AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
In females the sex chromosomes are the same (XX). • In males the…
Full specification wording
Sex determination: In females the sex chromosomes are the same (XX). • In males the chromosomes are different (XY).
Human body cells normally contain 23 pairs of chromosomes.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 3 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Human body cells normally contain 23 pairs of chromosomes. One pair, the sex chromosomes, determines whether an individual develops as female or male. In females the two sex chromosomes are the same, both X, so the pair is written XX. In males the two sex chromosomes are different: one X and one Y, written XY. The Y chromosome carries the gene that triggers testes development, so its presence normally leads to a male phenotype. Gametes are haploid: every egg carries one X, while sperm carry either X or Y. At fertilisation, an X-bearing sperm gives XX and an X-bearing egg with a Y-bearing sperm gives XY. This explains why sex is determined by the sperm and why the chance of each sex is approximately equal.
What earns the marks
- Human body cells contain 23 pairs of chromosomes, including one pair of sex chromosomes.
- In females the sex chromosomes are the same, both X, so the pair is written XX.
- In males the sex chromosomes are different, one X and one Y, so the pair is written XY.
- The Y chromosome carries the gene that triggers testes development, so XY normally produces a male phenotype.
- Egg cells all carry one X chromosome, while sperm cells carry either an X or a Y chromosome.
- Sex is determined at fertilisation by whether an X-bearing or a Y-bearing sperm fuses with the egg.
Common mistakes and how to put them right
Writing the male pair as YX rather than XY.
Put it right: The conventional order is X first, so correct it to XY.
Thinking that females have two identical chromosomes in every pair.
Put it right: Only the sex chromosome pair is XX, while the other 22 pairs are autosomes.
Believing the mother determines the sex of the child.
Put it right: The egg always contributes X, so the sperm's X or Y chromosome determines sex.
Exam tips
- Use the exact symbols XX and XY when describing the sex chromosome pairs.
- State clearly that gametes are haploid and carry only one sex chromosome.
- Link the Y chromosome to the development of testes and the male phenotype rather than saying Y simply means male.
Try the lesson
The start of the Sex determination: In females the sex chromosomes are the same (XX). • In males the chromosomes are different (XY). lesson, made from checked teaching notes for this specification point.
Have a go first
Which pair of sex chromosomes is found in a human female?
Listen
Human body cells normally contain 23 pairs of chromosomes. One pair, the sex chromosomes, determines whether an individual develops as female or male. In females the two sex chromosomes are the same, both X, so the pair is written XX. In males the two sex chromosomes are different: one X and one Y, written XY. The Y chromosome carries the gene that triggers testes development, so its presence normally leads to a male phenotype. Gametes are haploid: every egg carries one X, while sperm carry either X or Y. At fertilisation, an X-bearing sperm gives XX and an X-bearing egg with a Y-bearing sperm gives XY. This explains why sex is determined by the sperm and why the chance of each sex is approximately equal. A strong answer earns marks for these points. Human body cells contain 23 pairs of chromosomes, including one pair of sex chromosomes. In females the sex chromosomes are the same, both X, so the pair is written XX. In males the sex chromosomes are different, one X and one Y, so the pair is written XY. The Y chromosome carries the gene that triggers testes development, so XY normally produces a male phenotype. Exam tip: Use the exact symbols XX and XY when describing the sex chromosome pairs.
Key words
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Sex determination
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Haploid gametes and fertilisation
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Genotype and phenotype
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See every definition
- Sex determination
- Whether an individual develops as female or male is determined by one pair of chromosomes, the sex chromosomes.
- Haploid gametes and fertilisation
- Gametes are haploid, so every egg carries one X while sperm carry either X or Y, and sex is determined at fertilisation by which sperm fuses with the egg.
- Genotype and phenotype
- The Y chromosome carries the gene that triggers testes development, so XY normally produces a male phenotype.
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