Chromosome, gene, allele — the hierarchy
Chromosome — a very long DNA molecule in the nucleus, carrying genetic information as genes. Gene — a length of DNA that codes for a protein. Allele — an alternative form of a gene, sitting at the same position (locus) on a chromosome and coding for a slightly different version of the same protein. Size order: chromosome gene allele. A human diploid cell has 23 pairs of chromosomes, one of each pair from each parent, so it carries two alleles of every gene.
Sex chromosomes — eggs all X, sperm X or Y
Sex chromosomes — one of the 23 pairs in a human cell; the other 22 pairs are autosomes. Female = XX, male = XY. Every egg carries a single X, because the mother is XX. Sperm carry either an X or a Y, in equal numbers, because the father is XY. So the sperm decides the sex: an X-sperm gives XX (female), a Y-sperm gives XY (male). Each conception is an independent 1 in 2 (50%) chance either way, never influenced by the sex of earlier children.
(Extended) Gene expression — DNA controls proteins
Gene expression — DNA controls a cell by controlling which proteins it makes: enzymes that control metabolic reactions, membrane carriers that control what crosses the membrane, and receptor proteins such as neurotransmitter receptors at a synapse. Almost all body cells hold the same genes, having arisen by mitosis from one fertilised egg, but each cell expresses only the genes for the specific proteins it needs — which is why a muscle cell and a nerve cell differ.
Drawn from real examiner reports.
Same genes, different expression
(Extended) Asked why a liver cell and a skin cell differ, candidates answer that the cells contain different genes. They do not — every body cell carries the same complete set. What differs is which genes are expressed, or switched on, in each cell type. Examiners want the word expressed; listing the different proteins alone does not secure the mark.
Chromosome > gene > allele order
The examiner digest records this size order as not well known. A chromosome is a long DNA molecule with many genes strung along its length, and each of those genes exists in a population as two or more alleles. A chromosome is therefore far larger than a gene, and a gene is never larger than the chromosome carrying it — the order always runs chromosome, then gene, then allele.
Chromosome vs gene vs allele size order not known (cambridge-0610.md Section 17, w22 Paper 31 Q2b).
A gene is not an allele
The two terms are not interchangeable. A gene is a length of DNA that codes for a protein; an allele is one alternative version of that gene. Reversing the relationship — writing that a gene is an alternative form of an allele — is a stock multiple-choice distractor. Every gene can have several alleles, and a different allele is not a different gene.
Chromosome vs gene vs allele size order not known (cambridge-0610.md Section 17, w22 Paper 31 Q2b).
DNA is not a protein
Candidates describe DNA as made of amino acids, or as a protein. DNA is a nucleic acid: it codes for proteins, which are built separately from amino acids at the ribosome. Never write that DNA is broken down into a protein or turns into a protein. The only correct link is that a gene, a length of DNA, codes for one specific protein.
DNA described as "made of amino acids" or "a protein" (cambridge-0610.md Section 4, w22 Paper 41 Q4a; s23 Paper 23 Q8) — the same confusion recurs when candidates explain how DNA controls cell function in Section 17.
Sex-linked alleles need X superscripts
For colour blindness, candidates put the allele on the Y chromosome, or wrote plain letters such as Bb with no sex chromosomes at all. The Y carries no colour-vision allele, so the allele must sit on an X: a carrier female is and an affected male is . Gamete boxes must show the allele on its chromosome, not a bare X or Y.
Colour-blindness alleles placed on Y chromosomes, or sex chromosomes used in place of alleles in gamete boxes (cambridge-0610.md Sections 16 and 17, s22 Paper 42 Q3c iii; w22 Paper 31 Q2b i; s22 Papers 41/42 Q3c).
The mother cannot determine the sex
The mother is XX, so every egg she makes carries an X — her gamete is fixed and cannot vary the outcome. Only the father is XY, so only his sperm can supply either an X or a Y. Answers that say the egg decides the sex, or that describe the father's variable contribution without ever stating that eggs are always X, lose the mark.
Earlier children do not shift the odds
After three daughters, candidates argue that the next child is more likely to be a boy because one is due. That is the gambler's fallacy. X-carrying and Y-carrying sperm are produced in equal numbers at every conception, so the chance stays at exactly 1 in 2 — earlier births change nothing about which sperm fertilises the next egg.
Sex determination: every child has a 50:50 chance regardless of previous children (cambridge-0610.md Section 17, s23 Paper 11 Q34).
Give both halves of a definition
Mark schemes want two elements. Gene = a length of DNA (the unit) that codes for a protein (the function). Allele = an alternative form (the relationship) of a gene (what it is a form of). Dropping either half, or defining a gene as "part of a chromosome", scores nothing.
One mark per link in the chain
For "explain" questions, write each link as its own sentence: a gene (a length of DNA) codes for a specific protein; that protein carries out a specific function; that function determines a characteristic or activity of the cell. Cambridge normally awards one mark per link.
Show the gametes in a sex cross
Set the eggs out as all X and the sperm as half X, half Y before you fill in the boxes, then say in words that the father's sperm determines the sex. A cross with no gametes shown, or with whole parental genotypes written into the gamete boxes, loses marks.
Expression answers: say switched on
For (Extended) differentiation questions, run the chain: the same genes are present in every body cell; only the genes needed are expressed; so different proteins are made; so structure and function differ. The word expressed, or switched on/off, is what examiners want.
Chromosome: a structure made of DNA (deoxyribonucleic acid) found in the nucleus, carrying genetic information organised into genes.
Gene: a length of DNA that codes for a protein.
Allele: an alternative form of a gene.
Size-order (largest to smallest): chromosome gene allele. A chromosome contains many genes; each gene may exist as two or more alleles.
Definition trap: do not describe DNA as "made of amino acids" or "a protein" — DNA is a nucleic acid that codes for proteins; it is not itself a protein.
Human chromosome number: a human diploid cell contains 23 pairs of chromosomes (22 pairs of autosomes + 1 pair of sex chromosomes).
Define a gene.
(a) Define a gene.
(b) Define an allele.
(c) State the relative size order of a chromosome, a gene and an allele, from largest to smallest.
