ESAT Biology: Genetics, DNA and inheritance, set 3

• Answer all questions. No calculator.
• Each question has exactly one correct answer.
• This is a demanding-difficulty set with longer, multi-step questions; aim for a little over 100 seconds on each question.

1. 11 mark

   A student claims: 'Every cell that contains DNA must store that DNA inside a nucleus.' A bacterium (a prokaryotic cell) is known to contain DNA but has no nucleus. Which statement correctly evaluates the student's claim in light of this fact?

   1. A The claim is correct: bacteria do have a nucleus, but one so small that standard light microscopy cannot resolve it as a distinct structure.
   2. B The claim is correct: only eukaryotic cells contain true DNA, since a bacterium's chromosome is built from RNA rather than DNA.
   3. C The claim is false: the nucleus stores DNA only in eukaryotic cells, and this bacterium keeps its DNA free in the cytoplasm instead.
   4. D The claim is false, but for the wrong reason: bacteria contain no genetic material at all, so there is no DNA for a nucleus to store.
2. 21 mark

   A gene controlling coat colour in a species of rabbit is located at a particular position on one of its chromosomes. This gene has two versions: one that produces black fur and one that produces brown fur. Which statement correctly distinguishes the 'gene' from an 'allele' of that gene, using the terms as defined in the specification?

   1. A The gene is the section of DNA at that position that codes for coat colour; each specific version, black-producing or brown-producing, is a separate allele of it.
   2. B A gene and an allele name the exact same thing, so a chromosome carrying this coat-colour gene can only ever be described as carrying one single allele, never two different versions of it.
   3. C The allele is the entire chromosome carrying the coat-colour gene, while the gene itself is just one single base among the many thousands that make up that same chromosome.
   4. D The gene for coat colour is inherited only from the mother's side of the family, while any allele for coat colour is inherited only through the father's side.
3. 31 mark

   A recessive condition is caused by allele a; the dominant allele A does not cause the condition. Two unaffected parents, both known to be carriers (Aa), already have one child with the condition. What is the probability that their next TWO children will both be unaffected?

   1. A 3/4
   2. B 1/16
   3. C 3/8
   4. D 9/16
4. 41 mark

   Two unaffected parents are both known carriers (Aa) of a recessive condition. Considering only their offspring who are NOT affected by the condition, what is the probability that one of these unaffected offspring is a carrier (Aa) rather than homozygous dominant (AA)?

   1. A 1/2
   2. B 2/3
   3. C 1/4
   4. D 3/4
5. 51 mark

   A living organism develops from a single fertilised egg cell into a multicellular adult through repeated mitotic cell division. Different cells in the adult body (for example, muscle cells and skin cells) look and function very differently from one another. Which statement about the genome of these different body cells is correct?

   1. A Each cell type must contain a physically different genome from the others: a muscle cell's DNA is assumed to carry a muscle-specific set of genes entirely absent from a skin cell's DNA.
   2. B Only the cells of the very early embryo share the fertilised egg's original genome; every mitotic division afterwards is assumed to discard whichever genetic material the resulting cell type does not need.
   3. C Every body cell contains the same genome as the original fertilised egg; different cell types arise because different genes in that shared genome are switched on or off.
   4. D The genome stays fixed only in reproductive cells, sperm and egg; the genome inside ordinary body cells is assumed to keep changing throughout the organism's life as new tissues form.
6. 61 mark

   In a molecule of double-stranded DNA, thymine makes up 22 per cent of all the bases. This DNA molecule contains 5000 bases in total. How many of these bases are guanine?

   1. A 2800
   2. B 1100
   3. C 2200
   4. D 1400
7. 71 mark

   A single-stranded DNA (ssDNA) virus has a genome consisting of one strand of 3200 nucleotides. A researcher synthesises the complementary strand to convert this into double-stranded DNA (dsDNA), with no nucleotides added or removed from the original strand. How many nucleotides does the resulting double-stranded molecule contain in total?

   1. A 6400
   2. B 3200
   3. C 1600
   4. D 4800
8. 81 mark

   A gene codes for a polypeptide that is 148 amino acids long. In addition to the triplets coding for these amino acids, the gene also contains one stop codon (a triplet of three bases that signals the end of the polypeptide but does not itself code for an amino acid). How many bases make up the complete coding sequence of this gene, including the stop codon?

   1. A 444
   2. B 445
   3. C 447
   4. D 441
9. 91 mark

   A mutation changes a single base within a gene. Because more than one DNA triplet can code for the same amino acid, this particular base change does not alter which amino acid is coded for at that position in the polypeptide. Based on this information, which outcome for the organism's phenotype is most likely?

   1. A The phenotype will definitely change: any mutation occurring anywhere within a gene is assumed to always alter the amino acid sequence and therefore always determines the phenotype.
   2. B The phenotype is unlikely to change here, since the polypeptide's full amino acid sequence, and so the protein's resulting shape and function, remain unaffected.
   3. C The phenotype will only change if this particular mutation happens to occur in a reproductive cell that is later passed on, rather than in an ordinary body cell during the organism's lifetime.
   4. D The phenotype cannot possibly be affected here, because a mutation is assumed to only ever occur in DNA lying outside every gene, never within one.
10. 101 mark

    Below are four events in a genetic engineering procedure used to make bacteria produce a human protein, listed in a scrambled order: (i) DNA ligase joins the extracted human gene into the opened plasmid, forming a recombinant plasmid. (ii) The recombinant plasmid is taken up by a bacterium. (iii) Restriction enzymes cut the human gene out of human DNA and cut open a bacterial plasmid, both at the same specific base sequence. (iv) The bacterium reproduces repeatedly, and its descendants use the inserted gene to produce the human protein. Which order (from first to last) correctly reflects how this procedure actually proceeds?

    1. A iii, i, ii, iv
    2. B i, iii, ii, iv
    3. C iii, ii, i, iv
    4. D iv, iii, i, ii
11. 111 mark

    A crop plant is genetically engineered to produce its own pesticide, reducing the need for chemical pesticide spraying. Which of the following identifies a genuine, recognised risk of this application of genetic engineering, rather than a benefit?

    1. A The genetically engineered crop will always produce a higher yield than any non-engineered crop of the same species, regardless of growing conditions.
    2. B Because the crop makes its own pesticide, farmers growing it will never need to use any chemical inputs of any kind, including fertiliser.
    3. C Reducing chemical pesticide spraying removes all risk to insect populations in the area surrounding the crop, since the plant's own pesticide cannot affect insects at all.
    4. D The inserted gene could potentially spread into wild, related plant species through cross-pollination, with consequences that are difficult to predict or reverse.
12. 121 mark

    Bone marrow contains adult stem cells capable of differentiating into the different types of blood cell (red blood cells, white blood cells and platelets), but not into unrelated cell types such as nerve or muscle cells. A patient with a blood disorder receives a bone marrow transplant from a matched donor. Which statement correctly classifies these bone marrow stem cells and explains a genuine limitation of using them for this treatment?

    1. A These are totipotent stem cells, exactly like those of the very early embryo; because totipotent cells can develop into an entirely new organism, this transplant risks a new individual growing inside the patient.
    2. B These are multipotent adult stem cells; since they can only differentiate into a limited, related range of cell types, they could not be used to treat a condition needing a completely unrelated cell type, such as damaged nerve tissue.
    3. C These are pluripotent stem cells, identical in potential to embryonic stem cells taken from a very early embryo, so bone marrow could equally well be used to treat any condition needing any cell type at all.
    4. D These stem cells have no limitations of any kind, since any adult stem cell, once isolated from the body, can be redirected by doctors in the laboratory to differentiate into whatever cell type a treatment requires.
13. 131 mark

    Over many generations, breeders of a particular pedigree dog breed have only ever bred from dogs within that same breed, deliberately selecting for a small set of desired physical traits each generation. As a result, this breed now shows an unusually high rate of certain inherited health conditions compared with mixed-breed dogs. Which of the following best explains this outcome in terms of selective breeding's effect on genetic variation?

    1. A Repeatedly breeding from a narrow set of individuals has reduced the range of alleles in the population, raising the chance that harmful recessive alleles pair up in offspring.
    2. B Selective breeding has increased the breed's genetic variation over time, since each new litter of puppies inherits a fresh combination of alleles through the shuffling that occurs at reproduction, which keeps refreshing the breed's overall gene pool with each generation.
    3. C The increased rate of inherited conditions must instead be caused by a brand new mutation that arose independently within this breed, entirely unrelated to the narrow way the breed has actually been bred over the generations.
    4. D Because natural selection and selective breeding are treated here as exactly the same process, the higher rate of inherited conditions is taken to show that these harmful traits must actually be beneficial to the dogs in their natural environment.
14. 141 mark

    A species of moth exists in two colour forms: pale and dark. Before a nearby factory opened, most tree trunks in the moths' habitat were pale-coloured with lichen, and pale moths were far more common than dark moths, since pale moths were better camouflaged from birds. After the factory began operating, decades of soot blackened the tree trunks, killing the lichen. Over the following decades, the proportion of dark moths in the population increased sharply, while the proportion of pale moths fell. Which explanation is most consistent with the specification's account of natural selection?

    1. A Individual pale moths are assumed to have changed their own colour to dark in direct response to the blackened tree trunks during their lifetime, and to have then passed this newly acquired dark colouration on to their offspring.
    2. B The soot released by the factory is assumed to have directly rewritten the DNA of every single moth in the population in an identical way, causing the entire population to turn dark-coloured all at the same time.
    3. C Both colour forms already existed through genetic variation before the trunks changed; pale moths then became easier for predators to spot, so dark moths survived and bred more successfully.
    4. D The proportion of dark moths increased only because dark moths happened to begin reproducing faster than pale moths, for reasons entirely independent of the change in tree trunk colour or of predation risk.
15. 151 mark

    In a particular plant species, flower colour is controlled entirely by a single gene: one allele gives red flowers and is dominant over the other allele, which gives white flowers when homozygous, with no other colours possible. In the same species, overall plant height varies smoothly from very short to very tall, is influenced by many different genes acting together, and is also affected by the amount of light, water and nutrients each individual plant receives. Which of the following correctly matches each characteristic to its type(s) of variation?

    1. A Flower colour shows continuous variation, since red and white are two ends of a colour range; plant height shows discontinuous variation, since every plant can be sorted into a small number of fixed height categories.
    2. B Both flower colour and plant height must be caused only by environmental factors, since a plant's characteristics are always determined by the conditions it is grown in rather than by its genes.
    3. C Flower colour shows discontinuous variation caused by environmental factors alone, since being grown in different conditions could cause a plant to switch between having red or white flowers; plant height shows continuous variation caused by genetic factors alone.
    4. D Flower colour shows discontinuous variation caused only by genetic factors (a single gene with clear-cut categories); plant height shows continuous variation caused by both genetic and environmental factors acting together.

Worked solutions

Every question below carries the reasoning, not just the answer. The official material for this test publishes a correct option letter and nothing else.

1. Question 1Answer: C

   1. Spec B4.1 states the nucleus is the site of the genetic material in eukaryotic cells specifically, not in cells in general.
   2. A bacterium is a prokaryotic cell: it does contain DNA, as the question states, but this DNA lies free within the cytoplasm, with no nuclear membrane enclosing it.
   3. This shows the student's general claim is false: DNA does not have to be stored inside a nucleus, only eukaryotic DNA is.
   4. So option C correctly evaluates the claim using the eukaryotic versus prokaryotic distinction.
   • Why not A: Invents an unobservably tiny bacterial nucleus to rescue the claim; prokaryotic cells are defined by having no nuclear membrane at all, not by having one too small to see under a light microscope.
   • Why not B: Wrongly claims bacterial genetic material is RNA rather than DNA; bacteria do use DNA as their genetic material, so the real distinction is where the DNA sits, not what molecule it is.
   • Why not D: Contradicts the question's own premise that the bacterium contains DNA, then wrongly concludes no storage-location question arises because there is supposedly no genetic material at all.

2. Question 2Answer: A

   1. Spec B4.2 requires knowing the terms gene and allele, among others.
   2. A gene is a section of DNA, at a particular position on a chromosome, that codes for a particular characteristic (here, coat colour).
   3. An allele is one particular version of that gene's DNA sequence; here, the black-producing and brown-producing versions are two different alleles of the same coat-colour gene.
   4. So option A correctly distinguishes the two terms; B, C and D each misassign what a gene or an allele actually is.
   • Why not B: Treats 'gene' and 'allele' as interchangeable, missing that a gene is the general section of DNA for a characteristic while an allele is one specific version of it, so a chromosome can carry two different alleles of the same gene.
   • Why not C: Wrongly scales 'allele' up to an entire chromosome and shrinks 'gene' down to a single base, when both terms describe sections of DNA at an intermediate scale, not either extreme.
   • Why not D: Invents an unsupported rule tying a gene and its allele to different specific parents; both alleles of a gene are simply inherited, one from each parent, without either term being tied to a single parent's side.

3. Question 3Answer: D

   1. Since the couple already has one child with the condition, both parents must be heterozygous carriers, Aa (spec B4.3).
   2. For an Aa x Aa cross, each child independently has a 3/4 probability of being unaffected (genotype AA or Aa) and a 1/4 probability of being affected (aa), and this probability is the same for every pregnancy regardless of previous children.
   3. For the next two children to BOTH be unaffected, the two independent probabilities are multiplied together: 3/4 x 3/4 = 9/16.
   4. So the probability that the next two children are both unaffected is 9/16, answer D.
   • Why not A: Gives the probability that a single next child is unaffected, without accounting for needing BOTH of the next two children to be unaffected; fails to combine the probability across two independent events.
   • Why not B: Squares the probability of the affected (recessive) outcome, 1/4, instead of the unaffected outcome, giving the probability that both next children are affected rather than both unaffected.
   • Why not C: Multiplies 3/4 by 1/2 instead of by 3/4 again, incorrectly halving for a second child rather than squaring the single-child probability across two independent events.

4. Question 4Answer: B

   1. Spec B4.3 requires interpreting family trees/pedigrees and expressing outcomes as probabilities.
   2. Two unaffected carrier parents (Aa x Aa) produce offspring in the ratio 1 AA : 2 Aa : 1 aa.
   3. Restricting attention to only the unaffected offspring removes the aa individuals, leaving a ratio of 1 AA : 2 Aa among the 3 remaining parts.
   4. So the probability that an unaffected child of this cross is a carrier (Aa) is 2 out of these 3 equally likely outcomes, 2/3, answer B.
   • Why not A: Assumes the two possible unaffected genotypes, AA and Aa, are equally likely, ignoring that a standard Aa x Aa cross produces Aa twice as often as AA (a 1 AA : 2 Aa : 1 aa ratio).
   • Why not C: Gives the probability of being affected (aa) from the original 1 AA : 2 Aa : 1 aa cross, rather than the probability of being a carrier among only the unaffected offspring.
   • Why not D: Gives the overall probability of being unaffected from the original cross (3 out of 4 genotypes), rather than the probability of specifically being a carrier once it is already known the child is unaffected.

5. Question 5Answer: C

   1. Spec B5.1 defines the genome as the full set of an organism's genetic material (DNA), contained within its chromosomes.
   2. Mitosis produces genetically identical daughter cells, so every body cell arising from repeated mitotic division of the original fertilised egg carries a copy of the same genome.
   3. Cell types differ in appearance and function because different genes from this same shared genome are switched on in different cells, not because their DNA content differs.
   4. So option C is correct: the genome is shared across body cells, and differentiation is a matter of gene expression, not genome content.
   • Why not A: Assumes different cell functions require physically different DNA content in each cell type, a muscle-specific gene set entirely missing from skin cells, rather than differential expression from one shared genome.
   • Why not B: Invents a process in which mitosis discards unneeded DNA as development proceeds; mitosis is instead defined by producing genetically identical daughter cells throughout the organism's life, embryo included.
   • Why not D: Confuses the stability of the genome in body cells with reproductive cells, and invents ongoing genome change in ordinary body cells that spec B5.1's definition of the genome does not support.

6. Question 6Answer: D

   1. In double-stranded DNA, adenine pairs with thymine and guanine pairs with cytosine, so A equals T and G equals C in amount (spec B5.2).
   2. If T is 22 per cent, then A is also 22 per cent, so A and T together make up 44 per cent of all bases, leaving 56 per cent for G and C combined.
   3. Since G equals C, each makes up half of that 56 per cent, so G is 28 per cent of all the bases.
   4. 28 per cent of the 5000 total bases is 1400, so the number of guanine bases is 1400, answer D.
   • Why not A: Treats guanine plus cytosine together (56 per cent) as if this whole amount were guanine alone, forgetting that guanine and cytosine are present in equal amounts and so must be halved.
   • Why not B: Wrongly assumes guanine's percentage equals thymine's given percentage directly, applying the 'A pairs with T' equal-amounts rule to guanine and thymine instead of to guanine and cytosine.
   • Why not C: Doubles thymine's given percentage (to 44 per cent, the combined A and T share) and then treats that combined share as if it were guanine's percentage, instead of subtracting from 100 per cent first.

7. Question 7Answer: A

   1. Spec B5.2 states that double-stranded DNA is a polymer of two strands forming a double helix.
   2. The original ssDNA strand of 3200 nucleotides is unchanged; a new complementary strand of equal length, also 3200 nucleotides, is synthesised alongside it.
   3. The resulting double-stranded molecule contains both strands' nucleotides together: 3200 + 3200 = 6400.
   4. So the dsDNA molecule contains 6400 nucleotides in total, answer A.
   • Why not B: Treats the resulting double-stranded molecule as if it still contained only the nucleotides of the original single strand, ignoring the complementary strand added to form the double helix.
   • Why not C: Halves the original strand's nucleotide count instead of doubling it, confusing 'double-stranded' with splitting the original single strand in half rather than adding a complete second strand alongside it.
   • Why not D: Adds only half of the original strand's length again, as if the synthesised complementary strand were shorter than the strand it pairs with, rather than an equal-length second strand.

8. Question 8Answer: C

   1. Spec B5.3 states that nucleotide bases in a gene are read as triplets, each triplet coding for one amino acid.
   2. Coding for 148 amino acids requires 148 x 3 = 444 bases.
   3. The stop codon is an additional triplet of three bases that does not itself code for an amino acid, so it adds 3 more bases: 444 + 3 = 447.
   4. So the complete coding sequence, including the stop codon, is 447 bases, answer C.
   • Why not A: Gives only the number of bases needed to code for the 148 amino acids (148 x 3), omitting the extra triplet needed for the stop codon.
   • Why not B: Adds only a single extra base for 'the stop signal' rather than a full extra triplet of three bases, undercounting the stop codon by two bases.
   • Why not D: Subtracts the stop codon's three bases from the amino-acid total instead of adding them on, treating the stop codon as though it were removed from the coding sequence rather than appended to it.

9. Question 9Answer: B

   1. Spec B5.3 describes the triplet code, and since there are 64 possible triplets but only 20 amino acids, more than one triplet can code for the same amino acid.
   2. Here, the base change described does not alter which amino acid is coded for at that position, so the polypeptide's amino acid sequence is unchanged.
   3. Since a protein's three-dimensional shape, and therefore its function, is determined by its amino acid sequence (spec B5.3), an unchanged sequence means the protein's shape and function are unlikely to be affected.
   4. This matches spec B5.4's statement that most mutations have no effect on phenotype, so option B is correct.
   • Why not A: Misstates spec B5.4, which says most mutations have no effect on phenotype and only occasionally determine it; it wrongly claims a mutation inside a gene always alters the amino acid sequence and always determines the phenotype.
   • Why not C: Introduces irrelevant factors, which type of cell the mutation occurs in and whether it is passed on; whether the amino acid sequence itself changes is what determines whether the phenotype is affected, regardless of cell type.
   • Why not D: Contradicts the scenario's own premise that the mutation occurs within a gene, and invents an unsupported restriction that mutations can only ever occur outside every gene.

10. Question 10Answer: A

    1. Spec B6.1 describes taking a copy of a gene from one organism's DNA and inserting it into another organism's DNA, naming restriction enzymes and ligase as the enzymes involved.
    2. Restriction enzymes must act first: they cut the gene out of human DNA and cut open the plasmid at matching sequences, event (iii).
    3. Only once both pieces of DNA have been cut can DNA ligase join the gene into the opened plasmid, forming a recombinant plasmid, event (i).
    4. This recombinant plasmid is then taken up by a bacterium, event (ii), which reproduces and uses the gene to make the protein, event (iv), so the correct order is iii, i, ii, iv, answer A.
    • Why not B: Places ligation, event (i), before the restriction enzyme cutting step, event (iii); this is impossible, since there is nothing to join together until the gene has been cut out and the plasmid has been opened.
    • Why not C: Has the bacterium take up the plasmid, event (ii), before DNA ligase has joined the gene into it, event (i), meaning the bacterium would take up an unmodified, still-open plasmid rather than the finished recombinant plasmid.
    • Why not D: Places protein production and bacterial reproduction, event (iv), right at the very start, before the gene has even been cut out, joined to a plasmid, or taken up by any bacterium.

11. Question 11Answer: D

    1. Spec B6.1 requires explaining the benefits and risks of genetic engineering.
    2. A recognised risk of releasing a genetically modified crop is that its inserted gene may spread to wild relative plants through cross-pollination (gene flow), with effects on wild populations that are difficult to predict or reverse.
    3. Claims of guaranteed higher yield, elimination of all chemical inputs, or zero risk to any insect are overstated or invented benefits, not risks, and are not supported by the specification.
    4. So the genuine risk described is option D.
    • Why not A: Presents an absolute, unconditional claim of guaranteed higher yield, which is not a risk at all, and is also an overgeneralisation the specification does not support.
    • Why not B: Wrongly assumes that producing one specific pesticide eliminates the need for every chemical input, including unrelated products such as fertiliser, overgeneralising from a narrow property of the modified gene.
    • Why not C: Wrongly assumes the plant's built-in pesticide cannot affect any insects at all, when a real risk of this technology is that it may also harm non-target, beneficial insect species, not only pests.

12. Question 12Answer: B

    1. Spec B6.2 states adult stem cells are multipotent, differentiating into a limited number of related cell types; here, bone marrow stem cells are limited to the blood cell lineage.
    2. This is a genuine limitation: a condition needing an unrelated cell type, such as nerve tissue, cannot be treated using bone marrow stem cells alone.
    3. Totipotent cells (very early embryos) and pluripotent cells (most embryonic stem cells) have far wider potential than the multipotent adult stem cells described here.
    4. So option B correctly classifies these cells and states a genuine limitation of using them for this treatment.
    • Why not A: Wrongly classifies bone marrow stem cells as totipotent, when the specification restricts totipotency to the very earliest embryonic cells, and invents an implausible risk of a whole new organism developing from a bone marrow transplant.
    • Why not C: Wrongly classifies bone marrow stem cells as pluripotent, equating adult stem cells with early embryonic ones, when the specification distinguishes pluripotent embryonic stem cells from the narrower multipotent adult stem cells.
    • Why not D: Invents an unlimited redirect-to-any-cell-type ability for adult stem cells that directly contradicts the multipotent classification given in the scenario itself, which is limited to the blood cell lineage.

13. Question 13Answer: A

    1. Spec B6.3 requires understanding the impact of selective breeding on populations.
    2. Breeding repeatedly from a narrow set of individuals within one breed reduces the range of alleles present in that population's gene pool, rather than refreshing or increasing it.
    3. With less genetic variation, there is a higher chance that two parents both carry, and pass on, the same harmful recessive allele, increasing the proportion of offspring with the associated inherited condition.
    4. This is a direct consequence of selective breeding narrowing variation, not a new unrelated mutation, and it is not evidence that the harmful trait is adaptive, so option A is correct.
    • Why not B: Mistakes the reshuffling of already-existing alleles between individuals at reproduction for an increase in overall variation; choosing parents repeatedly from a narrow set of individuals reduces, rather than refreshes, the range of alleles in the population.
    • Why not C: Invents an unrelated new mutation as the cause, ignoring the direct, well-established link between narrowing a breeding population and an increased chance of harmful recessive alleles pairing up.
    • Why not D: Conflates selective breeding, which is human-directed, with natural selection, which is environment-directed, and wrongly assumes a trait must be adaptive simply because it has become common, when here it is common only because of reduced genetic variation.

14. Question 14Answer: C

    1. Spec B7.1 states there is usually extensive genetic variation within a population, and describes evolution as a change in inherited characteristics over time through natural selection of variants best suited to the environment.
    2. Both the pale and dark colour forms already existed in the moth population due to genetic variation, before the tree trunks changed colour.
    3. Once the trunks darkened, pale moths became easier for predators to spot and were eaten more often, while dark moths were better camouflaged and survived and reproduced more successfully.
    4. This is natural selection acting on existing variation, not an acquired characteristic or a uniform pollutant-caused change, so option C is correct.
    • Why not A: Describes an acquired characteristic, a colour change occurring within one moth's lifetime that is then inherited, rather than pre-existing genetic variation being acted on by natural selection across generations.
    • Why not B: Invents a uniform, factory-caused genetic change striking every moth identically and at once, rather than differential survival acting on variation that already existed in the population before the trunks darkened.
    • Why not D: Removes the environmental selection pressure, predation risk linked to camouflage, from the explanation entirely, attributing the shift to an unexplained general reproductive advantage instead.

15. Question 15Answer: D

    1. Spec B4.3 states that some phenotypes result from single-gene inheritance, and spec B7.2 distinguishes genetic and environmental variation.
    2. Flower colour here is controlled by a single gene with clear dominant and recessive categories (red or white only), so it shows discontinuous variation with a purely genetic cause.
    3. Plant height is influenced by many genes acting together and by environmental factors (light, water and nutrients), producing a smooth range of values: continuous variation with both a genetic and an environmental contribution.
    4. So option D correctly matches each characteristic to its variation type and cause.
    • Why not A: Swaps the two variation types, wrongly calling the two-category flower colour 'continuous' and the smoothly ranging height 'discontinuous'.
    • Why not B: Denies any genetic basis for either characteristic, contradicting the single-gene basis explicitly given for flower colour and the multi-gene basis given for height.
    • Why not C: Wrongly attributes flower colour's two clear-cut categories to environmental conditions rather than to the single gene described, and wrongly claims height is caused by genetic factors alone, ignoring the stated light, water and nutrient influence.