AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Sex determination: In males the chromosomes are different (XY).
Human body cells are diploid with 23 pairs of chromosomes.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Human body cells are diploid with 23 pairs of chromosomes. The sex chromosomes form one pair. In males the two sex chromosomes are not the same: one is a large X chromosome and the other is a much smaller Y chromosome, written XY. Because X and Y are different in size and gene content, they are not truly homologous, although they still pair and separate during meiosis. A male therefore produces two types of sperm in roughly equal numbers: half carry X and half carry Y. If an X sperm fertilises an egg the zygote is XX and female; if a Y sperm fertilises an egg the zygote is XY and male. This is why the father's sperm determines the sex of the offspring.
What earns the marks
- Human body cells contain 23 pairs of chromosomes, including one pair of sex chromosomes.
- In males the sex chromosomes are different: one X and one smaller Y, written XY.
- The X and Y chromosomes differ in size and the genes they carry, so they are not a matching homologous pair.
- During meiosis the X and Y separate, so a male produces roughly equal numbers of X-bearing and Y-bearing sperm.
- Fertilisation by a Y sperm gives an XY zygote, which develops into a male.
- The father's sperm, not the mother's egg, determines whether the offspring is male or female.
Common mistakes and how to put them right
Writing that males have 'two different chromosomes called XY but they are homologous' —.
Put it right: They differ in size and gene content.
Saying males produce only Y sperm —.
Put it right: Half the sperm carry X and half carry Y.
Claiming the egg determines sex —.
Put it right: An X egg plus a Y sperm gives a male, while an X egg plus an X sperm gives a female.
Exam tips
- Draw a genetic diagram with parental genotypes XX and XY, showing gametes X, X and X, Y and the resulting XX and XY offspring.
- State the expected ratio of male to female offspring as approximately 1:1 and link it to the equal numbers of X and Y sperm.
- Use the terms 'sex chromosome', 'gamete' and 'fertilisation' accurately to gain credit for explanation.
Try the lesson
The start of the Sex determination: 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 male?
Listen
Human body cells are diploid with 23 pairs of chromosomes. The sex chromosomes form one pair. In males the two sex chromosomes are not the same: one is a large X chromosome and the other is a much smaller Y chromosome, written XY. Because X and Y are different in size and gene content, they are not truly homologous, although they still pair and separate during meiosis. A male therefore produces two types of sperm in roughly equal numbers: half carry X and half carry Y. If an X sperm fertilises an egg the zygote is XX and female; if a Y sperm fertilises an egg the zygote is XY and male. This is why the father's sperm determines the sex of the offspring. A strong answer earns marks for these points. Human body cells contain 23 pairs of chromosomes, including one pair of sex chromosomes. In males the sex chromosomes are different: one X and one smaller Y, written XY. The X and Y chromosomes differ in size and the genes they carry, so they are not a matching homologous pair. During meiosis the X and Y separate, so a male produces roughly equal numbers of X-bearing and Y-bearing sperm. A common slip: Writing that males have 'two different chromosomes called XY but they are homologous' — the error is treating X and Y as a matching pair. Instead, they differ in size and gene content. Exam tip: Draw a genetic diagram with parental genotypes XX and XY, showing gametes X, X and X, Y and the resulting XX and XY offspring.
See it
Key words
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Sex chromosomes
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Meiosis
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Fertilisation
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Zygote
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See every definition
- Sex chromosomes
- The pair of chromosomes that in males are not the same: one is a large X chromosome and the other is a much smaller Y chromosome, written XY.
- Meiosis
- The process during which the X and Y chromosomes separate, so a male produces roughly equal numbers of X-bearing and Y-bearing sperm.
- Fertilisation
- The joining of a sperm and an egg, so that an X sperm gives an XX zygote and a Y sperm gives an XY zygote.
- Zygote
- The cell formed by fertilisation, which is XX and female if an X sperm fertilises an egg and XY and male if a Y sperm does.
Label the diagram
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1Box 1
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