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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 — From a Sequence of Bases to a Working Cell
Total: 12 marks
(a)[5]
Explain how a protein is made in a cell, starting from the gene. Your answer should refer to the nucleus, to mRNA and to a ribosome.
Model Answer — 1(a)
the gene stays in the nucleus [1]
mRNA is a copy of the gene, made in the nucleus [1]
the mRNA moves to the cytoplasm [1]
the mRNA passes through a ribosome [1]
the ribosome assembles amino acids into a protein, in the order set by the sequence of bases in the mRNA [1]
⚠ If you missed marks here: Five steps, five marks, and then stop. The chemical detail of how the bases are read is not on this syllabus and cannot be credited, so it costs you time you needed elsewhere. The mark most often lost is the first: an answer that lets the gene travel out of the nucleus contradicts the syllabus in its opening sentence.
(b)[4]
A neurone and a cell from the lining of the trachea in the same person are compared. State how their genes compare, and explain how the two cells can be so different. In your answer refer to one protein that each of these cells needs.
Model Answer — 1(b)
they contain the same genes [1]
because both descend by mitosis from the same zygote, and mitosis produces genetically identical cells [1]
they differ because different genes are expressed in each — a cell only makes the proteins it needs [1]
a neurone needs receptors for neurotransmitters; a ciliated cell in the trachea needs the proteins that build its cilia [1]
⚠ If you missed marks here: The idea that a specialised cell has discarded the genes it does not use is intuitive and wrong: mitosis copies the entire set. The last mark is testing whether you learned the syllabus list of protein kinds — enzymes, membrane carriers and receptors for neurotransmitters — rather than just the word enzyme.
(c)[3]
A couple already have three daughters. Explain why the probability that their next child is a boy is still 1 in 2.
Model Answer — 1(c)
a female is XX, so every egg cell carries an X chromosome [1]
a male is XY, so half his sperm carry an X and half carry a Y; the sex depends on which sperm fertilises the egg cell [1]
each fertilisation is independent, so the children they already have do not change the probability [1]
⚠ If you missed marks here: Two things to avoid. The first is writing that the father “decides” the sex — describe the mechanism, which sperm arrives, not an intention. The second is the gambler’s idea that a run of daughters makes a son more likely; nothing about a previous fertilisation reaches the next one.
Question 2 — Two Processes, Neither of Them Named
Total: 12 marks
Fig. 2.1 shows two processes, X and Y, taking place in the same animal. Neither is named for you.
(a)[4]
Name process X and process Y, and give the evidence from Fig. 2.1 for each.
Model Answer — 2(a)
X is mitosis [1]
because it produces two cells, each with the same number of chromosomes (4) as the starting cell [1]
Y is meiosis [1]
because it produces four cells, each with half the number of chromosomes (2) [1]
⚠ If you missed marks here: Half the marks here are for the evidence, so naming both processes and stopping gives you two out of four. Read the numbers off the diagram and quote them. An answer that identifies a process purely from the number of cells, without checking the chromosomes, has only done half the reasoning.
(b)[3]
State how the cells produced by process Y differ genetically from those produced by process X, and explain why that difference matters to a species.
Model Answer — 2(b)
the cells produced by Y are genetically different from one another; those produced by X are genetically identical [1]
the cells from Y are gametes, so the offspring formed from them show variation [1]
variation means that if the environment changes, some individuals may be better able to survive [1]
⚠ If you missed marks here: The phrases genetically different and genetically identical are the marked ones and neither “the same” nor “similar” is accepted. In the last mark be careful with the wording: variation means some individuals may survive, not that the species changes itself to fit its surroundings.
(c)[3]
State three roles of process X in an adult mammal that has finished growing.
Model Answer — 2(c)
repair of damaged tissues [1]
replacement of cells, for example red blood cells or the lining of the gut [1]
growth of individual tissues that continue to enlarge, such as hair and nails, and asexual reproduction in organisms that carry it out [1]
⚠ If you missed marks here: The word adult in the stem is doing work: growth of the whole organism has stopped, so replacement is the role to lead with rather than growth. Naming an example alongside the role is what secures the mark, because it shows you know what replacement actually looks like.
(d)[2]
A student writes: “Stem cells divide by meiosis, which is why they can become several different kinds of cell.” Explain what is wrong with this statement.
Model Answer — 2(d)
stem cells divide by mitosis, which produces genetically identical diploid cells [1]
meiosis halves the chromosome number and produces gametes; a haploid cell could not develop into a body cell [1]
⚠ If you missed marks here: A flat contradiction earns one mark; the second is for explaining why meiosis could not do the job. Getting into the habit of saying why the wrong answer is impossible, rather than only what the right one is, is worth marks right across this topic.
Question 3 — Black, Brown and an Unexpected Litter
Total: 12 marks
(a)[6]
In rabbits, black coat (B) is dominant to brown coat (b). Use a full genetic diagram to predict the offspring of a cross between two heterozygous black rabbits. Set out every stage.
Model Answer — 3(a)
parental phenotypes: black × black [1]
parental genotypes: Bb × Bb [1]
gametes: B and b from each parent, circled [1]
a correctly drawn Punnett square with the gametes on the outside [1]
offspring genotypes: 1 BB : 2 Bb : 1 bb [1]
offspring phenotypes and ratio: 3 black : 1 brown [1]
⚠ If you missed marks here: Note the two separate marks for the genotype ratio 1 : 2 : 1 and the phenotype ratio 3 : 1. Giving only one of them costs the other, and giving the genotype ratio when the question asked for phenotypes is a common way to lose a mark you had already earned.
(b)[3]
The two rabbits produce a litter of five, and all five are black. A student concludes that at least one parent must have been homozygous. Explain why the student is wrong.
Model Answer — 3(b)
a 3 : 1 ratio is a probability: each offspring independently has a 3 in 4 chance of being black [1]
it is not a rule about how a particular litter must turn out; fertilisation is random so small samples vary [1]
five black offspring from Bb × Bb is not unlikely, so the result gives no reason to change the parental genotypes [1]
⚠ If you missed marks here: Ratios describe expectations across many offspring, not a guarantee for one litter. The reverse error is just as common: seeing 4 black and 1 brown and declaring that the ratio has been proved. Neither a small agreement nor a small disagreement settles anything on its own, and the reason is always random fertilisation.
(c)[3]
A brown rabbit is bred with one of the black offspring from the litter. Half the offspring of this second cross are brown. Give the genotype of that black rabbit and explain how the result shows it.
Model Answer — 3(c)
the black rabbit is Bb [1]
the brown parent is bb and can only produce b gametes, so every offspring receives a b from it [1]
the brown offspring are bb, so they must also have received a b from the black parent, which proves it carries one [1]
⚠ If you missed marks here: This is a test cross, even though the question never uses the phrase. The reasoning marks are for tracing where each allele in a brown offspring came from. Simply asserting “because half are brown, it must be Bb” states the pattern without explaining it and scores one of the three.
Question 4 — Blue Chickens
Total: 12 marks
(a)[2]
In one breed of chicken, birds with the genotype FBFB are black, birds with FWFW are white, and birds with FBFW are blue. Name the type of inheritance shown and explain how the genotypes tell you this.
Model Answer — 4(a)
codominance [1]
because the heterozygote FBFW has its own phenotype that is neither black nor white — both alleles contribute to the phenotype [1]
⚠ If you missed marks here: The evidence for codominance is always the same: three phenotypes rather than two, with the heterozygote looking like neither homozygote. Take care not to describe blue as a mixture or a blend — both alleles are fully expressed, in different feathers.
(b)[5]
Use a full genetic diagram to predict the offspring of a cross between two blue chickens.
Model Answer — 4(b)
parental phenotypes: blue × blue [1]
parental genotypes: FBFW × FBFW [1]
gametes: FB and FW from each parent, circled [1]
Punnett square giving FBFB, FBFW, FBFW, FWFW [1]
phenotype ratio: 1 black : 2 blue : 1 white [1]
⚠ If you missed marks here: Writing 3 : 1 here is the error to avoid, and it happens because the hand has been trained on monohybrid crosses. With codominance the heterozygote is visible, so the 1 : 2 : 1 pattern does not collapse. Keep the superscripts small and distinct from the capital F.
(c)[3]
A breeder wants to produce a flock in which every bird is blue. Explain which cross would achieve this, and why crossing two blue birds would not.
Model Answer — 4(c)
cross a black bird (FBFB) with a white bird (FWFW) [1]
each parent is homozygous, so each can produce only one kind of gamete, and every chick must be FBFW and therefore blue [1]
crossing two blue birds would give 1 black : 2 blue : 1 white, so only about half the chicks would be blue [1]
⚠ If you missed marks here: This is the practical use of codominance and it turns the usual logic upside down: two parents of the wrong colours reliably give the colour you want, while two parents of the right colour do not. The mark is in the gametes — homozygous parents have only one kind to give.
(d)[2]
A blue chicken is crossed with a white chicken. State the expected ratio of the offspring, and explain your answer.
Model Answer — 4(d)
1 blue : 1 white [1]
the blue parent is FBFW and gives FB or FW; the white parent is FWFW and can only give FW, so the offspring are half FBFW (blue) and half FWFW (white) [1]
⚠ If you missed marks here: No black chicks are possible here, because there is only one FB allele in the whole cross and a black bird needs two. Checking whether a phenotype is even reachable before you write it down is a fast way to catch an error in the square.
Question 5 — Halving and Restoring
Total: 10 marks
(a)[3]
Define meiosis, and state where in a flowering plant it takes place.
Model Answer — 5(a)
a reduction division in which the chromosome number is halved from diploid to haploid [1]
it produces cells that are genetically different from one another [1]
in a flowering plant it takes place in the anthers and the ovules, where gametes are produced [1]
⚠ If you missed marks here: The commonest incomplete answer is “meiosis makes gametes”, which is true and scores at most one mark, because it never mentions the halving that makes it a reduction division. Naming a stage of meiosis earns nothing — the stages are not on this syllabus.
(b)[4]
A plant has 22 chromosomes in the nucleus of a leaf cell. Complete the reasoning: state how many chromosomes are in the nucleus of a pollen grain and in the nucleus of a cell of the embryo inside a seed of that plant, explaining each answer.
Model Answer — 5(b)
a pollen grain nucleus has 11 chromosomes [1]
because a pollen grain carries the male gamete nucleus, which is haploid after meiosis [1]
a cell of the embryo has 22 chromosomes [1]
because the embryo developed from a zygote, formed when two haploid nuclei fused at fertilisation, 11 + 11 = 22 [1]
⚠ If you missed marks here: Take the diploid number from a body cell first — a leaf cell — and then work outwards. The step people rush is the embryo: an embryo is built from a zygote by mitosis, so it is diploid, and treating it as a gamete because it is small and new is a real error.
(c)[3]
Explain why meiosis is necessary in a species that reproduces sexually.
Model Answer — 5(c)
fertilisation is the fusion of the nuclei of two gametes, so the zygote receives the chromosomes of both [1]
if the gametes were diploid the zygote would have twice the diploid number, and the number would double every generation [1]
meiosis halves the number so that fertilisation restores it, keeping the chromosome number of the species constant between generations [1]
⚠ If you missed marks here: Three marks for three linked statements, and the middle one is the one that turns a description into an explanation. Note that a second, separate reason exists — meiosis produces genetically different gametes and so creates variation — and it is worth adding if you have room.
Question 6 — A Condition That Skips a Generation
Total: 12 marks
Fig. 6.1 shows the inheritance of red-green colour blindness in a family. Squares are males, circles are females, and a shaded symbol means that person is colour blind.
(a)[3]
Using Fig. 6.1, state two features of the pattern of inheritance that suggest this condition is sex-linked, and explain what sex-linked means.
Model Answer — 6(a)
all the affected individuals are male — the condition is much more common in one sex [1]
the condition skips a generation, passing through unaffected females [1]
sex-linked means the gene responsible is located on a sex chromosome, which makes the characteristic more common in one sex than the other [1]
⚠ If you missed marks here: The definition mark is about where the gene is, not about who is affected — the sex bias is the consequence, not the definition. Deciding that a feature is sex-linked before you start is what tells you to use XB, Xb and Y rather than B and b, so this is the first decision in the question, not the last.
(b)[3]
Give the genotypes of individuals I-2 and II-1 in Fig. 6.1, and explain how you deduced them.
Model Answer — 6(b)
I-2 is XBXb, a carrier [1]
II-1 is XBXb, a carrier [1]
each has a colour-blind son, and a son receives his X chromosome from his mother, so each mother must carry an Xb while having normal vision herself [1]
⚠ If you missed marks here: The reasoning mark is the one to protect. Writing “she is a carrier” describes the answer rather than justifying it; the justification is that her son’s only X came from her, so the allele must have been in her. That single argument solves most sex-linked pedigrees.
(c)[4]
Individual II-1 has children with a man who is colour blind. Use a genetic diagram to show the possible children, and state the ratio of their phenotypes.
Model Answer — 6(c)
parental genotypes: XBXb × XbY [1]
gametes: XB and Xb from the mother, Xb and Y from the father, circled [1]
Punnett square giving XBXb, XbXb, XBY, XbY [1]
phenotypes: carrier daughter, colour-blind daughter, son with normal vision, colour-blind son — ratio 1 : 1 : 1 : 1 [1]
⚠ If you missed marks here: This is the only kind of cross that can produce a colour-blind daughter, because she needs an Xb from each parent, which means an affected father. If your square has no colour-blind daughter in it, check that you gave the father an Xb rather than an XB.
(d)[2]
Explain why a man cannot be a carrier of this condition, and state which of his children can receive the allele from him.
Model Answer — 6(d)
a man has only one X chromosome and his Y carries no allele for this gene, so whatever is on his X is expressed — he is either affected or not, never a carrier [1]
he passes his X only to his daughters; his sons receive his Y, so a son can never receive this allele from him [1]
⚠ If you missed marks here: These two facts between them dispose of the most persistent misconception about sex linkage, that a condition runs from father to son. If a question describes a “carrier father”, the question is testing you: that father is affected.
Question 7 — Three Crosses in a Greenhouse
Total: 10 marks
(a)[4]
In a tomato plant, hairy stems (H) are dominant to smooth stems (h). Three crosses were carried out. Cross 1, hairy × hairy, gave 96 hairy and 31 smooth. Cross 2, hairy × smooth, gave 58 hairy and 61 smooth. Give the genotypes of both parents in cross 1 and of the hairy parent in cross 2, explaining your reasoning.
Model Answer — 7(a)
cross 1 gives approximately 3 : 1 (96 ÷ 31 is about 3.1) [1]
a 3 : 1 ratio requires both parents to be heterozygous, Hh × Hh [1]
cross 2 gives approximately 1 : 1 [1]
the smooth parent must be hh, so a 1 : 1 ratio means the hairy parent is Hh [1]
⚠ If you missed marks here: Do the division before you write anything: the ratio, not the raw counts, identifies the parents. Two of these four marks are for the arithmetic. Note that cross 2 is a test cross even though the question does not call it one, because the smooth parent must be homozygous recessive.
(b)[3]
A third cross, hairy × smooth, gave 124 hairy plants and no smooth plants. State the most likely genotype of the hairy parent and explain why this result does not prove it.
Model Answer — 7(b)
the hairy parent is most likely HH [1]
because if it were Hh, about half the offspring would be expected to be smooth, and none were [1]
but a heterozygous parent could give 124 hairy offspring by chance, so the result makes HH very likely rather than certain — the conclusion is safer the more offspring there are [1]
⚠ If you missed marks here: Cambridge marks the difference between proves and strongly suggests. A recessive offspring proves heterozygous; the absence of one never proves homozygous, however many offspring there are. Writing “this proves the parent is HH” loses the third mark on its own.
(c)[3]
Explain why a plant grower cannot tell by looking at a hairy tomato plant whether it is HH or Hh, and state what this means for the answer to a question asking for the genotype of such a plant.
Model Answer — 7(c)
hairy is caused by a dominant allele, which is expressed whenever it is present [1]
so HH and Hh give the same phenotype, and the phenotype cannot distinguish them [1]
the correct answer to such a question is “HH or Hh”, unless other information in the question rules one out [1]
⚠ If you missed marks here: The link between the phenotype and the genotype runs one way only. From a genotype you can always name the phenotype; from a phenotype showing the dominant feature you usually cannot name the genotype. Writing the uncertainty down is a full-credit answer, not a hedge — and guessing one is marked wrong even when it happens to be right.
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