Chromosomes, genes and proteins — Cambridge IGCSE Biology
Chromosomes, genes and alleles, how sex is inherited, how genes control protein production, gene expression, and haploid and diploid nuclei.
Revision notes
4 short notes, in the order of the specification. Each one in short:
A gene is a length of DNA that codes for a protein, and an allele is an alternative form of a gene. Chromosomes in the nucleus are made of DNA, and every gene has a fixed position on a chromosome. Different alleles, such as for red or white flowers, have slightly different DNA, so they code for slightly different proteins.
Sex is inherited through one pair of sex chromosomes: females are XX and males are XY. Every egg carries an X chromosome, but half the sperm carry X and half carry Y. So the sex of a baby depends on which sperm fertilises the egg, giving a 1 : 1 chance of a boy or a girl.
How genes control proteins
SupplementThe sequence of bases in a gene determines the sequence of amino acids used to make a specific protein. The gene stays in the nucleus, and a copy called messenger RNA (mRNA) is made. The mRNA moves to the cytoplasm and passes through a ribosome, which assembles amino acids into the protein in the order set by the bases.
Gene expression, haploid and diploid
SupplementGene 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.
Exam questions
7 exam-style questions (21 marks), each with its mark scheme.
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14 cards: flip them, mark what you knew, and practise the rest.
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Quick check questions
Define a gene.
Show answer
A length of DNA that codes for a protein.
Define an allele.
Show answer
An alternative form of a gene.
Which sex chromosomes do males and females have?
Show answer
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.
Frequently asked questions
What is the difference between a gene and an allele?
A gene is a length of DNA that codes for a protein, while an allele is an alternative form of a gene. For example, the gene for flower colour may have one allele for red flowers and another for white flowers. Different alleles have slightly different DNA, so they code for slightly different proteins.
How is the sex of a baby determined?
The sex of a baby is determined by which sperm fertilises the egg. Females are XX, so every egg carries an X chromosome. Males are XY, so half the sperm carry X and half carry Y. An X sperm gives a girl (XX) and a Y sperm gives a boy (XY), a 1 : 1 chance.
How do genes control the making of proteins?
Supplement Genes control protein production through their sequence of bases, which determines the sequence of amino acids in a protein. The gene stays in the nucleus, and a copy called mRNA is made. The mRNA moves to the cytoplasm and passes through a ribosome, which assembles amino acids into the protein in the correct order.
Why do different cells make different proteins?
Supplement Different cells make different proteins because only some of their genes are expressed. Most body cells contain the same genes, but each cell makes only the specific proteins it needs. For example, a pancreas cell makes insulin, while a skin cell has the insulin gene but does not make it. This lets cells do different jobs.
What is the difference between haploid and diploid?
Supplement A haploid nucleus contains a single set of chromosomes, while a diploid nucleus contains two sets. Human gametes, sperm and egg cells, are haploid with 23 chromosomes. Human body cells such as skin cells are diploid, with 23 pairs or 46 chromosomes, one chromosome of each pair from the mother and one from the father.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026