Inquiry Question 1: How does mutation introduce new alleles into a population?
Explain how a range of mutagens operate, including but not limited to: electromagnetic radiation sources, chemicals, naturally occurring mutagens; and classify different types of mutation including point, silent, frameshift and chromosomal mutations
A focused answer to the HSC Biology Module 6 dot point on classifying mutations. Covers point mutations (substitution, insertion, deletion), silent vs missense vs nonsense, frameshift effects on reading frame, chromosomal mutations (deletion, duplication, inversion, translocation, non-disjunction), and the somatic vs germ-line distinction.
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What this dot point is asking
NESA wants you to classify mutations into the standard categories and explain the structural difference between each type. Most exam questions ask you to compare two types, often with a named example.
A second skill the markers test hard is the somatic vs germ-line distinction: whether a mutation is inherited depends on the cell it happens in (gamete vs body cell), not on its molecular type. Several recent HSC questions (2019, 2025) turn entirely on getting that right, so treat it as core, not a footnote.
The answer
A mutation is a permanent, heritable change to the DNA sequence of an organism. Mutations are classified by scale (point vs chromosomal), by effect on the protein product (silent, missense, nonsense, frameshift), and by location (somatic vs germ-line, which decides whether the change is passed on).
Point mutations
A point mutation changes a single base pair in the DNA. There are three structural sub-types.
- Substitution
- One base is replaced by another (e.g. A to G). The reading frame is unchanged; at most one codon is altered.
- Insertion
- An extra base is inserted into the sequence.
- Deletion
- A base is removed from the sequence.
Insertions and deletions of one or two bases shift the reading frame of the ribosome, so all codons downstream are read in the wrong groups of three. This is called a frameshift mutation.
Classifying substitutions by effect
Substitutions are further classified by what they do to the protein.
| Type | Effect on codon | Effect on protein |
|---|---|---|
| Silent | New codon codes for the same amino acid | None (the genetic code is degenerate) |
| Missense | New codon codes for a different amino acid | One amino acid changed |
| Nonsense | New codon is a stop codon (UAA, UAG, UGA) | Truncated, usually non-functional |
Worked example. Sickle cell anaemia is a single substitution (A to T) in the beta-globin gene, changing codon 6 from GAG to GTG. This is a missense mutation: glutamic acid becomes valine. The altered haemoglobin polymerises under low oxygen, deforming red blood cells.
Frameshift mutations
A frameshift is caused by an insertion or deletion of a number of bases not divisible by three. Every codon downstream of the mutation is shifted, so the amino acid sequence past that point is essentially random and a premature stop codon usually appears within a few codons. The resulting protein is truncated and non-functional.
Worked example. Many cystic fibrosis alleles involve deletions in the CFTR gene. The most common, ΔF508, deletes three bases (one codon) and is technically an in-frame deletion, but other CF alleles are true frameshifts that abolish CFTR function entirely.
Chromosomal mutations
A chromosomal mutation changes the structure or number of whole chromosomes. These affect many genes at once.
Structural chromosomal mutations
- Deletion. A segment of the chromosome is lost (e.g. cri-du-chat syndrome, partial deletion of chromosome 5).
- Duplication. A segment is copied so that two copies are present on the same chromosome.
- Inversion. A segment breaks off, flips and rejoins in reverse orientation.
- Translocation. A segment moves from one chromosome to a non-homologous chromosome (e.g. the Philadelphia chromosome in chronic myeloid leukaemia, a translocation between chromosomes 9 and 22).
Numerical chromosomal mutations (aneuploidy)
These arise from non-disjunction during meiosis, where homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate.
- Trisomy 21 (Down syndrome). Three copies of chromosome 21.
- Monosomy X (Turner syndrome). A single X chromosome (XO).
- 47,XXY sex-chromosome aneuploidy (Klinefelter syndrome). Two X and one Y chromosome.
Somatic vs germ-line mutations: who inherits it?
This is the distinction NESA tests most often, and it is decided by where the mutation happens, not by its molecular type. A substitution, a frameshift or a chromosomal mutation can each be either somatic or germ-line.
- A somatic mutation occurs in a body cell (skin, gut, blood-forming cell, etc.). It is copied only to that cell's daughter cells by mitosis, so it forms a patch of altered cells in the individual only. It is not passed to offspring, and it dies with the individual. Somatic mutations can still be serious for that person - most cancers begin as somatic mutations - but they never enter the gene pool.
- A germ-line mutation occurs in a gamete (egg or sperm) or a cell that forms gametes. Because the affected gamete can fertilise, the mutation is passed to offspring and is then present in every cell of that offspring. Germ-line mutations are the ones that introduce new alleles into a population and so matter for evolution.
The population consequence follows directly: only germ-line mutations change allele frequencies, because only they are inherited. A somatic mutation, however dramatic, leaves the next generation untouched. (Plants blur this slightly - a somatic mutation in a shoot can end up in flowers and seeds - but in animals the somatic/germ-line line is sharp, and that is what the HSC tests.)
Examples in context
- Example 1. Sickle cell anaemia, a single missense mutation
- A single A to T substitution in the sixth codon of the HBB gene changes the codon GAG (glutamic acid) to GTG (valine) in beta-haemoglobin. This one amino acid swap replaces a polar acidic residue with a hydrophobic one, causing haemoglobin molecules to polymerise into long fibres under low-oxygen conditions and distorting red blood cells into the characteristic sickle shape. The mutation is missense, not silent, because the amino acid identity changes. Sickle cell carriers (heterozygotes) are largely asymptomatic and gain some resistance to Plasmodium falciparum malaria, a classic example of balancing selection that maintains the mutated allele in African and Mediterranean populations. As a germ-line mutation it is inherited, which is why it persists in those populations.
- Example 2. Down syndrome and non-disjunction in chromosome 21
- Down syndrome (trisomy 21) is a chromosomal mutation, not a point mutation. It arises when a homologous pair of chromosome 21 fails to separate during meiosis I (or sister chromatids fail to separate in meiosis II), producing a gamete with two copies of chromosome 21. After fertilisation, the resulting zygote has 47 chromosomes (2n + 1). Australian Bureau of Statistics data show roughly 1 in 700 live births in Australia have Down syndrome, with maternal-age-related non-disjunction the dominant cause (risk rises from 1 in 1500 at age 20 to 1 in 100 at age 40). The phenotype reflects extra dosage of all the roughly 230 genes on chromosome 21.
- Example 3. A somatic translocation, the Philadelphia chromosome
- Chronic myeloid leukaemia arises when a translocation between chromosomes 9 and 22 occurs in a single haematopoietic (blood-forming) stem cell, fusing the BCR and ABL genes into a BCR-ABL oncogene that drives uncontrolled cell division. Because the mutation is somatic - it happened in a body cell, not a gamete - it is serious for the individual but is not inherited: the patient's children do not carry it. This shows how a single mutation can be devastating to one person yet have no effect on the population's gene pool.
Exam-style practice questions
Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2025 HSC3 marksA CAMT mutation produced the amino acid sequence Glutamine - Tyrosine - Isoleucine - Aspartic acid. The same DNA fragment sequenced from an unaffected individual has the template strand GTC ATA CAG CTG. Using the codon chart, explain the type of mutation which causes CAMT. (CAMT = congenital amegakaryocytic thrombocytopenia.)Show worked answer →
Work from the template strand to the mRNA/amino acids, compare the two sequences, then name the mutation with reference to the data. Sample answer: The unaffected amino acid sequence is Glutamine-Tyrosine-Valine-Aspartic acid. The codon GUC (coding for Valine) is replaced by AUC, which produces Isoleucine. Because only a single nucleotide is changed, CAMT is a point mutation (also accepted: substitution / missense). Marks: 3 = explains the mutation type AND refers to the data (codon change); 2 = outlines the mutation with reference to the data; 1 = some relevant information. A common error was being unable to use the codon chart to identify amino acids from the mRNA sequence.
Source: NESA 2025 HSC Biology examination and marking guidelines.
2022 HSC3 marksExplain the cause of a type of chromosomal mutation. (Birth defects in humans can be caused by chromosomal abnormalities.)Show worked answer →
Identify a chromosomal mutation type, then explain its cause (not just describe it). Sample answer: One type of chromosomal mutation is a numerical abnormality - more or fewer chromosomes than the normal diploid number. This is caused during meiosis by non-disjunction: a pair of homologous chromosomes (or sister chromatids) fails to separate/segregate, so some gametes end up with one chromosome too many and others with one too few. Other accepted chromosomal mutations: deletions, duplications, inversions or translocations. Marks: 3 = identifies a type AND explains its cause; 2 = outlines the cause; 1 = some relevant information. Common errors: treating point mutations as chromosomal, and describing the type rather than explaining the cause.
Source: NESA 2022 HSC Biology examination and marking guidelines.
2025 HSC4 marksA and B are two separate mutations shown on a diagram of germ-line and somatic cells in two related individuals. Analyse how mutations A and B affect the genetic information present in cells U, V, W and X. (Mutation A is a germline mutation; mutation B is a somatic mutation.)Show worked answer →
Classify each mutation by where it occurs, then trace which cells it can reach. Sample answer: Mutation A is a germline mutation - it will not be present in Individual 1 but could be passed to its offspring, including Individual 2. Mutation B is a somatic mutation - it will only be present in some cells of Individual 2 and cannot be passed to offspring. As a result: cell U has neither A nor B; cell V could have A but not B; cell W could have both A and B; cell X could carry A. Marks: 4 = thorough analysis accounting for the genetic differences in the cells; 3 = sound analysis; 2 = some understanding OR sound analysis of one mutation; 1 = some relevant information. The common error was not linking the somatic and germline mutations to their impact on the specific cells U, V, W, X.
Source: NESA 2025 HSC Biology examination and marking guidelines.
2019 HSC3 marksComplete the table to show the differences between somatic and germ-line mutations (rows: Location; Effect on offspring; Example).Show worked answer →
Contrast the two mutation types across all three rows. Sample answer:
| Somatic mutation | Germ-line mutation | |
|---|---|---|
| Location | Body cells (not gametes) | Sex cells / gametes only |
| Effect on offspring | Not passed to offspring | May be passed to offspring |
| Example | Mutation in skin-cell DNA leading to skin cancer | Mutation in a sex cell leading to haemophilia |
Marks: 3 = table correctly completed; 2 = substantially correct table; 1 = some relevant information. The key discriminator is correctly stating that a somatic mutation occurs in body cells and is not inherited, whereas a germ-line mutation occurs in gametes and can be inherited.
Source: NESA 2019 HSC Biology examination and marking guidelines.
Practice questions
Original practice questions graded from foundation to exam level, each with a full worked solution. Try them before revealing the solution.
foundation2 marksDefine a point mutation and name its three structural sub-types.Show worked solution →
1 mark - definition. A point mutation is a change to a single base (or base pair) in the DNA sequence.
1 mark - the three sub-types. Substitution (one base replaced by another), insertion (an extra base added) and deletion (a base removed).
Both the single-base definition and all three named sub-types are needed for full marks. Listing the sub-types without the definition, or vice versa, caps at 1 mark.
foundation2 marksDistinguish between a missense mutation and a nonsense mutation.Show worked solution →
1 mark - missense. A missense substitution changes a codon so that it codes for a different amino acid, altering one amino acid in the protein.
1 mark - nonsense. A nonsense substitution changes a codon into a stop codon (UAA, UAG or UGA), ending translation early and producing a truncated, usually non-functional protein.
The discriminator is "different amino acid" (missense) versus "premature stop / truncated protein" (nonsense). Defining only one caps at 1 mark.
foundation3 marksA mutation deletes a single base near the start of a gene's coding sequence. Explain why this is likely to have a severe effect on the protein produced.Show worked solution →
- 1 mark - reading frame shifted
- Deleting one base (a number not divisible by three) shifts the reading frame, so the ribosome reads every codon downstream in the wrong groups of three.
- 1 mark - downstream sequence altered
- Every amino acid after the deletion is therefore likely to be different from the original (the downstream sequence is effectively scrambled).
- 1 mark - early position / premature stop
- Because the deletion is near the start, almost the whole protein is affected, and a premature stop codon usually appears within a few codons, truncating the protein so it is non-functional.
The three marks reward: identifying the frameshift, the downstream scrambling, and the consequence (truncation/non-functional protein), with credit for noting the early position maximises the damage.
core4 marksCompare a substitution mutation with a frameshift mutation in terms of their cause and their effect on the protein produced. Use an example of each.Show worked solution →
Award up to 4 marks for a genuine comparison (cause AND effect for both) supported by examples.
- Cause (1-2 marks)
- A substitution replaces one base with another, leaving the total number of bases - and the reading frame - unchanged. A frameshift is caused by an insertion or deletion of a number of bases not divisible by three, which changes the number of bases and shifts the reading frame.
- Effect on the protein (1-2 marks)
- A substitution alters at most one codon, so its effect is local: it can be silent (no change), missense (one amino acid changed) or nonsense (premature stop). A frameshift alters every codon downstream of the mutation, so the amino acid sequence past that point is essentially scrambled and a premature stop usually appears, giving a truncated, non-functional protein.
- Examples (built into the above)
- Substitution: sickle cell anaemia (GAG to GTG, glutamic acid to valine). Frameshift: a single-base deletion or insertion in a coding sequence, or a frameshift CFTR allele in cystic fibrosis.
A response that describes only one mutation type, or gives effects without the cause, does not reach full marks. The discriminator is "reading frame unchanged, one codon" (substitution) versus "reading frame shifted, all downstream codons" (frameshift).
core4 marksDistinguish between somatic and germ-line mutations, and explain why only one of them affects the allele frequencies of a population.Show worked solution →
- 1 mark - somatic location/inheritance
- A somatic mutation occurs in a body cell (not a gamete); it is passed only to that cell's daughter cells by mitosis, so it affects the individual only and is not inherited by offspring.
- 1 mark - germ-line location/inheritance
- A germ-line mutation occurs in a gamete (egg or sperm) or a gamete-forming cell, so it can be passed to offspring and is present in every cell of that offspring.
- 1 mark - link to population
- Allele frequencies describe the alleles passed between generations. Only germ-line mutations enter gametes and are inherited, so only they add new alleles to the next generation.
- 1 mark - somatic excluded with reason
- A somatic mutation dies with the individual (it never enters a gamete), so it cannot change the population's allele frequencies - even if it causes a serious effect such as cancer in that person.
Full marks need the location difference, the inheritance difference, and the explicit reason only germ-line mutations are heritable. Treating "somatic" as simply "less serious" misses the mark; the point is heritability.
core5 marksCompare the causes, processes and effects of structural chromosomal mutations (deletion, duplication, inversion and translocation).Show worked solution →
- 1 mark - shared cause/process
- All four are structural chromosomal mutations: a chromosome breaks and the broken segment is lost, copied or rejoined incorrectly (often during meiosis or after DNA damage), affecting many genes at once.
- 1 mark - deletion
- A segment of the chromosome is lost, so the genes it carried are missing (e.g. cri-du-chat syndrome, a partial deletion of chromosome 5). Effect: loss of gene products, usually harmful.
- 1 mark - duplication
- A segment is copied, so two copies are present on the same chromosome. Effect: extra dosage of those genes.
- 1 mark - inversion
- A segment breaks, flips and rejoins in reverse on the same chromosome. Effect: gene order reversed; genes at the breakpoints may be disrupted.
- 1 mark - translocation
- A segment moves to a non-homologous chromosome (e.g. the Philadelphia chromosome, a translocation between chromosomes 9 and 22 in chronic myeloid leukaemia). Effect: genes relocated; a fusion gene/product may form.
The discriminator across the four is what happens to the segment: lost (deletion), copied (duplication), reversed in place (inversion), moved to another chromosome (translocation). Each must be tied to its effect for full marks.
exam6 marksA geneticist studies two single-base mutations in the same gene. Mutation X changes a codon from CTG to CTA (both still code for leucine). Mutation Y deletes one base from the same codon. Compare the likely effects of mutations X and Y on the protein, and justify which mutation is more likely to be passed on to offspring if each occurred in a skin cell rather than a gamete.Show worked solution →
Target a sequenced response: classify each mutation, predict its protein effect, then apply the somatic/germ-line rule.
- Classify and predict (2-3 marks)
- Mutation X is a silent (synonymous) substitution: CTG and CTA both specify leucine, so the genetic code's degeneracy means the protein is unchanged and there is likely no phenotypic effect. Mutation Y is a single-base deletion, a frameshift: removing one base (not divisible by three) shifts the reading frame, so every codon downstream is altered, a premature stop usually appears, and the protein is truncated and non-functional - a far more severe effect than X.
- Compare the severity (1 mark)
- X is effectively harmless at the protein level; Y is likely to abolish protein function. So Y has by far the greater effect on the protein, despite both being changes to a single base.
- Apply the somatic/germ-line rule (2 marks)
- Whether a mutation is inherited depends on the cell type, not the mutation type. If either mutation occurs in a skin (somatic) cell, it is passed only to that cell's daughter cells and dies with the individual - it is not passed to offspring. Only a mutation occurring in a gamete (germ-line) can be inherited. Therefore neither X nor Y is heritable when it occurs in a skin cell; both would only be passed on if they arose in a gamete.
The trap is to assume the "milder" or the "more severe" mutation is the one inherited. Full marks require recognising that heritability is determined by where (somatic vs germ-line) the mutation occurs, independent of its molecular type.
exam7 marksMutations are the ultimate source of new alleles, yet most mutations have no effect on a population. Using examples of different mutation types, assess the claim that the type and location of a mutation determine its significance for an individual and for a population.Show worked solution →
"Assess" requires a supported judgement weighing how mutation type (its molecular effect) and location (somatic vs germ-line) each shape significance, with a clear conclusion.
- Type determines the effect on the individual (2-3 marks)
- Mutation type sets how much the protein changes. A silent substitution (e.g. CTG to CTA, still leucine) changes nothing. A missense substitution can be mild or severe (e.g. sickle cell anaemia, GAG to GTG, one amino acid swap with major consequences). A nonsense or frameshift mutation usually abolishes the protein (truncation). Chromosomal mutations (deletion, duplication, inversion, translocation, or non-disjunction giving trisomy 21) affect many genes at once and are often severe or lethal. So type strongly predicts the effect on the individual.
- Location determines significance for the population (2-3 marks)
- Significance for a population depends on heritability, which is set by location. A somatic mutation - however severe for the individual, such as a translocation in a stem cell causing leukaemia - is not inherited and cannot change allele frequencies; it dies with the individual. Only a germ-line mutation enters a gamete, is passed on, and so introduces a new allele into the population's gene pool, where selection and drift then act.
- Judgement (1-2 marks)
- A supported conclusion: the claim is well founded - type governs the effect on the individual's phenotype, while location governs whether the mutation matters to the population. A severe somatic mutation can be devastating for one person yet evolutionarily irrelevant, whereas even a mild germ-line mutation can persist and spread. Both dimensions are needed to judge a mutation's full significance. An answer that addresses only type, or only location, or omits an explicit judgement, caps below full marks.