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This paper covers the whole of Topic 17. Like a real Cambridge paper it ranges across every sub-topic — 17.1 chromosomes, genes and proteins, 17.2 mitosis and stem cells, 17.3 meiosis, 17.4 monohybrid inheritance, and 17.5 codominance, ABO blood groups and sex linkage — and it mixes them inside single questions. All three Topic 17 papers do; they differ in the angle they come at it from, not in what they cover.
Question 1 — The Same Genes in Every Cell
Total: 12 marks
(a)[3]
Define the terms gene, allele and genotype.
Model Answer — 1(a)
a gene is a length of DNA that codes for a protein [1]
an allele is an alternative form of a gene [1]
a genotype is the genetic make-up of an organism, in terms of the alleles present [1]
⚠ If you missed marks here: Three memorised sentences for three marks — the cheapest marks anywhere in this topic, and the ones most often thrown away by paraphrasing. “An allele is a type of chromosome” is the single most common wrong definition on Topic 17 papers; an allele is a version of a gene.
(b)[4]
A stem cell in the bone marrow of a person divides many times. Some of the daughter cells become red blood cells and others become white blood cells. Explain how cells with such different structures can be produced from one stem cell.
Model Answer — 1(b)
the stem cell divides by mitosis, which produces genetically identical cells [1]
so every daughter cell contains the same genes as the stem cell and as each other [1]
the cells become different because different genes are expressed in each of them [1]
a cell only makes the proteins it needs, so many of its genes are not expressed [1]
⚠ If you missed marks here: The wrong answer that feels right is that specialised cells “lose the genes they do not need”. Mitosis copies the whole set of chromosomes, so nothing can be lost. Note also that a stem cell divides by mitosis and never by meiosis, whatever the range of cells it can produce.
(c)[5]
A change in one base in a gene coding for a membrane carrier protein means the carrier no longer works. (i) Explain why the protein no longer works. (ii) Explain what effect this would have on the cell. (iii) Name two other kinds of protein that a change of this sort could affect.
Model Answer — 1(c)
the sequence of bases determines the sequence of amino acids in the protein [1]
a different sequence of amino acids gives the protein a different shape, and the shape is what allows it to work [1]
the cell could no longer carry out active transport of the substance that carrier moves [1]
so that substance could no longer be moved against its concentration gradient into or out of the cell [1]
two other kinds: enzymes and receptors for neurotransmitters [1]
⚠ If you missed marks here: Part (iii) is testing whether you learned the syllabus list of protein kinds rather than only the word enzyme. And in (ii), naming the process is not enough on its own — say what active transport does, which is move a substance against its concentration gradient using energy from respiration.
Question 2 — Round Seeds and Wrinkled Seeds
Total: 12 marks
(a)[6]
In pea plants, round seeds (R) are dominant to wrinkled seeds (r). Use a full genetic diagram to predict the offspring of a cross between two heterozygous round-seeded plants. Set out every stage.
Model Answer — 2(a)
parental phenotypes: round × round [1]
parental genotypes: Rr × Rr [1]
gametes: R and r from each parent, circled [1]
a correctly drawn Punnett square with the gametes on the outside [1]
offspring genotypes: 1 RR : 2 Rr : 1 rr [1]
offspring phenotypes and ratio: 3 round : 1 wrinkled [1]
⚠ If you missed marks here: The two lines candidates omit most often are the circled gametes and the offspring phenotypes written as words. Each is a whole mark for about five seconds of writing, and both are easy to check before you move on.
(b)[3]
One of the round-seeded offspring is crossed with a wrinkled-seeded plant, and about half the resulting seeds are wrinkled. Give the genotype of that offspring plant, and explain how the result shows it.
Model Answer — 2(b)
the offspring plant is Rr [1]
the wrinkled plant is rr and can only produce r gametes, so every seed receives an r from it [1]
the wrinkled seeds are rr, so they must also have received an r from the round-seeded plant, which proves it carries one [1]
⚠ If you missed marks here: This is a test cross even though the phrase is not used. The reasoning marks come from tracing where each allele in a wrinkled seed must have come from. Asserting “half are wrinkled so it must be Rr” restates the observation and scores one of three.
(c)[3]
The cross in part (a) produced 312 round seeds and 104 wrinkled seeds. Explain whether these results agree with your prediction, and explain why the numbers are not exactly 312 : 104 in every repeat of the experiment.
Model Answer — 2(c)
312 ÷ 104 is exactly 3, so the results are approximately 3 : 1 and agree with the prediction [1]
a ratio is a probability: each seed independently has a 3 in 4 chance of being round [1]
fertilisation is random, so the actual numbers vary by chance around the expected ratio, especially when the sample is small [1]
⚠ If you missed marks here: A large sample like this one comes out very close to the prediction, and a small one does not — that is the point being tested. Do not claim the ratio is “exactly 3 : 1” as a biological law: it is exactly 3 : 1 in these data by coincidence, and the expected ratio remains a probability.
Question 3 — Four Blood Groups, Three Alleles
Total: 12 marks
(a)[4]
Blood group in humans is controlled by three alleles: IA, IB and IO. State the relationship between the three alleles, and give all the possible genotypes for blood groups A and AB.
Model Answer — 3(a)
IA and IB are codominant with each other [1]
IO is recessive to both of them [1]
group A: IAIA or IAIO [1]
group AB: IAIB only [1]
⚠ If you missed marks here: Group A has two possible genotypes and group AB has one, and knowing which groups are certain is what makes ABO questions fast. If you gave only one genotype for group A you have missed the heterozygous possibility, which is the one that makes surprising children possible.
(b)[5]
A man of blood group AB has children with a woman of blood group A whose father was blood group O. Use a full genetic diagram to show the possible blood groups of their children, and give the ratio.
Model Answer — 3(b)
the woman must be IAIO, because her father was group O and could only pass on IO [1]
parental genotypes: IAIB × IAIO [1]
gametes: IA and IB from the man, IA and IO from the woman, circled [1]
Punnett square giving IAIA, IAIO, IAIB, IBIO [1]
phenotypes and ratio: 2 group A : 1 group AB : 1 group B [1]
⚠ If you missed marks here: The first mark is a deduction, not a lookup: her father being group O forces an IO into her. Notice the phenotype ratio is 2 : 1 : 1 rather than 1 : 1 : 1 : 1, because two different genotypes both give group A — count phenotypes, not boxes.
(c)[3]
Explain why no child of this couple can be blood group O, and explain why a couple who are both blood group A could have a child of blood group O.
Model Answer — 3(c)
group O requires the genotype IOIO, so a child must receive an IO from each parent [1]
the man is IAIB and has no IO allele to pass on, so no child of his can be group O [1]
two group A parents could both be IAIO, in which case each could pass on IO and a child could be IOIO [1]
⚠ If you missed marks here: The two halves of this question test the same idea from opposite directions: what matters is not what a parent shows but which alleles they can pass on. That is why a blood group can appear in a child that neither parent has, and why a group present in a parent can be impossible in a child.
Question 4 — A Colour-Blind Mother
Total: 12 marks
(a)[3]
Explain what is meant by a sex-linked characteristic, and state why red-green colour blindness is more common in males than in females.
Model Answer — 4(a)
a feature in which the gene responsible is located on a sex chromosome, which makes the characteristic more common in one sex than the other [1]
the allele is on the X chromosome and the Y carries no allele for this gene [1]
a male has only one X, so a single recessive allele is expressed, whereas a female needs two copies [1]
⚠ If you missed marks here: The definition is about where the gene sits; the sex bias is the consequence and belongs in the second half of the answer. Putting the allele on the Y is the fatal error, because it would make colour blindness impossible in females, and colour-blind women exist.
(b)[5]
A woman who is colour blind has children with a man who has normal vision. Use a full genetic diagram to show their possible children, using the symbols XB, Xb and Y.
Model Answer — 4(b)
parental phenotypes: colour-blind female × male with normal vision [1]
parental genotypes: XbXb × XBY [1]
gametes: Xb from the mother (only one kind), XB and Y from the father, circled [1]
Punnett square giving XBXb and XbY [1]
phenotypes: every daughter is a carrier with normal vision, and every son is colour blind — ratio 1 : 1 [1]
⚠ If you missed marks here: The mother is homozygous, so she can only make one kind of gamete — if you wrote two different gametes underneath her, check the genotype above them. This cross gives a result that surprises people: every son is affected and no daughter is, which is the exact opposite of the more familiar carrier-mother cross.
(c)[2]
Explain why every son of this couple is colour blind while no daughter is.
Model Answer — 4(c)
each son receives his Y chromosome from his father and his only X from his mother, and her every X carries Xb, so every son is colour blind [1]
each daughter receives her mother’s Xband her father’s XB, so she has normal vision and is a carrier [1]
⚠ If you missed marks here: The general rule to carry away is that a son’s X always comes from his mother, so for any sex-linked feature his phenotype is decided entirely by her. His father contributes only a Y, and therefore contributes nothing at all to his son for a sex-linked gene.
(d)[2]
One of the daughters later has children with a colour-blind man. State the probability that a daughter of that couple is colour blind, and explain your answer.
Model Answer — 4(d)
the cross is XBXb × XbY, giving XBXb, XbXb, XBY and XbY [1]
two of the four boxes are daughters and one of those two is XbXb, so the probability is 1 in 2 [1]
⚠ If you missed marks here: The denominator is the whole difficulty. One box in four is a colour-blind daughter, so the answer for a child would be 1 in 4; but the question asked about a daughter, and only two boxes are daughters. Underline the word daughter, son or child before you count.
Question 5 — Why the Number Has to Fall
Total: 10 marks
(a)[3]
State three ways in which meiosis differs from mitosis.
Model Answer — 5(a)
meiosis produces four cells; mitosis produces two [1]
meiosis halves the chromosome number, from diploid to haploid; mitosis maintains it [1]
⚠ If you missed marks here: Each difference must state both sides to count as a difference. “Meiosis makes four cells” is half of one and is usually not credited alone. Avoid giving the location of each division as a difference — where a process happens is a consequence of what it does, not a difference in the process itself.
(b)[4]
A species of beetle has 20 chromosomes in its body cells. Explain what happens to the chromosome number when the beetle produces gametes and when two of those gametes meet at fertilisation. Give the numbers at each stage.
Model Answer — 5(b)
during meiosis the chromosome number is halved from 20 to 10 [1]
so each gamete nucleus is haploid, with one set of chromosomes [1]
at fertilisation the nuclei of two gametes fuse, 10 + 10 = 20 [1]
so the zygote is diploid and the chromosome number of the species is kept constant from generation to generation [1]
⚠ If you missed marks here: Give the numbers — two of these marks depend on them. Cambridge also marks the phrase fusion of the nuclei of two gametes; “the sperm joins the egg” is loose, because fertilisation is defined by what happens to the nuclei, not to the cells.
(c)[3]
The four cells produced by meiosis are genetically different from one another. Explain why this matters to a species living in an environment that is changing.
Model Answer — 5(c)
because the gametes are genetically different, the offspring show variation [1]
if the environment changes, some individuals may have features that let them survive where others do not [1]
those individuals can reproduce and pass on their alleles, so the species can continue [1]
⚠ If you missed marks here: Be careful with the wording in the second mark: individuals do not change themselves to suit a new environment, and the variation is present before the change happens. Write “some may already have features that let them survive” rather than “they adapt to the change”.
Question 6 — A Family of Guinea Pigs
Total: 12 marks
Fig. 6.1 shows the inheritance of coat texture in a family of guinea pigs. Smooth coat (D) is dominant to rough coat (d). Squares are males, circles are females, and a shaded symbol means the animal is rough-coated.
(a)[2]
State, with a reason, whether the allele for a rough coat is dominant or recessive.
Model Answer — 6(a)
recessive [1]
because I-1 and I-2 are both smooth-coated but their daughter II-1 is rough-coated — a dominant allele would have to be expressed in at least one parent [1]
⚠ If you missed marks here: One mark for the conclusion and one for the reason, and the reason has to describe the pattern rather than merely assert it. “It skips a generation” is a weaker version that examiners often refuse on its own; name the parents and the child.
(b)[4]
Using the symbols D and d, give the genotypes of individuals I-2, II-1, II-3 and III-2, explaining any that cannot be given exactly.
Model Answer — 6(b)
I-2 is Dd — she is smooth-coated but her daughter II-1 is rough-coated, so she must carry d [1]
II-1 is dd — she shows the recessive phenotype [1]
II-3 is Dd — he is smooth-coated but his son III-1 is rough-coated [1]
III-2 is DD or Dd — the pedigree does not distinguish them [1]
⚠ If you missed marks here: Work outwards from certainty: individuals showing the recessive phenotype are the only ones whose genotype is fixed by the diagram alone. The last mark is the one most often lost by guessing; “DD or Dd” is exactly what a mark scheme prints, and choosing one is wrong even when it turns out to be right.
(c)[3]
II-3 and II-4 are expecting another offspring. Use a genetic diagram to find the probability that it is rough-coated.
Model Answer — 6(c)
both II-3 and II-4 must be Dd, because they are smooth-coated but their son III-1 is rough-coated [1]
a Punnett square for Dd × Dd gives 1 DD : 2 Dd : 1 dd [1]
one box in four is dd, so the probability is 1 in 4 (25%) [1]
⚠ If you missed marks here: The probability is not affected by the offspring they already have — each fertilisation is independent, so having one rough-coated offspring does not make the next one less likely. The first mark is for establishing the parental genotypes; skipping straight to a number leaves the answer unjustified.
(d)[3]
Explain how the pedigree shows that coat texture is not carried on the X chromosome, and state one other observation in a pedigree that would suggest a feature is sex-linked.
Model Answer — 6(d)
a female showing a sex-linked recessive feature must receive a recessive allele from both parents, including from her father, whose only X she inherits [1]
II-1 is rough-coated and her father I-1 is smooth-coated, so the allele cannot be on the X chromosome [1]
a pedigree suggesting sex linkage would show many more affected males than females, or the feature passing through unaffected mothers from an affected grandfather to a grandson [1]
⚠ If you missed marks here: Learn the ruling-out move: an affected female whose father is unaffected disproves X-linked recessive inheritance in a single sentence. It is worth scanning any pedigree for that pairing before you write anything, because it settles a whole part of the question in two lines.
Question 7 — Drooping Leaves and Inherited Change
Total: 10 marks
(a)[4]
In a species of shrub, erect leaves (E) are dominant to drooping leaves (e). A heterozygous erect plant is crossed with a drooping plant. Use a genetic diagram to predict the offspring, and give the ratio.
Model Answer — 7(a)
parental genotypes: Ee × ee [1]
gametes: E and e from the erect plant, e only from the drooping plant, circled [1]
Punnett square giving Ee, ee, Ee, ee [1]
phenotype ratio: 1 erect : 1 drooping [1]
⚠ If you missed marks here: Writing 3 : 1 out of habit is the commonest error in this kind of question, and it comes from not reading the parental genotypes. The drooping plant is homozygous, so it produces only one kind of gamete — if you drew two different gametes underneath it, that is where the answer went wrong.
(b)[3]
A gardener notices that one shrub has unusually large leaves after being given extra fertiliser, and takes cuttings from it hoping the new plants will also have large leaves. Explain whether the new plants will have large leaves.
Model Answer — 7(b)
the cuttings are produced by mitosis, so the new plants are genetically identical to the parent shrub [1]
the large leaves were caused by the environment (the extra fertiliser), not by a change in the genes [1]
so the new plants will only have large leaves if they are given the same extra fertiliser — the feature is not inherited [1]
⚠ If you missed marks here: This is the distinction between a feature caused by genes and one caused by conditions, and it is examined more often than students expect. An environmental effect on an individual does not change its genes, so it cannot be passed on — not through cuttings and not through gametes either.
(c)[3]
A change does occur in the DNA of a cell in one of the shrub’s leaves. Explain why this change cannot be passed to the plant’s offspring, and state where in the plant a change would have to occur for it to be inherited.
Model Answer — 7(c)
a leaf cell divides by mitosis and produces only more body cells, which are never passed to offspring [1]
a change can only be inherited if it is present in a gamete, because the offspring develops from a zygote formed when two gamete nuclei fuse [1]
so the change would have to occur in a cell in the anthers or the ovules, where meiosis produces gametes [1]
⚠ If you missed marks here: The route is the answer: meiosis, gamete, fertilisation, zygote, every cell of the offspring. If a change never enters that route it can never be inherited, however dramatic its effect on the individual. This is the same logic as part (b) approached from the genetic side rather than the environmental one.
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