AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Sex determination: Ordinary human body cells contain 23 pairs of chromosomes.
Chromosomes are thread-like structures of DNA found in the nucleus.
Study notes
From the checked teaching notes for this specification point.
The idea
Chromosomes are thread-like structures of DNA found in the nucleus. In ordinary human body cells, chromosomes exist as homologous pairs, giving 23 pairs in total, which is 46 chromosomes. One chromosome of each pair is inherited from the mother and one from the father. The pairs are numbered, and the first 22 pairs are autosomes; the 23rd pair is the sex chromosomes. Body cells are diploid, meaning they have two of each chromosome. Gametes are different: sperm and egg cells are haploid and contain 23 single chromosomes, not 23 pairs. At fertilisation, the sperm nucleus fuses with the egg nucleus, restoring the diploid number of 46 chromosomes, or 23 pairs, in the zygote.
What earns the marks
- State that ordinary human body cells are diploid, containing two copies of each chromosome.
- Identify the total number of chromosomes as 46, arranged as 23 homologous pairs.
- Explain that one chromosome in each pair comes from the mother and one from the father.
- Distinguish body cells, which have 23 pairs, from gametes, which have 23 single chromosomes.
- Link the restoration of 23 pairs at fertilisation to the fusion of haploid sperm and egg nuclei.
Common mistakes and how to put them right
Writing that body cells contain 23 chromosomes rather than 23 pairs.
Put it right by stating 23 pairs, which equals 46 chromosomes.
Confusing diploid and haploid numbers.
Put it right by remembering body cells are diploid (2n = 46) and gametes are haploid (n = 23).
Claiming that all 23 pairs are identical.
Put it right by noting that the pairs are homologous but may carry different alleles, and the 23rd pair differs between males and females.
Exam tips
- Use the phrase '23 pairs' precisely, and add '46 chromosomes in total' to show the numerical link.
- When a question asks about gametes, contrast their haploid number with the diploid number of body cells.
- Sketch a simple cell with 23 paired lines and label one chromosome from each parent to support your written answer.
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 there in total in an ordinary human body cell?
Listen
Chromosomes are thread-like structures of DNA found in the nucleus. In ordinary human body cells, chromosomes exist as homologous pairs, giving 23 pairs in total, which is 46 chromosomes. One chromosome of each pair is inherited from the mother and one from the father. The pairs are numbered, and the first 22 pairs are autosomes; the 23rd pair is the sex chromosomes. Body cells are diploid, meaning they have two of each chromosome. Gametes are different: sperm and egg cells are haploid and contain 23 single chromosomes, not 23 pairs. At fertilisation, the sperm nucleus fuses with the egg nucleus, restoring the diploid number of 46 chromosomes, or 23 pairs, in the zygote. A strong answer earns marks for these points. State that ordinary human body cells are diploid, containing two copies of each chromosome. Identify the total number of chromosomes as 46, arranged as 23 homologous pairs. Explain that one chromosome in each pair comes from the mother and one from the father. Distinguish body cells, which have 23 pairs, from gametes, which have 23 single chromosomes. A common slip: Writing that body cells contain 23 chromosomes rather than 23 pairs. Instead, stating 23 pairs, which equals 46 chromosomes. Exam tip: Use the phrase '23 pairs' precisely, and add '46 chromosomes in total' to show the numerical link.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Sex determination: Ordinary human body cells contain 23 pairs of chromosomes.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge