Question 1
Paper 4 short-answer style1 markState the number of chromosomes in a normal human body cell. (1 mark)
Model answer
46 chromosomes (23 pairs).
Why this scores
One mark for '46'.
Launching your learning experience…
Detailed notes on Inheritance for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
DNA → genes → proteins → traits. The hierarchy that links your genome to what you look like.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Each level zooms in. DNA molecule → chromosomes → genes (sections) → alleles (versions) → proteins → traits.
DNA.
Chromosome.
Gene.
Allele.
Protein.
Worked qualitative. Why do CHROMOSOMES form pairs in body cells but not in gametes?
Cambridge tip. State 'GENE = section of DNA coding for a protein'. Cambridge marks this directly.
Two stages. DNA stays in the nucleus; mRNA carries the message to ribosomes.
Stage 1 — TRANSCRIPTION (in the NUCLEUS).
Stage 2 — TRANSLATION (at a RIBOSOME, in cytoplasm).
Why mRNA?
Worked qualitative. Why does a mutation in DNA usually change the protein?
Cambridge tip. Cambridge IGCSE wants the OUTLINE — DNA → mRNA → ribosome → protein. Don't go too deep into the molecular detail.
Body cells: 46. Gametes: 23. Sex: XX (female) or XY (male).
In humans:
| Cell | Chromosomes | Notes |
|---|---|---|
| Body cell (diploid) | 46 | 23 PAIRS |
| Sperm or egg (gamete, haploid) | 23 | One of each pair |
| Zygote | 46 | 23 from each parent — diploid restored |
Sex chromosomes (the 23rd pair).
Worked qualitative. Why is the SEX of a baby determined by the FATHER, not the mother?
Cambridge tip. Memorise 46 (body) / 23 (gamete). Cambridge often gives a chromosome count and asks 'is this body cell or gamete?'.
Verbatim phrases and definitions Cambridge mark schemes credit.
Chromosomes/genes/proteins appear Paper 4 (5-7 marks: hierarchy, protein synthesis outline, chromosome numbers). Examiner reports flag students saying chromosomes ARE genes, and missing 'codes for protein'.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (17.2); 0610/42 Oct/Nov 2024 — Q17 (DNA, proteins); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on chromosomes, genes and proteins, written exactly the way a tutor would explain them at the board.
Question
Describe the relationship between DNA, a chromosome, a gene and an allele.
Step-by-step solution
Step 1
A chromosome is a long, coiled molecule of DNA (with protein).
Step 2
A gene is a section of DNA on a chromosome that codes for one protein.
Step 3
An allele is a particular version of a gene.
Step 4
So: a chromosome is made of DNA; it carries many genes; each gene can exist as different alleles.
Answer
A chromosome is a coiled DNA molecule; a gene is a section of that DNA coding for a protein; an allele is a version of a gene.
Question
State the number of chromosomes in a human (a) body cell, (b) gamete and (c) zygote.
Step-by-step solution
Step 1
(a) A body cell is diploid and has 46 chromosomes (23 pairs).
Step 2
(b) A gamete is haploid and has 23 chromosomes (one of each pair).
Step 3
(c) A zygote is formed when two gametes fuse, so it has 23 + 23 = 46 chromosomes (diploid restored).
Answer
Body cell: 46. Gamete: 23. Zygote: 46.
Examiner tip
46 is the diploid number (23 PAIRS); 23 is the haploid number. Do not confuse the number with the number of pairs.
Question
Explain how a gene controls a characteristic of an organism.
Step-by-step solution
Step 1
A gene is a length of DNA that carries the code for making a particular protein.
Step 2
The protein made could be a structural protein (e.g. part of a cell) or an enzyme that controls a chemical reaction.
Step 3
These proteins then produce the characteristic — for example, an enzyme that makes a pigment gives a flower its colour.
Step 4
So genes control characteristics by controlling which proteins are made in the cells.
Answer
A gene carries the code for a protein; the protein (structural or an enzyme) produces a characteristic. So genes control characteristics by controlling which proteins are made.
Question
All the body cells of an organism contain the same genes, yet they do different jobs. Explain how the cells come to have the same genes, and why they are still different.
Step-by-step solution
Step 1
All the body cells are produced from the original zygote by mitosis, which copies the DNA exactly.
Step 2
So every body cell contains the same complete set of genes.
Step 3
However, only some of the genes are 'switched on' (used) in each type of cell.
Step 4
Different cells use different genes, so they make different proteins and become specialised for different jobs, even though they share the same genes.
Answer
All cells are made by mitosis from the zygote, so they have the same genes. They differ because each cell type only uses (switches on) some of those genes, making different proteins.
Question
Outline how the information in a gene is used to make a protein.
Step-by-step solution
Step 1
The sequence of bases in the gene (DNA) is the code for the order of amino acids in a protein.
Step 2
A copy of the gene is made as a molecule of messenger RNA (mRNA) in the nucleus.
Step 3
The mRNA passes out of the nucleus to a ribosome in the cytoplasm.
Step 4
At the ribosome the code is read in groups of three bases, and amino acids are joined in the correct order to build the protein.
Answer
The base sequence of the gene codes for the amino acid order. The gene is copied into mRNA, which goes to a ribosome, where amino acids are joined in the coded order to make the protein.
Examiner tip
Cambridge wants: DNA base sequence codes for amino acid order; mRNA copy carries the code to a ribosome; amino acids joined in order.
Question
Explain how a change (mutation) in the base sequence of a gene could change the protein the gene makes.
Step-by-step solution
Step 1
The sequence of bases in the gene determines the order of amino acids in the protein.
Step 2
A mutation is a change in this base sequence.
Step 3
This can change which amino acids are joined, so the protein has a different sequence of amino acids.
Step 4
A different sequence can give the protein a different shape, so it may not work properly — for example, an enzyme whose active site has changed shape may no longer fit its substrate.
Answer
A mutation changes the gene's base sequence, which changes the order of amino acids, so the protein has a different shape and may not work properly (e.g. an enzyme's active site changes).
Examiner tip
Link the base change → amino acid order → protein shape → altered function.
High-scoring sample answers for chromosomes, genes and proteins on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
State the number of chromosomes in a normal human body cell. (1 mark)
Model answer
46 chromosomes (23 pairs).
Why this scores
One mark for '46'.
Explain the relationship between a gene and a chromosome. (2 marks)
Model answer
A chromosome is a long molecule of DNA, and a gene is a section (length) of the DNA on a chromosome. Each chromosome carries many genes.
Why this scores
Two marks: gene is a section of DNA on a chromosome; a chromosome carries many genes.
Explain how a gene controls a characteristic of an organism. (3 marks)
Model answer
A gene is a length of DNA that carries the code for making a particular protein. The protein produced — which may be a structural protein or an enzyme — then produces the characteristic (for example, an enzyme that makes a pigment determines colour). So a gene controls a characteristic by controlling which protein is made.
Why this scores
Three marks: gene codes for a protein; protein is structural or an enzyme; the protein produces the characteristic.
All the body cells of a person contain the same genes, but they are specialised for different jobs. Explain how this is possible. (4 marks)
Model answer
All the body cells are produced from the original zygote by mitosis, which makes exact copies of the DNA, so every cell contains the same complete set of genes. However, in each type of cell only some of the genes are switched on (used). Because different cell types use different genes, they make different proteins, which makes them specialised for different jobs — even though they all carry the same genes.
Why this scores
Four marks: cells made by mitosis from the zygote; same genes in all; only some genes used/expressed in each; different proteins → different specialised cells.
Describe how the information in a gene is used to make a protein. (5 marks)
Model answer
The sequence of bases in the gene (DNA) is a code that determines the order of amino acids in the protein. A copy of the gene is made as a molecule of messenger RNA (mRNA) in the nucleus. The mRNA leaves the nucleus and attaches to a ribosome in the cytoplasm. At the ribosome, the base sequence of the mRNA is read in groups of three bases, and amino acids are joined together in the order specified by the code to build the protein.
Why this scores
Five marks: base sequence codes for amino acid order; mRNA copy made; mRNA goes to a ribosome; read in threes; amino acids joined in the coded order to make the protein.
Explain how a mutation in a gene can lead to the production of a non-functioning enzyme. (6 marks)
Model answer
A gene carries a code in the form of a sequence of bases, and this base sequence determines the order of amino acids in the protein the gene makes. A mutation is a change in the base sequence of the gene. Because the base sequence has changed, when the gene is used to make the protein, the amino acids may be joined in a different order, so the protein has a different sequence of amino acids. The order of amino acids determines the way the protein folds and the shape it takes. If the protein is an enzyme, a change in its shape can change the shape of its active site, so that it is no longer complementary to its substrate. The substrate can then no longer fit into the active site, so the enzyme cannot catalyse its reaction and does not work properly. In this way a mutation in a gene can lead to a non-functioning enzyme.
Why this scores
Up to 6 marks: gene base sequence codes for amino acid order; mutation changes the base sequence; changes the order of amino acids; changes the protein's shape; for an enzyme, the active site shape changes; substrate no longer fits / enzyme does not work.
Definitions to memorise and the exact keywords mark schemes credit for chromosomes, genes and proteins answers — sharpened from recent examiner reports for the 2026 0610 sitting.
A long, coiled molecule of DNA. Humans have 46 (23 pairs) in each body cell.
The genetic material, made of two strands of nucleotides in a double helix; carries the code for making proteins.
A length of DNA that codes for a protein. Its base sequence determines the amino acid order of the protein.
A molecule that carries a copy of a gene's code from the DNA in the nucleus to a ribosome.
The structure where mRNA is read and amino acids are joined together to make a protein.
The traps other students keep falling into on chromosomes, genes and proteins questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
Both are inheritance terms.
How to avoid it
A chromosome contains MANY genes; a gene is a small section of a chromosome.
Why it happens
23 is the gamete (haploid) number.
How to avoid it
Body cells have 46 (23 PAIRS); gametes have 23. Don't confuse the number with the number of pairs.
Why it happens
The cells look and act very differently.
How to avoid it
All body cells contain the SAME genes (made by mitosis from the zygote); they differ because each uses only some of those genes.
The things students keep getting wrong in this sub-topic, answered.