That ratio should be described as a probability rather than a guarantee.
Revision notes
The sex chromosomes
Of the 23 pairs of chromosomes, one pair determines sex.
Females have two X chromosomes, XX. Males have one X and one Y chromosome, XY. The other 22 pairs carry the genes for all other characteristics.
Which parent determines sex
All eggs carry an X chromosome, because the mother has only X chromosomes to pass on.
Sperm may carry either an X or a Y, because the father has one of each. The sex of the offspring is therefore determined by the sperm.
The Punnett square
Crossing XX with XY gives XX, XX, XY and XY.
Two of the four outcomes are female and two are male, so there is a 50 per cent probability of each. This is a probability for each pregnancy, not a guarantee about a family of four.
Key points
- Sex is determined by one pair of chromosomes.
- Females are XX and males are XY.
- All eggs carry an X chromosome.
- Sperm carry either an X or a Y.
- The sperm determines the sex of the offspring.
- The probability of each sex is 50 per cent.
Worked examples
Example 1
Explain why the sex of a baby is determined by the father rather than the mother. [3 marks]
Model answer
The mother has two X chromosomes, so all her eggs carry an X (1 mark). The father has one X and one Y chromosome, so half his sperm carry an X and half carry a Y (1 mark). Whether the sperm that fertilises the egg carries an X or a Y therefore determines the sex of the baby (1 mark).
Example 2
Use a Punnett square to show the probability of a couple having a girl. [3 marks]
Model answer
Crossing XX with XY gives the possible offspring XX, XX, XY and XY (2 marks). Two of the four outcomes are XX, so the probability of a girl is 1 in 2, or 50 per cent (1 mark).
Example 3
A couple have three sons. Explain the probability that their fourth child will be a girl. [2 marks]
Model answer
The probability is still 50 per cent (1 mark), because each fertilisation is an independent event and previous births do not affect the chromosome carried by the next sperm (1 mark).
Common mistakes
Saying the mother determines the sex.
All eggs carry X, so it is the sperm that decides.
Treating a 50 per cent probability as a guarantee.
Each pregnancy is independent; three sons does not make a daughter more likely.
Writing the female as XY.
Females are XX; males are XY.
Forgetting the other 22 pairs.
Only one pair determines sex; the rest carry other genes.
Exam tips
- Draw the Punnett square — it usually carries marks.
- Explain sex determination through which gamete carries what.
- Say probability rather than certainty.
- Remember each pregnancy is an independent event.
Key terms
- Sex chromosome
- The chromosome pair determining sex, XX or XY.
- Gamete
- A sex cell — sperm or egg.
- Probability
- The likelihood of an outcome, here 50 per cent for each sex.
- Independent event
- An event whose outcome is unaffected by previous ones.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.