AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Ordinary human body cells contain 23 pairs of chromosomes.
Full specification wording
Sex determination: Ordinary human body cells contain 23 pairs of chromosomes.
In males, the X and Y sex chromosomes are not fully homologous.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 1 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In males, the X and Y sex chromosomes are not fully homologous. Initial cellular context is established: ordinary human body cells are diploid with 23 pairs or 46 chromosomes. Standard processing applies to autosomes: 22 pairs are homologous. The final transformation: the 23rd pair determines sex. Females (XX) have a homologous pair, but males (XY) do not. Body cells divide by mitosis to maintain 23 pairs. Gametes are haploid, containing 23 single chromosomes. Fertilisation restores the diploid state.
For example: A sperm and egg fuse to form a zygote with 46 chromosomes.
What earns the marks
- In males, the X and Y sex chromosomes are not fully homologous.
- Ordinary human body cells are diploid, containing 23 pairs (46 chromosomes total).
- 22 pairs are homologous autosomes; the 23rd pair are sex chromosomes (XX or XY).
- Gametes are haploid (23 single chromosomes); fertilisation restores the diploid number.
Common mistakes and how to put them right
Saying all 23 pairs are homologous.
Put it right: Male XY chromosomes are not fully homologous.
Saying body cells contain 23 chromosomes.
Put it right: Body cells contain 23 pairs, which is 46 chromosomes.
Saying gametes contain 23 pairs.
Put it right: Gametes contain 23 single chromosomes, so they are haploid.
Exam tips
- Specify that only 22 pairs are always homologous (autosomes).
- Always give the total chromosome count (46) alongside the pair count (23) to show the diploid state.
Try the lesson
The start of the Sex determination: Ordinary human body cells contain 23 pairs of chromosomes. lesson, made from checked teaching notes for this specification point.
Have a go first
How many chromosomes are found in an ordinary human body cell?
Listen
In males, the X and Y sex chromosomes are not fully homologous. Initial cellular context is established: ordinary human body cells are diploid with 23 pairs or 46 chromosomes. Standard processing applies to autosomes: 22 pairs are homologous. The final transformation: the 23rd pair determines sex. Females (XX) have a homologous pair, but males (XY) do not. Body cells divide by mitosis to maintain 23 pairs. Gametes are haploid, containing 23 single chromosomes. Fertilisation restores the diploid state. For example, a sperm and egg fuse to form a zygote with 46 chromosomes. A strong answer earns marks for these points. In males, the X and Y sex chromosomes are not fully homologous. Ordinary human body cells are diploid, containing 23 pairs (46 chromosomes total). 22 pairs are homologous autosomes; the 23rd pair are sex chromosomes (XX or XY). Gametes are haploid (23 single chromosomes); fertilisation restores the diploid number. A common slip: Saying all 23 pairs are homologous. Instead, male XY chromosomes are not fully homologous. Exam tip: Specify that only 22 pairs are always homologous (autosomes).
Key words
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Diploid and haploid
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Homologous chromosomes
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Sex determination
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See every definition
- Diploid and haploid
- Ordinary human body cells are diploid, containing 23 pairs or 46 chromosomes, while gametes are haploid, containing 23 single chromosomes.
- Homologous chromosomes
- 22 pairs are homologous autosomes, but in males the X and Y sex chromosomes are not fully homologous.
- Sex determination
- The 23rd pair determines sex: females (XX) have a homologous pair, but males (XY) do not.
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