Gene — a length of DNA that codes for a protein
A chromosome is a thread-like structure made of DNA (deoxyribonucleic acid), and the genetic information it carries is organised into genes. The mark-scheme definition of a gene has two elements: a length of DNA, and codes for a protein. An allele is an alternative form of that gene, sitting at the same position on the same chromosome. Scale order: chromosome (many genes) > gene (one protein) > allele (one version of that gene).
Sex inheritance — the sperm decides
A human female is XX and a human male is XY; the sex chromosomes are one pair out of the 23. Because the mother is XX, every egg carries a single X. Because the father is XY, he makes X-sperm and Y-sperm in equal numbers. At fertilisation an X-sperm gives XX (female) and a Y-sperm gives XY (male), so the expected ratio is 1 female : 1 male. The egg never varies, so it is the type of sperm that determines the sex of the child.
Haploid and diploid nuclei (Extended)
A haploid nucleus contains a single set of chromosomes, unpaired — gametes are haploid. A diploid nucleus contains two sets, a pair of each type (homologous pairs), one member of each pair from each parent — ordinary body cells are diploid. A human diploid body cell has 46 chromosomes arranged as 23 pairs; a human gamete has 23. Fertilisation of a haploid egg by a haploid sperm restores 46 in the zygote.
Drawn from real examiner reports.
Gene ≠ "controls a characteristic"
Candidates write "a part of a chromosome" or "something that controls a characteristic", and drop the two elements the mark scheme wants: a length of DNA, and codes for a protein. The protein is the point being tested: the gene codes for a protein, and it is that protein which produces the observable characteristic. Naming the characteristic alone skips the step.
Grounding: shared digest docs/exam-digests/cambridge-0654.md, B16 section -- "Define a gene = a length of DNA that codes for a protein ... poorly defined (June 2022 Paper 31 Q4d; November 2023 Paper 32 Q4ci)."
Allele ≠ "a type of gene"
Candidates repeat the word gene — "an allele is a gene for a characteristic" — without the idea of alternatives. The definition needs both halves: an alternative (different) form, of a gene. Different alleles of one gene occupy the same position on the same chromosome and code for slightly different versions of the same protein.
Grounding: shared digest docs/exam-digests/cambridge-0654.md, B16 section -- "an allele = an alternative form of a gene -- both poorly defined (June 2022 Paper 31 Q4d; November 2023 Paper 32 Q4ci)."
46 chromosomes, not 23
Asked for the chromosome number of a human diploid body cell, candidates answer 23 — that is the haploid gamete number, correct only for a sperm or an egg. A human body cell has 46 chromosomes, arranged as 23 pairs. Read which cell type the question names before answering: body cell = 46, gamete = 23.
Grounding: shared digest docs/exam-digests/cambridge-0654.md, B16 section -- "A human diploid body cell has 46 chromosomes (23 pairs) ... (June 2022 Paper 41 Q4c)."
"Sex chromosomes" wants letters
When a question asks which sex chromosomes a human male has, the answer is XY — two letters, not a number. Candidates have answered 23, 48, 46, or named a gamete instead. Split the two question types: "state the number of chromosomes" wants a number; "state the sex chromosomes" wants X and/or Y. Male XY, female XX.
Grounding: shared digest docs/exam-digests/cambridge-0654.md, B16 section -- "sex chromosomes of a male = XY (candidates gave 23, 48, a gamete name, or 46 when XY was asked) (June 2022 Paper 42 Q1bii; November 2023 Paper 32 Q4cii)."
Mothers do not make Y eggs
In a sex-determination genetic diagram, candidates often show the mother producing both X and Y gametes. She is XX, so every egg she makes carries X and only X — the two gamete types belong to the father alone. Giving the mother two gamete types makes the whole diagram wrong, even if the 1:1 ratio underneath it happens to be quoted correctly.
Sex ratio is 1:1, not 3:1
The 3:1 ratio comes from a monohybrid cross between two heterozygotes, where a dominant allele masks a recessive one. There is no dominance between X and Y — XX and XY are simply two equally likely combinations of the father's two sperm types with the mother's single egg type. Sex determination is therefore always 1:1, whatever the monohybrid rule says.
"23 pairs" is a human figure only
Given an organism with a stated diploid number, candidates still answer using the human 23 pairs. Use the number in the question: the number of pairs is half the diploid number, and a gamete carries half the diploid number too. A fruit fly (Drosophila melanogaster) with a diploid number of 8 has 4 pairs and 4 chromosomes in each gamete.
Scaffold: inheritance of sex
Answer in a fixed order: mother XX, so all eggs carry X; father XY, so his sperm are X or Y in equal numbers; the egg is fertilised at random by one of them; the outcomes are XX (female) and XY (male) in a 1:1 ratio; therefore it is the sperm that decides the sex.
Match define, state and describe
"Define" (gene, allele) needs the exact two-element wording — an example alone scores zero. "State" (chromosome composition or number) needs one short fact. "Describe" (inheritance of sex, haploid/diploid nucleus) needs the mechanism, not a one-word label.
Give 46 and 23 pairs together
When asked to state the chromosome number and its arrangement in a human body cell, give both figures — 46 chromosomes, arranged as 23 pairs. Quoting only one leaves half the answer on the table, and 23 written on its own reads to an examiner as the gamete number.
Check your genetic diagram
Before moving on, check five things: mother labelled XX and father XY the right way round; one egg type (X); two sperm types (X and Y) in equal numbers; both sperm rows of the grid filled in; and the ratio written out as 1 female : 1 male.
Cambridge 0654 spec reference: Section B16 "Inheritance", sub-topic B16.1 (Core + Extended/Supplement). This leaf covers: what chromosomes are made of; the definitions of gene and allele; the inheritance of sex in humans via XX/XY chromosomes (Core); and haploid vs diploid nuclei, plus human chromosome-pair numbers (Extended/Supplement).
Out of scope for B16.1 (belongs to later leaves, do not use here): mitosis, meiosis, chromosome replication before cell division (B16.2); dominant/recessive monohybrid crosses and pedigree diagrams (B16.3).
| Term | Mark-scheme-precise definition |
|---|---|
| Chromosome | A thread-like structure made of DNA (deoxyribonucleic acid), which carries genetic information in the form of genes |
| Gene | A length of DNA that codes for a protein |
| Allele | An alternative form of a gene |
| Haploid nucleus (Extended) | A nucleus containing a single set of chromosomes |
| Diploid nucleus (Extended) | A nucleus containing two sets of chromosomes (a pair of each type) |
| Human diploid body cell | Contains 46 chromosomes, arranged as 23 pairs |
| Human gamete (haploid) | Contains 23 (unpaired) chromosomes -- half the diploid number |
The hierarchy to remember: DNA molecule chromosome (a length of coiled DNA) gene (a specific length of that DNA, coding for one protein) allele (one version of a gene).
Sex determination in humans: the sex chromosomes are in a female and in a male. All eggs carry ; sperm carry or in a 1:1 ratio -- so the father's sperm determines the sex of the child, not the mother's egg.
Define a gene.
Define each of the following terms:
(a) gene
(b) allele
(c) State what a chromosome is made of.
(3 marks)
