IB Biology · Theme D Continuity and change · Molecules
D1.3 Mutation and gene editing
A gene mutation changes a gene's base sequence: a substitution, an insertion or a deletion. Substitutions alter at most one codon; insertions and deletions can shift the whole reading frame. Mutation is random and the source of all new alleles; CRISPR now makes edits on purpose.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
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Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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D1.3.1 Three kinds of gene mutation: substitution, insertion, deletion
A gene mutation is a change in the base sequence of a gene.
A substitution swaps one base for another; the gene stays the same length.
An insertion adds bases and makes the gene longer; a deletion removes bases.
Chromosome mutations are different: they change whole chromosomes, not a sequence within one.
Students often think two sequences differing at many positions must show many substitutions. In fact one deleted base shifts every later base, so the whole tail looks different.
Students often think anything that changes the amount of DNA is a chromosome mutation. In fact a deletion inside a gene is a gene mutation.
D1.3.2 A substitution may change one amino acid, or none
A substitution changes at most one codon; the reading frame is untouched.
Because the code is degenerate, the new codon may specify the same amino acid.
Or it changes one amino acid, or creates a stop codon that shortens the polypeptide.
A SNP is a position where individuals differ by one base, from a past substitution.
Most SNPs lie in non-coding DNA or are silent; they are common variation, not disease.
Students often think every substitution changes the polypeptide. In fact a synonymous substitution gives another codon for the same amino acid and changes nothing.
Students often think a substitution shifts the reading frame. In fact the base count is unchanged, so only one codon is altered.
D1.3.3 Insertions and deletions usually wreck the polypeptide
Inserting or deleting bases not in multiples of three causes a frameshift.
Every codon after the mutation is read wrongly, and a premature stop is likely.
Codons before the mutation are read exactly as before.
A major insertion or deletion adds or removes many amino acids; folding usually fails.
Students often think deleting one base changes only one amino acid. In fact every amino acid after that point is likely to change.
Students often think every insertion causes a frameshift. In fact inserting a multiple of three bases adds whole codons and keeps the frame.
D1.3.4 Mutagens, and copying errors, cause mutation
A mutagen is a chemical or physical agent that raises the mutation rate.
Chemical mutagens include benzo[a]pyrene in tobacco smoke, nitrosamines, mustard gas and nitrous acid.
Mutagenic radiation is ionising radiation (X-rays, gamma, alpha, beta) and ultraviolet light.
Mutations also arise with no mutagen, from errors in DNA replication or repair.
Students often think DNA copies perfectly unless a mutagen acts. In fact replication and repair errors cause mutations on their own.
Students often think all radiation is mutagenic. In fact radio waves, microwaves and visible light lack the energy to alter DNA.
D1.3.5 Mutation is random, but not evenly spread
Mutations can occur anywhere in a genome, regardless of whether the change would help.
No natural mechanism is known that changes a chosen base to alter a trait.
Some bases mutate more often: a mutation hotspot such as methylated cytosine changing to thymine.
Students often think organisms mutate the genes they need when conditions change. In fact mutation is random with respect to usefulness.
Students often think random means every base has an equal chance. In fact some bases, such as methylated cytosines, mutate far more often.
D1.3.6 Germ cell mutations are inherited; somatic mutations stay in the body
A germ cell mutation is in a gamete or a cell that forms gametes.
It can pass to offspring and then sits in every one of their cells.
A somatic mutation is copied only into that cell's descendants by mitosis.
Somatic mutations in genes controlling the cell cycle can lead to cancer.
Students often think any mutation in the body can be passed on. In fact only germ cell mutations reach the next generation.
Students often think any somatic mutation makes a cancer cell. In fact only mutations in the genes controlling cell division do.
D1.3.7 Mutation is where all new alleles come from
New alleles arise only by gene mutation; meiosis and fertilisation only reshuffle existing ones.
Most mutations are harmful or neutral for the individual.
Over the long term, mutation supplies the variation that natural selection acts on.
Commercial genetic tests read SNPs and report risk associations, not diagnoses.
Without expert interpretation, a risk association is easily misread as certainty; other genes, environment and lifestyle also matter.
Students often think most mutations are beneficial because mutation drives evolution. In fact only a small minority help, but those matter over time.
Students often think a disease-linked variant means the disease is certain. In fact it changes the probability; the outcome also depends on environment and chance.
D1.3.8 Knock a gene out to see what it does HL
A gene knockout changes a gene so it makes no functional product.
Comparing the knockout's phenotype with a normal organism shows what the gene does.
Knockout libraries exist for model species such as mouse, yeast and Arabidopsis.
The gene may stay in the chromosome; it is inoperative, not removed.
Students often think a gene must be cut out to be knocked out. In fact it need only be made inoperative; the DNA can remain.
Students often think no phenotype change means no function. In fact another gene may compensate, or the function may show only in other conditions.
D1.3.9 CRISPR–Cas9 cuts DNA where a guide RNA leads it HL
A guide RNA of about 20 bases, complementary to the target, leads Cas9 there.
Cas9 cuts both DNA strands there; the guide RNA itself cuts nothing.
The cell's own repair reseals the break, disrupting the gene or copying a supplied template.
One success: editing a patient's blood stem cells to make fetal haemoglobin in sickle cell disease.
Countries regulate genome editing differently, so there is an international effort to harmonise the rules; some uses raise ethical issues that must be settled first.
Students often think Cas9 recognises the target sequence itself. In fact the guide RNA's base pairing decides where Cas9 cuts.
Students often think a safe, working edit is automatically ethical. In fact consent, effects on future generations and fair access remain issues.
D1.3.10 Two hypotheses for why some sequences barely change HL
A conserved sequence is similar across a species or group of species.
A highly conserved sequence stays similar over long evolutionary time: histone and rRNA genes.
Hypothesis one, functional requirement: mutations occur normally but selection removes almost every change.
Hypothesis two, slower mutation rate: fewer mutations arise there in the first place.
The two can be told apart by measuring how often new mutations actually appear in the sequence.
Students often think conservation means mutations do not happen there. In fact they may happen at the normal rate and be removed by selection.
Students often think a sequence stays the same because it is unused. In fact the most conserved sequences have the most demanding functions.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 Part of the base sequence of a gene is GAT TCA GGC ATT before a mutation and GAT TCG GCA TT after it. Which type of gene mutation has occurred?
Answer and reasoning
A deletion of one base, which has moved every later base one position along — Aligning the two sequences shows that the A in the second codon is missing and every base after it has moved up one place; the mutant is one base shorter. This is a deletion.
A substitution of the base A by G at the sixth position in the sequence — A student who compares the sequences position by position sees A become G and calls it a substitution. A substitution leaves the length unchanged, but the mutant sequence has 11 bases instead of 12: one base has been deleted and the rest have shifted.
A chromosome mutation, because the total amount of DNA has changed — A student who sorts mutations by whether DNA is gained or lost picks this. A change of one base within a gene is a gene mutation (a deletion); chromosome mutations alter the number or structure of whole chromosomes.
A substitution that has shifted the reading frame of later codons — A student who attaches frameshift to every mutation picks this. The frame has shifted, but only because a base was removed; a substitution cannot shift the frame because it does not change the number of bases.
2 In mRNA the codons GCU, GCC, GCA and GCG all code for alanine. A base substitution in a gene changes the codon GCU in its mRNA to GCC. What is the effect on the polypeptide?
Answer and reasoning
Alanine is replaced by a different amino acid at that one position — A student who treats codons and amino acids as one-to-one picks this. Alanine has four codons, and GCU to GCC is a change between two of them, so the amino acid is unchanged.
None: the same amino acid, alanine, is placed at that position — GCC codes for alanine just as GCU does, so the substitution is silent. This is a consequence of the degeneracy of the genetic code: a base substitution may or may not change a single amino acid.
The reading frame shifts, changing all amino acids after it — A student who thinks any mutation shifts the frame picks this. A substitution replaces one base with another and leaves the number of bases the same, so the frame is not shifted.
Every amino acid in the polypeptide is changed to another one — A student who both attaches a frameshift to every mutation (as in m04) and pictures a frameshift as scrambling the whole polypeptide picks this. Even a frameshift leaves earlier codons untouched, and a substitution affects at most the single codon it lies in.
3 Why is a polypeptide likely to cease functioning when several hundred bases are deleted from the middle of its gene?
Answer and reasoning
Only because a deletion of any size shifts the reading frame of every later codon — A student who thinks every deletion is a frameshift picks this. If the number of bases deleted is a multiple of three the frame is unchanged, yet the polypeptide still loses many amino acids and is likely to cease to function for that reason.
So many amino acids are missing that the polypeptide cannot fold into its working shape — A major deletion removes a large section of the coding sequence, so a long run of amino acids is absent from the polypeptide. Even if the reading frame happens to be preserved, a polypeptide missing so much of its sequence is unlikely to fold into a functional shape.
Only the amino acids coded by the deleted bases are missing; the rest still folds as normal — A student who assumes the effect of a deletion is confined to the mutation point, as for a single-base change, picks this. Several hundred bases code for a hundred or more amino acids (three bases per amino acid), and losing that much of the sequence prevents the polypeptide from folding into its functional shape, even if the reading frame is preserved.
The amino acids coded before the deletion are also changed by the loss of bases — A student who thinks any change spreads back along the polypeptide picks this. Codons before the deletion are read exactly as before; the damage lies in the large section that is missing and, if the frame is shifted, in the sequence after it.
4 Which statement about the causes of gene mutation is correct?
Answer and reasoning
Gene mutations arise only in cells that are exposed to a mutagen — A student who knows mutagens as the cause of mutation picks this. Mutagens raise the rate, but replication and repair errors produce a background rate of mutation in cells that have met no mutagen at all.
Microwaves and radio waves are mutagenic just like X-rays — A student who treats all radiation as dangerous to DNA picks this. Only ionizing radiation, such as X-rays and gamma rays, and ultraviolet light have enough energy to alter DNA; microwaves and radio waves do not.
Replication errors can cause mutation without a mutagen — Gene mutation is caused by mutagens and by errors in DNA replication or repair. DNA polymerase occasionally pairs a base wrongly, and a small fraction of these errors escape proofreading and repair and become permanent mutations.
Proofreading by DNA polymerase corrects every error — A student who turns a very low error rate into a zero error rate picks this. Proofreading and repair correct most errors, but not all, which is why replication errors are a genuine cause of mutation.
5 Which statement about randomness in mutation is correct?
Answer and reasoning
Mutations can occur anywhere in the genome, but some bases have a higher probability of mutating than others — Mutation is random in the sense that no natural mechanism directs a change to a particular base for the purpose of changing a trait; it is not uniform, because the chemistry of some bases and sequences (for example methylated cytosine) makes them mutation hotspots.
Mutations can occur anywhere in the genome, and every base has an identical probability of mutating — A student who reads random as equally likely picks this. Randomness refers to the lack of direction towards a useful outcome; some bases mutate much more often than others.
Mutations occur in the genes an organism needs to change when its environment changes — A student who thinks organisms mutate in response to need picks this. No natural mechanism is known for making a deliberate change to a particular base with the purpose of changing a trait.
Mutations occur only at those bases in the genome that a mutagen has chemically damaged — A student who ties every mutation to a mutagen picks this. Errors in replication or repair cause mutations at bases that no mutagen has touched, anywhere in the genome.
6 Which statement about mutations in germ cells and somatic cells is correct?
Answer and reasoning
A mutation in a skin cell can be inherited by the person's children — A student who treats all of a person's DNA as heritable picks this. Skin cells are somatic cells: their DNA is never passed to a gamete, so the mutation dies with the individual.
Every mutation in a somatic cell causes that cell to become cancerous — A student who equates somatic mutation with cancer picks this. Cancer follows mutations in the genes that control the cell cycle; most somatic mutations are silent or harmless.
A mutation in a germ cell can be passed on to the offspring — Germ cells give rise to gametes, so a mutated gene in a germ cell can be passed to the next generation, where it will be present in every cell of the offspring. This is how mutation adds inherited variation to a species.
A mutation in one liver cell spreads to every body cell — A student who generalises from inherited mutations picks this. A somatic mutation is copied only into the descendants of the cell in which it occurred, not into other cells of the body.
7 Why is gene mutation described as the original source of all genetic variation?
Answer and reasoning
New alleles arise only by mutation; meiosis and fertilization just recombine alleles that already exist — Every allele of every gene began as a mutation in a germ cell of some ancestor. Meiosis and fertilization produce new combinations of alleles, which adds variation among individuals, but they cannot produce a new allele.
Crossing over in meiosis creates new alleles, and mutation adds to the variation they produce — A student who thinks crossing over makes new genes picks this. Crossing over exchanges existing alleles between chromatids; it produces new combinations, not new alleles.
Most mutations are beneficial, so each one adds a useful new variant to the species — A student who thinks mutation is mostly helpful picks this. Most mutations are harmful or neutral for the individual; mutation is the source of variation regardless of whether a particular mutation is useful.
Organisms generate new alleles whenever their environment demands a new trait from them — A student who believes mutation is directed picks this. Mutations occur at random with respect to their usefulness; the environment selects among variants that already exist.
8 Researchers want to investigate the function of gene Z in mice. They obtain a strain from a knockout library in which gene Z is inoperative. How should they use it? HL
Answer and reasoning
Conclude that Z has no function if the knockout mice appear normal — A student who reads a null result at face value picks this. An unchanged phenotype may mean another gene compensates or that the function is needed only under conditions not tested, so it does not show that Z is functionless.
Check first that the whole of gene Z has been cut out of the chromosome — A student who thinks knockout means physical removal picks this. A gene is knocked out when it is changed so that it is inoperative; its DNA may still be present.
Compare the phenotype of the knockout mice with that of normal mice — Gene knockout investigates function by making the gene inoperative and observing what changes. Any difference between the knockout and normal mice, in structure, physiology or behaviour, is evidence about what the product of gene Z normally does.
Block the protein made by gene Z in adult mice and observe the effect — A student who confuses inhibiting a protein with knocking out a gene picks this. The knockout strain makes no functional product of Z in any cell, so there is no protein to block; the comparison is between organisms.
9 In CRISPR–Cas9 gene editing, what determines the position in the genome at which the DNA is cut? HL
Answer and reasoning
The base sequence of the guide RNA, which pairs with the complementary sequence in the target DNA — The guide RNA contains about 20 bases complementary to the target. Base pairing between the guide RNA and the DNA holds Cas9 at that one position, where Cas9 cuts both strands. Changing the guide RNA sequence changes the site that is cut.
The active site of the Cas9 enzyme, which recognises the target DNA base sequence directly — A student who expects specificity to come from the enzyme picks this. Cas9 cuts wherever it is taken; the target is chosen by the guide RNA, not by the enzyme's active site.
The guide RNA, which cuts both strands of the DNA at the position where it binds to it — A student who gives the guide RNA the active role picks this. The guide RNA does locate the target, but it cannot cut DNA; the enzyme Cas9 makes the cut.
The sequence of the new bases that Cas9 itself inserts into the gene to rewrite it — A student who thinks Cas9 writes the new sequence picks this. Cas9 only cuts; any new sequence is copied in by the cell's repair processes, and the cut site is set by the guide RNA.
10 The gene for histone H4 codes for an almost identical amino acid sequence in peas and in cattle, whose lineages diverged over a billion years ago. Which pair of hypotheses could account for this? HL
Answer and reasoning
The DNA of the gene is protected so that no mutation can occur in it in either organism — A student who hears conserved as protected picks this. Mutations can occur anywhere in a genome; a highly conserved sequence is one in which changes either arise more slowly or are removed by selection, not one that cannot mutate.
The gene is not used in either organism, so nothing has changed it since they diverged — A student who reasons that unused things stay intact picks this. Sequences that are not used are free to accumulate mutations and diverge quickly; histone H4 is essential in every eukaryotic cell, which is why change is not tolerated.
Each organism deliberately restores any base that changes, because it needs the histone protein — A student who believes organisms direct changes to their DNA picks this. No natural mechanism is known for making a deliberate change to a particular base for the purpose of changing or preserving a trait.
Selection removes mutations that impair the protein, or the gene mutates more slowly — These are the two hypotheses for conserved sequences. Under the functional-requirement hypothesis mutations arise but almost any change to histone H4 is so damaging that carriers are eliminated by natural selection; under the slower-mutation-rate hypothesis fewer mutations arise in the sequence in the first place.
Read the ones marked not yet in Learn, then Verify.
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11 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 The same gene was sequenced in 1000 people. At one position 62% carry C and 38% carry T, and the two resulting codons code for the same amino acid. How should this difference be described?
Answer and reasoning
A rare mutation causing disease in the 38% group — A student who equates SNP with genetic disease picks this. A variant carried by 38% of people is common, not rare, and both versions code for the same amino acid, so there is no basis for calling it disease-causing.
A frameshift mutation that affects later codons — A student who thinks a substitution shifts the reading frame picks this. One base has been replaced by another, not added or removed, so every codon stays in place.
An SNP that leaves the polypeptide unchanged — A single base differs between individuals at one position: this is a single-nucleotide polymorphism, the result of a base substitution that has spread in the population. Because both codons specify the same amino acid, the polypeptide is unchanged.
A substitution that changes one amino acid — A student who assumes every codon change alters the amino acid picks this. The stem states that both codons code for the same amino acid, which is possible because of the degeneracy of the genetic code.
2 One base is deleted from the fourth codon of a gene coding for a polypeptide of 300 amino acids. What is the most likely consequence?
Answer and reasoning
Only the fourth amino acid is altered, so the polypeptide still works normally — A student who applies the substitution rule of one base, one amino acid picks this. A deletion moves every later base one place, so all the codons after the fourth are regrouped: this is a frameshift.
Every one of the 300 amino acids is changed, including the first three — A student who thinks a frameshift scrambles the whole polypeptide picks this. The ribosome reads the first three codons correctly before it reaches the mutation; only the sequence from the fourth codon onwards is altered.
The change is a chromosome mutation, so the gene is not transcribed at all — A student who classes any loss of DNA as a chromosome mutation picks this. The loss of a single base is a gene mutation; the gene is still transcribed and translated, but in a shifted reading frame.
Almost all of the polypeptide is altered and it ceases to function — Deleting one base shifts the reading frame from the fourth codon onwards, so the remaining 297 codons are regrouped and read as different amino acids until a premature stop codon is reached. A polypeptide altered this extensively is very likely to cease functioning.
3 Two copies of a gene are mutated at the same point in the coding sequence. In copy X three consecutive bases are inserted; in copy Y one base is inserted. Which comparison is correct?
Answer and reasoning
X keeps the reading frame so the codons after it are unchanged, but Y shifts the frame for every later codon — Three bases is a whole codon, so in X the codons after the insertion are grouped exactly as before: the polypeptide gains one amino acid (possibly with one adjacent amino acid altered) and the rest is unchanged. One extra base in Y is not a multiple of three, so every later codon is regrouped: a frameshift that is likely to abolish function.
Both X and Y shift the reading frame, so both of the polypeptides are likely to cease to function — A student who thinks every insertion causes a frameshift picks this. The frame shifts only when the number of bases inserted is not a multiple of three, so X, with three bases inserted, keeps the frame.
Both X and Y alter just one amino acid at the insertion point, leaving the rest unchanged — A student who assumes one base change means one amino acid change picks this. It is roughly true for X, but the single extra base in Y regroups every codon after it, changing the whole of the rest of the polypeptide.
Y changes every amino acid, including those before the insertion, whereas X adds just one amino acid — A student who thinks a frameshift alters the amino acids before the mutation as well picks this. The X clause is correct, but in Y the ribosome reads the codons before the insertion exactly as before; only the codons after it are regrouped.
4 Cultures of cells were exposed separately to (i) ultraviolet light, (ii) visible light and (iii) nitrous acid, a chemical that alters bases in DNA. Which treatments are expected to raise the mutation rate above the background rate?
Answer and reasoning
(i), (ii) and (iii), because light and chemicals both damage DNA — A student who assumes all radiation is mutagenic includes visible light. Visible light does not carry enough energy to alter DNA chemically, so it is not a mutagen; only the ultraviolet light and the chemical raise the mutation rate.
(iii) only, because radiation from non-radioactive sources is harmless — A student who thinks only nuclear radiation is mutagenic picks this. Ultraviolet light is a mutagenic form of radiation whatever its source; sunlight is the main cause of the mutations that lead to skin cancer.
None of them, because DNA repair enzymes correct all of the damage — A student who believes repair is perfect picks this. Repair enzymes correct most damage, but mutagens increase the damage so much that more errors escape, and the mutation rate rises measurably.
(i) and (iii), because both chemically alter the bases in DNA — Ultraviolet light is a mutagenic form of radiation (it causes adjacent thymine bases to bond together) and nitrous acid is a chemical mutagen that alters bases so that they mispair. Visible light does not have enough energy to change DNA.
5 A bacterial culture that has never been exposed to the antibiotic streptomycin is found to contain about one resistant cell per 10⁷ cells. When streptomycin is added, the resistant cells survive and the culture soon consists almost entirely of resistant cells. What do these observations show?
Answer and reasoning
The antibiotic caused the bacteria to mutate the gene that gives resistance because they needed it in order to survive — A student who believes mutation is directed by need picks this. The resistant cells existed before the antibiotic was added, so the antibiotic cannot have caused the mutation; it only selected the cells that already carried it.
The resistance mutation arose by chance before exposure, and the antibiotic then selected the cells carrying it — Resistant cells were already present at a low frequency in a culture that had never met the antibiotic, so the mutation must have occurred at random beforehand. Adding streptomycin did not create resistance; it killed the sensitive cells and left the resistant ones to multiply.
Most mutations in bacteria are beneficial, which is why the resistance spread through the culture so quickly — A student who thinks mutation is mostly helpful picks this. Resistance was present in only one cell in ten million, consistent with a rare beneficial mutation among many harmful or neutral ones; it spread because selection removed the sensitive cells.
The resistance allele was created by recombination during reproduction rather than by mutation — A student who credits reproduction with making new alleles picks this. Bacteria in the culture reproduce asexually, and in any organism recombination only reshuffles existing alleles: a new allele can arise only by mutation.
6 A mutation occurs in a gene that controls the cell cycle in one cell lining a man's colon. Which statement about the consequences is correct?
Answer and reasoning
The mutation will be passed on to all of the man's children — A student who thinks any mutation is heritable picks this. A colon cell is a somatic cell, so its mutation cannot enter a sperm cell and is not inherited.
The mutation will spread to every cell in the man's body — A student who expects a mutation to spread through the body picks this. The mutation is copied only into the descendants of the one mutated cell when it divides.
The cell has become cancerous, as does every mutated somatic cell — A student who equates somatic mutation with cancer picks this. Most somatic mutations do not cause cancer, and even a mutation in a cell-cycle gene raises the risk rather than guaranteeing a tumour.
The cell may divide uncontrollably and form a tumour — Genes controlling the cell cycle keep cell division under control. A mutation in such a gene in a somatic cell can allow the cell and its descendants to divide without restraint, which is how cancer begins; several such mutations are usually needed before a tumour forms.
7 A direct-to-consumer genetic test reports that a healthy woman carries an SNP associated with a risk of type 2 diabetes 1.5 times higher than that of people without it. Which interpretation is correct?
Answer and reasoning
She will certainly develop type 2 diabetes in later life, because she carries the genetic variant that causes the disease — A student who reads a risk as a prediction picks this. A 1.5-fold risk means the disease is somewhat more likely, not certain, and many carriers never develop it.
She is more likely than non-carriers to develop the disease, but may never do so; expert interpretation is needed — The result describes an altered probability, not an outcome. Whether she develops type 2 diabetes also depends on other genes, diet, activity and chance. This is the NOS point: without expert interpretation, information from commercial genetic tests can be problematic.
The test has diagnosed type 2 diabetes in her, so she should begin treatment for the disease immediately — A student who treats a genetic test like a blood test picks this. The test read her DNA variants; it did not measure her blood glucose or detect any present disease, and she is healthy.
The SNP is a rare disease-causing error in her DNA, so she has inherited a genetic disease from one parent — A student who equates SNP with a defective gene picks this. SNPs are common variants; this one shifts the probability of a complex disease and is not a disease-causing mutation.
8 Mice in which gene P has been knocked out show no detectable difference from normal mice under standard laboratory conditions. What is the best conclusion? HL
Answer and reasoning
P has no function, because removing its product produced no change in the mice's phenotype — A student who takes a null result as proof picks this. The knockout can only reveal functions that produce a difference under the conditions examined; redundancy and untested conditions can hide a real function.
The knockout did not work, because the sequence of P is still present in the genome — A student who thinks a knocked-out gene must be absent from the DNA picks this. A gene is knocked out when it is made inoperative; its sequence can remain in the chromosome.
The protein made by P is still active because only the gene, not the protein, was altered — A student who thinks knockout acts on the protein picks this. Making the gene inoperative means no functional protein is produced, so there is no active protein from P in the knockout mice.
P may have a function masked by another gene or shown only in untested conditions — Absence of a visible change does not show absence of function. Another gene with a similar product may compensate for the loss, the function may matter only under stress, infection or at other life stages, or the effect may be too subtle for the tests used.
9 Which of the following is an example of the successful use of CRISPR–Cas9 gene editing? HL
Answer and reasoning
Editing the skin cells of adults with sickle cell disease so that all their own children inherit the corrected gene — A student who thinks somatic edits are inherited picks this. Edits to skin cells are never passed to gametes, so this could not work; the real treatment edits the patient's own blood-forming cells for the patient's own benefit.
Editing the blood-forming stem cells of patients with sickle cell disease so that they produce fetal haemoglobin — In this treatment, approved in 2023, a patient's own blood-forming stem cells are edited with CRISPR–Cas9 so that they switch on production of fetal haemoglobin, which prevents red blood cells from sickling. The edited cells are returned to the patient.
Using Cas9 itself both to cut out the faulty bases and to write the corrected sequence into the gene — A student who thinks Cas9 both cuts and rewrites the gene picks this. Cas9 only makes a double-strand break; the change to the sequence is produced by the cell's own repair processes, with or without a supplied template, so this is not how any CRISPR treatment works.
Using guide RNA on its own to cut out the faulty base from the gene in a patient's blood cells — A student who gives the guide RNA the cutting role picks this. The guide RNA only locates the target by base pairing; without Cas9 no cut is made and no edit occurs.
10 In 2018 a scientist announced the birth of babies whose genomes had been edited with CRISPR–Cas9 at the embryo stage. The work was condemned by scientists worldwide and the scientist was later imprisoned under his own country's laws. Why is there an international effort to harmonize the regulation of genome editing? HL
Answer and reasoning
A single worldwide law on genome editing exists and only needs to be enforced — A student who assumes a global authority already governs this picks this. No such law exists; the scientist was punished under national law, and it is precisely because rules differ between countries that harmonization is being sought.
Once embryo editing is shown to be safe there will be no ethical issue left to regulate — A student who equates ethical with safe picks this. Consent of future generations, fairness of access and the acceptability of altering human embryos are ethical issues that remain even if the procedure is safe.
Regulation is national, so a use banned in one country may be legal in another — Regulation is national, so what is prohibited in one country may be legal or unregulated in another and research could move to the least restrictive jurisdiction. Harmonizing regulation aims to ensure that the ethical issues raised by uses such as heritable embryo editing are addressed before implementation anywhere.
Editing an embryo is the same as editing adult somatic cells, so no new rules are needed — A student who thinks somatic edits are inherited sees no difference between the two. Edits to an embryo enter every cell including the germ line and are passed to descendants; edits to an adult's somatic cells are not, which is why embryo editing raises distinct ethical issues.
11 The rate at which new mutations appear per generation was measured directly in a highly conserved gene and in a gene that is not conserved. The rates were the same, but far fewer of the new mutations in the conserved gene persisted in later generations. Which hypothesis do the data support? HL
Answer and reasoning
Functional requirement: mutations arise at the normal rate but carriers are removed by natural selection — Equal rates of new mutation rule out the slower-mutation-rate hypothesis for this gene. Mutations arise as often as elsewhere but do not persist, which is what the functional-requirement hypothesis predicts: almost any change to the gene product is harmful, so carriers are eliminated and the original sequence is preserved.
Slower mutation rate: the conserved gene is protected from mutation, so fewer changes arise in it — A student who assumes conservation means mutations cannot occur picks this. The measured rate of new mutations was the same in both genes, so the data contradict a slower mutation rate here.
Lack of use: the conserved gene is inactive, so mutations in it have no effect and persist — A student who thinks conserved sequences are unused picks this. The data show the opposite: mutations in the conserved gene did not persist, which indicates strong selection against change, not indifference to it.
Beneficial mutation: most new mutations in the conserved gene improve it and so spread rapidly — A student who thinks most mutations are beneficial picks this. If the mutations were beneficial they would persist and spread; instead they disappeared, showing that they were harmful and selected against.
That was your twenty minutes. Real practice on D1.3 is past-paper questions marked against the mark scheme.
What the exam asks of D1.3
Paper 1A gives two sequences and asks you to classify the mutation, or asks which agents are mutagenic. Paper 1B gives a codon table and a mutated sequence; work through the codons and state the effect on the polypeptide. Paper 2 uses *distinguish* and *explain*: name the mutation type, then trace its effect through the reading frame to the protein. HL questions use *outline* for knockouts and CRISPR and *discuss* for conserved sequences; give both hypotheses and how they differ.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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