Gene expression, haploid and diploidSpec 17.1.9, 17.1.10, 17.1.11, 17.1.12
In short
Gene expression means a cell only makes the specific proteins it needs, so many of its genes are not expressed, which is why cells with the same genes can do different jobs. A haploid nucleus contains a single set of chromosomes, 23 in human gametes. A diploid nucleus contains two sets, 23 pairs or 46 chromosomes in humans.
Gene expression
Most body cells in an organism contain the same genes. However, a cell only makes the specific proteins it needs. Many genes in a particular cell are therefore not expressed. For example, a cell in the pancreas makes the protein insulin, but a skin cell has the gene for insulin and does not make it.
This is why cells can be different shapes and do different jobs, although they have the same genes.
Haploid and diploid nuclei
- Haploid nucleus
- A nucleus containing a single set of chromosomes.
- Diploid nucleus
- A nucleus containing two sets of chromosomes.
In a diploid cell there is a pair of each type of chromosome. One chromosome in each pair came from the mother and one came from the father. In a human diploid cell there are 23 pairs of chromosomes, which is 46 in total. Human gametes are haploid and have 23 chromosomes.
| Haploid | Diploid | |
|---|---|---|
| Sets of chromosomes | one | two |
| Number of chromosomes in a human cell | 23 | 46 (23 pairs) |
| Example | sperm cell, egg cell | skin cell, muscle cell |
Say 23 pairs for a diploid cell and 23 chromosomes for a haploid cell. '23 chromosomes' on its own does not tell the examiner whether you mean pairs or a single set.
Quick check
Define a gene.
Show answer
A length of DNA that codes for a protein.
Define an allele.
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An alternative form of a gene.
Which sex chromosomes do males and females have?
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Males are XY; females are XX.
Supplement What determines the sequence of amino acids in a protein?
Show answer
The sequence of bases in the gene.
Supplement How many pairs of chromosomes are in a human diploid cell?
Show answer
23 pairs.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026