⚡ Challenge Paper Preparation

Challenge Prep: Inheritance

IGCSE Biology 0610 — Topic 17 — Extended

Most of this topic is a procedure, not a fact, and Cambridge awards marks for the layout itself — parental phenotypes, parental genotypes, circled gametes, the Punnett square, offspring genotypes, offspring phenotypes, ratio. A correct ratio with no working scores one mark out of four. An allele is a version of a gene, never of a chromosome. Mitosis gives genetically identical cells; meiosis gives genetically different ones. There is no such thing as a male carrier of a sex-linked recessive, and a son never gets a sex-linked allele from his father. Twelve traps, six data-led walkthroughs, six lookalike pairs, a three-part concept map, six answers that read well and score badly, and ten full practice questions — every one aimed at a place where a sensible-sounding sentence earns nothing at all.

⚠️ Common Traps & Misconceptions

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Twelve traps that cost marks on Topic 17 challenge papers, spread across 17.1 to 17.5. Every one is an answer that sounds right and that mark schemes refuse.

⚠️ TRAP

Trap 1: Calling an allele a form of a chromosome

The Trap“An allele is a different type of chromosome that gives a different feature.” It sounds close enough to be safe, and it is the definition error examiners report most often in this topic.

The TruthAn allele is an alternative form of a gene. The sizes run chromosome > gene > allele: a chromosome is a structure made of DNA carrying thousands of genes; a gene is a length of DNA that codes for a protein; an allele is one of the possible versions of that one gene, sitting at the same position on each chromosome of a homologous pair.

Why It MattersThe definition is a mark on its own, and it also underpins everything else. If an allele were a chromosome, then Bb would describe two different chromosomes rather than two versions of one gene, and every Punnett square you draw would be describing the wrong thing.

Example Question“Define the terms gene and allele. [2]”

⚠️ TRAP

Trap 2: Writing that a dominant allele is stronger

The Trap“B is dominant because it is stronger than b and overpowers it.” Everybody writes this at least once. Mark schemes reject it every time.

The TruthA dominant allele is expressed if it is present in the genotype. A recessive allele is only expressed when no dominant allele of that gene is present. Nothing is competing, nothing is beaten, and the recessive allele is present and perfectly functional in a heterozygote — it is simply not expressed.

Why It MattersThe words “stronger”, “overpowers”, “beats” and “wins” are on the refusal list. Say expressed. It is one word and it converts a wrong answer into a right one. Note too that dominant does not mean common: plenty of recessive alleles are the ones most individuals carry.

Example Question“Explain what is meant by a recessive allele. [2]”

⚠️ TRAP

Trap 3: Swapping mitosis and meiosis

The Trap“Meiosis is used for growth and repair, and mitosis produces the gametes.” Written under time pressure, the two words look interchangeable and the whole answer inverts.

The TruthMitosis gives two genetically identical diploid cells and is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction. Meiosis gives four genetically different haploid cells and is used for the production of gametes and nothing else on this syllabus.

Why It MattersThis one contrast is worth more marks across a Topic 17 paper than any other single fact, and it is tested in almost every question that mentions cells at all. Anchor it on the number of cells: mitosis gives two, meiosis gives four.

Example Question“State two differences between mitosis and meiosis. [2]”

⚠️ TRAP

Trap 4: Dropping the word genetically

The Trap“Mitosis produces identical cells.” It looks complete, it is nearly complete, and it is regularly worth nothing.

The TruthThe marked phrase is genetically identical. Two cells can be identical in appearance without being genetically identical, and Cambridge is testing whether you know which kind of sameness matters. The mirror phrase for meiosis is genetically different.

Why It MattersThis is the cheapest mark in the topic and the one most often thrown away. Whenever you write identical or different about cells, ask yourself whether the word genetically is in front of it.

Example Question“Describe what is meant by mitosis. [2]”

⚠️ TRAP

Trap 5: Treating a 3 : 1 ratio as a promise about a real litter

The Trap“The ratio is 3 : 1, so out of the eight puppies exactly six will be black and two brown.” Or, worse, seeing four black puppies and concluding that the cross must have been different.

The TruthA ratio is a statement about probability. A 3 : 1 ratio means each offspring independently has a 3 in 4 chance of the dominant phenotype. Real numbers vary because fertilisation is random, and small samples vary a lot. Four black offspring from a Bb × Bb cross happens about a third of the time.

Why It MattersQuestions give you real counts — 43 and 17, or 61 and 19 — and ask what ratio they suggest. The mark is for the word approximately plus the reason: random fertilisation, and a small sample size. An answer that says the data disprove a 3 : 1 ratio loses both.

Example Question“A cross gave 61 tall and 19 dwarf plants. Suggest what this indicates about the parents, and explain why the numbers are not exact. [3]”

⚠️ TRAP

Trap 6: Writing the ratio and nothing else

The TrapThe question says “use a genetic diagram to predict the ratio” and is worth 4 marks. You know the answer instantly, so you write “3 : 1” and move on, feeling efficient.

The TruthThe marks are in the layout: parental phenotypes, parental genotypes, gametes (circled), the Punnett square, offspring genotypes, offspring phenotypes, ratio. The ratio is one of seven marking points, and “use a genetic diagram” is an instruction, not a suggestion.

Why It MattersThis single habit is worth more marks than any amount of extra content knowledge. It also protects you: if you slip in the last line, everything above it can still be credited. A wrong ratio with a full correct layout usually outscores a right ratio on its own.

Example Question“Use a genetic diagram to show the expected offspring of a cross between two heterozygous plants. [4]”

⚠️ TRAP

Trap 7: Letting the gene travel to the ribosome

The Trap“The gene leaves the nucleus and moves to a ribosome, where the protein is made.” It is the natural picture and it contradicts the first thing the syllabus says.

The TruthThe gene stays in the nucleus. What travels is mRNA, which is a copy of the gene. The mRNA is made in the nucleus, moves to the cytoplasm, and passes through a ribosome, which assembles amino acids into a protein in the order set by the sequence of bases in the mRNA.

Why It MattersSix steps, marked one by one, and the first of them is that the gene does not move. Answers that send the gene out lose that mark and usually the mRNA mark with it. Note also what to leave out: nucleotide structure and the mechanism of reading the bases are both off-syllabus and win nothing.

Example Question“Explain how a protein is made in a cell. [5]”

⚠️ TRAP

Trap 8: Using Bb notation for a sex-linked cross

The Trap“The mother is Bb and the father is BB, so…” It is the notation you have just spent an hour drilling, and here it destroys the answer.

The TruthA sex-linked allele sits on the X chromosome, so the notation has to show which chromosome carries it: X^B, X^b and Y. A carrier mother is X^BX^b, not Bb. The Y is written on its own with no superscript, because it carries no allele for this gene.

Why It MattersThe whole of sex linkage follows from where the alleles sit, so notation that hides that cannot express the answer. Writing X^BY^b is worse than Bb, because it invents an allele on the Y chromosome — and the absence of that allele is the reason the condition is commoner in males.

Example Question“Use a genetic diagram to show the possible children of a carrier mother and a colour-blind father. [5]”

⚠️ TRAP

Trap 9: Inventing a carrier male

The Trap“The father is a carrier, so he is not colour blind but can pass it on.” The word carrier is so familiar that it gets applied to everybody.

The TruthA carrier has an allele without expressing it, and that needs a second X chromosome to hide it on. A male is X^BY or X^bY: whatever is on his one X is expressed. There is no such thing as a male carrier of a sex-linked recessive. If a question describes a father who carries the allele, that father is affected.

Why It MattersThis is not a wording quibble — it is the reason the condition is commoner in males, and the reason a pedigree can be solved at all. When you meet an affected female, you can immediately state that her father must be affected too, which is often worth two marks on its own.

Example Question“Explain why red-green colour blindness is more common in males than in females. [3]”

⚠️ TRAP

Trap 10: Passing a sex-linked allele from father to son

The Trap“He is colour blind like his father.” It is the intuitive family story and it is genetically impossible.

The TruthA father gives his Y chromosome to every son and his X chromosome to every daughter. A son therefore never receives a sex-linked allele from his father. A colour-blind son always received the allele from his mother. A colour-blind father passes his X^b to all of his daughters, every one of whom is at least a carrier.

Why It MattersHalf of all sex-linkage questions turn on this. It is also the fastest way to test a pedigree: find an affected father with sons. If any son is affected, the feature is probably not X-linked recessive, because that allele cannot have come down that line.

Example Question“A colour-blind man has two sons and two daughters with a woman of normal vision who is not a carrier. State the phenotype of each child, giving reasons. [4]”

⚠️ TRAP

Trap 11: Treating codominance as a blend

The Trap“Red and white cattle give roan calves because the colours mix, so roan is halfway between them.” The picture of paint being stirred is the wrong picture.

The TruthCodominance means both alleles in a heterozygous organism contribute to the phenotype. A roan animal is covered in red hairs and white hairs together — both alleles are fully expressed, in different hairs, at the same time. Because the heterozygote has its own visible phenotype, the genotype ratio does not collapse: a C^RC^W × C^RC^W cross gives 1 red : 2 roan : 1 white for genotypes and phenotypes.

Why It MattersThe definition is the mark, and it is a definition about expression, not about mixing. The 1 : 2 : 1 phenotype ratio is what makes codominance recognisable in data: if you are shown three phenotypes in a 1 : 2 : 1 pattern, that is codominance and not ordinary dominance.

Example Question“Explain what is meant by codominance, using coat colour in cattle as an example. [3]”

⚠️ TRAP

Trap 12: Guessing a genotype that the pedigree does not decide

The TrapAn individual shows the dominant phenotype and has no affected relatives, and you write “BB” because you have to write something.

The TruthThe correct answer is BB or Bb, and mark schemes print exactly that. Work outwards from certainty instead: anyone showing the recessive phenotype is homozygous recessive; anyone showing the dominant phenotype who has a recessive-phenotype parent or child must be heterozygous; everyone else is genuinely undetermined.

Why It MattersWriting the uncertainty down is a positive, creditworthy answer, not a hedge. Guessing one of the two is marked wrong even in the cases where it happens to be right, because the question was testing whether you know what the diagram can and cannot tell you.

Example Question“State the genotype of individual II-2, explaining your answer. [2]”

🔍 Step-by-Step Walkthroughs

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Six challenge-level questions with real data, worked through in the order you should actually think about them. Try each part before revealing the next step.

Walkthrough 1 — A pedigree with nothing given away

Fig. W1 shows the inheritance of coat colour in a family of rabbits. Shaded symbols are brown; unshaded are black. (a) State, with a reason, whether the allele for brown is dominant or recessive. (b) Give the genotypes of I-1, II-1 and II-2. (c) Explain why the feature cannot be sex-linked recessive. [7]

Two unaffected parents with an affected child settles it

I-1 and I-2 are both black. Their daughter II-1 is brown. A feature that appears in a child but in neither parent cannot be caused by a dominant allele, because a dominant allele is expressed whenever it is present — at least one parent would have had to show it. So brown is caused by a recessive allele, and both parents must have been carrying it hidden. That is the whole of part (a), and it is worth two marks: the conclusion and the reason.

Start with everyone showing the recessive phenotype

Anyone who is brown must be bb — there is no other possibility. So II-1 is bb, and III-1 is bb. Now work outwards. I-1 is black, so he has at least one B; but his daughter II-1 is bb, so she received a b from him. He must therefore be Bb. The same argument makes I-2 Bb. Never start from the individuals you are unsure about; start from the ones the diagram forces.

II-2 is the trap

II-2 is black. His parents are Bb × Bb, so he could be BB or Bb, and he has no children shown who could settle it. The correct answer is BB or Bb — and that is a full-credit answer, not a hedge. Compare him with II-3, who is also black but has a brown son, III-1: II-3 must have given a b to that son, so II-3 is definitely Bb. Same phenotype, different amount of information.

One individual does all the work

If the brown allele were carried on the X chromosome and recessive, a brown female would have to be X^bX^b, which means she received an X^b from each parent — including from her father, whose only X she must have. Her father would therefore have to be X^bY and brown himself. II-1 is a brown female and her father I-1 is black. That is impossible for a sex-linked recessive, so the feature is not sex-linked.

The Answer(a) Recessive, because I-1 and I-2 are both black and their daughter II-1 is brown — a dominant allele would have to show in a parent [2]. (b) I-1 is Bb [1]; II-1 is bb [1]; II-2 is BB or Bb [1]. (c) An affected female must have received a recessive allele from her father as well as her mother, so if it were sex-linked her father would have to be brown; I-1 is black, so it cannot be sex-linked [2].

Examiner’s NotePart (c) is the mark most students never reach, and it needs only one individual and one sentence. Get into the habit of scanning a pedigree for an affected female with an unaffected father — that single pairing rules out X-linked recessive inheritance on its own. In part (b), writing BB for II-2 loses the mark even though it might be true.

Walkthrough 2 — Three crosses and a table of counts

In a species of tomato plant, hairy stems (H) are dominant to smooth stems (h). Three crosses were carried out and the offspring counted. Deduce the genotype of the hairy parent in each cross, and explain your reasoning. [6]

Cross	Parents	Offspring counts
1	hairy × hairy	96 hairy, 31 smooth
2	hairy × smooth	58 hairy, 61 smooth
3	hairy × smooth	124 hairy, 0 smooth

Divide by the smaller number

Cross 1: 96 ÷ 31 is about 3.1, so this is approximately 3 : 1. Cross 2: 58 and 61 are nearly equal, so approximately 1 : 1. Cross 3: every offspring is hairy, so all dominant. Do the arithmetic first — the ratio, not the raw counts, is what identifies the parents.

There are only three possibilities to check

A 3 : 1 ratio comes from Hh × Hh, so both parents in cross 1 are heterozygous. A 1 : 1 ratio from a hairy × smooth cross comes from Hh × hh, so the hairy parent in cross 2 is heterozygous. All-dominant offspring from a hairy × smooth cross means the hairy parent gave an H to every offspring, so it is HH.

Both are test crosses

The smooth parent must be hh, because smooth is the recessive phenotype. That means crosses 2 and 3 are test crosses: the recessive parent can only contribute h, so the offspring reveal exactly what the hairy parent contributed. Cross 2 shows that the hairy parent produced some h gametes, which proves Hh. Cross 3 shows no smooth offspring at all.

Two of the three are proofs; one is an inference

Crosses 1 and 2 prove Hh, because smooth offspring appeared, and a smooth offspring can only come from a parent carrying h. Cross 3 does not prove HH — a Hh parent could give 124 hairy offspring by chance, though the probability is vanishingly small. The honest wording is that the parent in cross 3 is almost certainly HH, and that a larger sample makes the conclusion safer.

The AnswerCross 1: Hh, because a 3 : 1 ratio requires both parents to be heterozygous, and the smooth offspring must each have received an h from both parents [2]. Cross 2: Hh, because a 1 : 1 ratio in a test cross means half the gametes from the hairy parent carried h [2]. Cross 3: HH is almost certain, because no smooth offspring appeared in 124; but the sample can never prove it absolutely [2].

Examiner’s NoteThe examiner is watching for the word “approximately” in step 1 and for a hedge in step 4. Absence of evidence is not proof, and Cambridge marks the difference between “proves” and “strongly suggests”. Notice also that you never needed to draw a Punnett square here — recognising the ratio was faster.

Walkthrough 3 — A family and their blood groups

The table shows the blood groups of a mother, a father and three children. One of the three children was adopted. (a) Give the genotypes of the mother and the father. (b) Deduce which child was adopted, explaining your reasoning fully. [6]

Person	Blood group
mother	AB
father	O
child 1	A
child 2	B
child 3	O

AB and O each have exactly one possible genotype

Group AB can only be I^AI^B, because both alleles must be present for both to be expressed. Group O can only be I^OI^O, because I^O is recessive to both of the others, so it can only show when nothing else is there. That is part (a) done, with no working needed.

The father is the constraint

The mother, I^AI^B, makes gametes carrying I^A or I^B. The father, I^OI^O, can only make gametes carrying I^O. Every child of theirs therefore receives an I^O from the father and either an I^A or an I^B from the mother.

Two combinations, two blood groups

The possible children are I^AI^O, which is group A, and I^BI^O, which is group B. Nothing else is available. Notice the surprise: no child of this couple can be group AB like the mother or group O like the father. Every child has a blood group that neither parent has.

Only one child is impossible

Child 1 is group A — possible. Child 2 is group B — possible. Child 3 is group O, which needs two I^O alleles, one from each parent. The mother has no I^O to give, so she cannot be child 3’s mother. Child 3 was adopted.

The Answer(a) Mother I^AI^B, father I^OI^O [2]. (b) Child 3 [1]. The mother can only pass on I^A or I^B, and the father can only pass on I^O, so every biological child must be I^AI^O (group A) or I^BI^O (group B) [2]. Group O requires I^OI^O, and the mother has no I^O allele to contribute [1].^O from the father, so no child can be group AB or group O.PARENTAL PHENOTYPESgroup AB×group OPARENTAL GENOTYPESIAIB×IOIOGAMETESIAIBIOIOIAIOIBIOIAIOIBIOIAIOIBIOOFFSPRING GENOTYPES1 IAIO1 IBIOOFFSPRING PHENOTYPESA : B : A : BRATIO1 A : 1 B

Examiner’s NoteAlways start an ABO question from the AB and O individuals, whose genotypes are certain, and use them to constrain everybody else. The reasoning mark here is for saying which allele is missing from a parent, not merely for naming the odd child out — “child 3, because O is not possible” would score one of the four available.

Walkthrough 4 — A colour-blindness pedigree and a probability

Fig. W4 shows red-green colour blindness in a family. (a) Give the genotypes of I-2 and II-1, using the symbols X^B and X^b. (b) II-1 and II-2 are expecting another child. State the probability that the child will be colour blind, and the probability that it will be colour blind if it is a boy. [6]

Every affected individual is male

I-1 is an affected male, II-3 is an affected male, III-1 is an affected male, and no female in the diagram is affected. A strong sex bias like that is the first sign of sex linkage, and it tells you immediately to abandon Bb notation and write X^B, X^b and Y instead. Choosing the notation is the first decision, and it decides everything downstream.

A son gets his X from his mother

II-3 is colour blind, so he is X^bY. His Y came from his father and his X came from his mother, so I-2 must carry an X^b. She is not colour blind herself, so she is a carrier: X^BX^b. Notice you did not need her father or anything else — one affected son is enough.

III-1 identifies his mother

III-1 is colour blind, so he is X^bY, and his X^b came from his mother II-1. II-1 has normal vision, so she too is a carrier: X^BX^b. This is what “skipping a generation” actually means — the allele has travelled through two unaffected women. II-2 has normal vision and is therefore X^BY.

Children and sons are different denominators

Cross X^BX^b × X^BY. The four boxes are X^BX^B, X^BX^b, X^BY and X^bY. Exactly one of the four children is colour blind, so the probability for a child is 1 in 4. But only two of the four are boys, and one of those two is colour blind, so the probability for a boy is 1 in 2. Both are correct answers to different questions.

The Answer(a) I-2 is X^BX^b [1] and II-1 is X^BX^b [1], because each has a colour-blind son who can only have received his X from his mother [1]. (b) The probability that the child is colour blind is 1 in 4 (25%) [1]; the probability that it is colour blind given that it is a boy is 1 in 2 (50%) [2].

Examiner’s NoteThe commonest lost mark in this question is answering 1 in 4 to both halves. Underline the word boy or son in the question stem before you look at your square. And notice the reasoning mark in (a): saying “she is a carrier” is not enough — the mark is for explaining that the son’s X must have come from her.

Walkthrough 5 — Four cells and a chromosome count

The table gives the number of chromosomes in the nuclei of four cells. Cells P, Q and R come from the same animal. (a) Name the type of division that produced cell Q, and explain your answer. (b) Explain how cell R came to have 38 chromosomes. (c) Explain why the count for cell S tells you nothing about the animal. [6]

Cell	Chromosomes in the nucleus
P — from the lining of the gut	38
Q — from an ovary, after division	19
R — a fertilised egg cell	38
S — from a root tip of the same species’ food plant	18

A gut lining cell is an ordinary body cell

Cell P is a body cell, so 38 is the diploid number for this animal — 19 pairs. Every body cell in the animal should have 38, and any cell with 19 has had its chromosome number halved. Establishing the diploid number first turns the rest of the question into arithmetic.

19 is half of 38

Cell Q has 19, which is haploid, so the chromosome number has been halved. Only meiosis halves it. The fact that the cell came from an ovary supports this — ovaries produce gametes — but the number is the evidence and the organ is only the context. Say “the number has been halved from 38 to 19, so it is meiosis”.

Two haploid nuclei, one diploid nucleus

Cell R is a fertilised egg cell, which is a zygote. It has 38 because fertilisation is the fusion of the nuclei of two gametes: a haploid egg nucleus with 19 fused with a haploid sperm nucleus with 19, giving 19 + 19 = 38. That is why meiosis has to halve the number in the first place — otherwise the total would double every generation.

S belongs to a different species

Cell S comes from a plant, which is a different species entirely, and chromosome number is not a measure of complexity or of relatedness. 18 in a plant tells you nothing about an animal with 38. Questions plant this kind of number to see whether you will try to compare across species — the only correct comment is that the two are unrelated.

The Answer(a) Meiosis [1], because the chromosome number has been halved from the diploid 38 to the haploid 19, and meiosis is the reduction division that produces gametes [2]. (b) The zygote formed when the nuclei of two haploid gametes fused at fertilisation, 19 + 19 = 38, restoring the diploid number [2]. (c) Cell S is from a different species, and the chromosome number of one species says nothing about another [1].

Examiner’s NoteThe mark in (a) is for the halving, not for the word meiosis on its own — an answer that just names the division scores one out of three. In (b) the phrase Cambridge wants is fusion of the nuclei of two gametes; “the sperm joined the egg” is loose and often only half credited.

Walkthrough 6 — One changed base and an enzyme that stops working

A bacterium normally makes an enzyme that breaks down a sugar. In one bacterium a single base in the gene for that enzyme is different, and the bacterium can no longer break the sugar down, although it still makes a protein of the usual length. (a) Explain, in terms of the gene and the protein, why the enzyme no longer works. (b) The bacterium still contains the gene. Explain why it still makes a protein at all. (c) Suggest one other kind of protein that a change like this could affect, and what the consequence would be. [7]

Bases set amino acids; amino acids set shape

The sequence of bases in a gene determines the sequence of amino acids in the protein it codes for. Change a base and you may change one amino acid. Different sequences of amino acids give proteins different shapes, so a changed amino acid can change the shape of the finished protein. Write the chain in that order and each link is a marking point.

Topic 5 arrives to finish the answer

An enzyme works because its active site has a shape complementary to its substrate. If the shape of the protein has changed, the substrate no longer fits the active site, so no enzyme-substrate complex forms and the sugar is not broken down. The whole of part (a) is the base-to-shape chain plus this one sentence about fit.

The protein-making steps are unchanged

A protein is still made because the gene is still there and the process is unaffected: mRNA is still made as a copy of the gene in the nucleus — or, in a bacterium, from its circular DNA — the mRNA still passes through a ribosome, and the ribosome still assembles amino acids into a protein. Only which amino acid goes in one position has changed, so the protein is the usual length but the wrong shape.

Three kinds of protein are named for you

DNA controls cell function by controlling the production of proteins, including enzymes, membrane carriers and receptors for neurotransmitters. So a change like this could alter a membrane carrier, meaning a particular ion or molecule can no longer be moved across the membrane by active transport; or a receptor for a neurotransmitter, meaning the neurotransmitter no longer fits and the impulse is not passed on at the synapse.

The Answer(a) The sequence of bases in the gene determines the sequence of amino acids [1]; a changed base changes one amino acid [1]; a different sequence of amino acids gives the protein a different shape [1]; the substrate no longer fits the active site, so the reaction is not catalysed [1]. (b) The gene is still present and is still copied into mRNA, which still passes through a ribosome where amino acids are still assembled into a protein — only one amino acid in the sequence is different [2]. (c) A membrane carrier, so a substance can no longer be moved across the membrane by active transport; or a receptor for a neurotransmitter, so the neurotransmitter no longer fits and the impulse is not passed on [1].

Examiner’s NotePart (a) is four separate marks for four separate links, and students routinely compress it into “the enzyme changed shape so it does not work”, which scores one. Write the chain out. Part (c) is testing whether you learned the three named kinds of protein rather than just the word enzyme — that list is in the syllabus for exactly this reason.

🔍 Spot the Difference

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Six pairs that look almost identical and have different answers. In this topic the distinction is nearly always where the marks live.

Question A

What does mitosis produce, and what is it for?

Two cells with the same chromosome number as the parent cell (diploid → diploid), genetically identical to it. Used for growth, repair of damaged tissues, replacement of cells and asexual reproduction.

Question B

What does meiosis produce, and what is it for?

Four cells with half the chromosome number (diploid → haploid), genetically different from the parent cell and from each other. Used for the production of gametes, and nothing else on this syllabus.

Key DifferenceThree facts differ — the number of cells, the chromosome number and whether the cells are identical — and each one is a separate mark. The stages of neither division are examinable, so any answer built on stage names is spending time on material that cannot be credited.

Question A

What is a genotype?

The genetic make-up of an organism, in terms of the alleles present. It is written in letters: Tt, bb, I^AI^O, X^BX^b.

Question B

What is a phenotype?

The observable features of an organism. It is written in words: tall, brown, group A, colour blind.

Key DifferenceThe link runs one way. From a genotype you can always name the phenotype; from a phenotype you often cannot name the genotype, because a black rabbit may be BB or Bb. That one-way gap is the reason the test cross exists, and it is why “BB or Bb” is so often the full-credit answer.

Question A

What does homozygous mean, and what is pure-breeding?

Two identical alleles of a particular gene: TT or tt. Two identical homozygous individuals bred together are pure-breeding — their offspring all show the same feature.

Question B

What does heterozygous mean?

Two different alleles of a particular gene: Tt. A heterozygous individual will not be pure-breeding, because it can produce two kinds of gamete.

Key DifferenceThe consequence is in the gametes, and that is where you use it. A homozygous parent makes only one kind of gamete; a heterozygous parent makes two. If your gamete line does not match the genotype above it, every line below is wrong — check that line first whenever a cross goes astray.

Question A

What is a gene?

A length of DNA that codes for a protein. A chromosome carries thousands of them, each at its own position.

Question B

What is an allele?

An alternative form of a gene. B and b are two alleles of one gene, found at the same position on each chromosome of a homologous pair.

Key DifferenceA gene is a place and a job; an allele is a version. They are the same length of DNA. Saying “B and b are two genes” is the error to avoid — there is one gene here, with two alleles, which is exactly why an individual has two of them and can pass on either.

Question A

In ordinary dominance, what does the heterozygote look like?

Exactly like the homozygous dominant. Bb looks the same as BB, so the 1 : 2 : 1 genotype ratio collapses into a 3 : 1 phenotype ratio.

Question B

In codominance, what does the heterozygote look like?

Like neither homozygote — it has its own visible phenotype, because both alleles contribute. C^RC^W is roan: red hairs and white hairs together. The ratio does not collapse; it stays 1 : 2 : 1.

Key DifferenceThe number of phenotypes is the giveaway. Two phenotypes in a 3 : 1 pattern means ordinary dominance; three phenotypes in a 1 : 2 : 1 pattern means codominance. If a question shows you data with three phenotypes, you should switch to superscript notation before you draw anything.

Question A

“What is the probability that their next child is colour blind?”

Count all four boxes of the Punnett square. For a carrier mother and a father with normal vision, one box in four is a colour-blind son, so the answer is 1 in 4.

Question B

“What is the probability that their next son is colour blind?”

Count only the boxes that are male. Two of the four boxes are sons, and one of those two is colour blind, so the answer is 1 in 2.

Key DifferenceSame square, same cross, two different denominators, and a factor of two between them. This is the most reliably lost mark in sex linkage. Underline the word child, son or daughter in the stem before you look at your square, and state which one you are answering.

🔗 Inheritance Concept Map

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Click each node. The whole topic is three frameworks: what the genetic material is and what it does, the two kinds of cell division, and one layout that answers five kinds of cross.

⭐ CORE FRAMEWORK 1

chromosome → gene → allele → protein → feature

The Four Sizes, and Which Word Goes Where ▶

A chromosome is a structure in the nucleus made of DNA, which carries genetic information in the form of genes. Chromosomes come in homologous pairs that carry the same genes in the same order.

A gene is a length of DNA that codes for a protein. An allele is an alternative form of a gene, found at the same position on each chromosome of a pair. That is why you have two alleles of every gene, and why a gamete carries only one of them.

A haploid nucleus has one set of chromosomes; a diploid nucleus has two. A human diploid cell has 23 pairs, so 46. These words describe the nucleus, not the cell.

How a Gene Actually Changes an Organism ▶

The sequence of bases in a gene determines the sequence of amino acids in a protein. Different sequences of amino acids give different shapes, and the shape decides what the protein can do.

Six steps, and no more: the gene stays in the nucleus; mRNA is a copy of the gene; it is made in the nucleus and moves to the cytoplasm; it passes through a ribosome; the ribosome assembles amino acids into a protein; the order of amino acids is set by the order of bases in the mRNA.

DNA controls cell function by controlling the production of proteins, including enzymes, membrane carriers and receptors for neurotransmitters. That is the bridge to Topic 5, Topic 3 and Topic 14 — and it is why a change in one gene can change a whole organism.

Most body cells contain the same genes; they differ because many genes are not expressed. A cell only makes the proteins it needs. Specialisation is not losing genes, it is switching most of them off.

⭐ CORE FRAMEWORK 2

two divisions: identical and diploid, or different and haploid

Mitosis: Two, Identical, Diploid ▶

Mitosis is nuclear division giving rise to genetically identical cells. Exact replication of the chromosomes happens before it, and during it the copies separate, maintaining the chromosome number.

Used for growth, repair of damaged tissues, replacement of cells and asexual reproduction. That last one is the link back to Topic 16 — asexual offspring are genetically identical because they are produced by mitosis.

Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised. They are not confined to embryos — bone marrow, skin and the lining of the gut all contain them, which is what makes those tissues replaceable for life.

Meiosis: Four, Different, Haploid ▶

Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid, giving genetically different cells. It is involved in the production of gametes.

The halving is not arbitrary. Fertilisation is the fusion of the nuclei of two gametes, so 23 + 23 = 46 and the chromosome number of the species stays constant. Without meiosis it would double every generation.

The genetic difference between gametes is where variation comes from, and it is why two children of the same parents are not alike. How meiosis creates that difference is not on this syllabus, and nor are the stages of either division.

⭐ CORE FRAMEWORK 3

one layout, five kinds of cross

The Seven Lines That Are Always the Same ▶

1 parental phenotypes (words) → 2 parental genotypes (letters) → 3 gametes, circled → 4 Punnett square → 5 offspring genotypes → 6 offspring phenotypes (words) → 7 ratio.

Cambridge awards marks for the layout itself. A correct ratio with no working typically scores one out of four; an incorrect ratio with a full correct layout usually scores three. Write all seven lines even when only the last one is asked for.

Two self-checks that catch nearly every slip: a homozygous parent gives one kind of gamete and a heterozygous parent gives two; and each gamete should appear in exactly two boxes of the square.

Choosing the Notation Before You Start ▶

One gene, a dominant and a recessive → monohybrid, letters B and b. Ratios to calculate: 3 : 1 (both parents heterozygous) and 1 : 1 (heterozygous × homozygous recessive). A homozygous dominant parent gives all the offspring the dominant phenotype, with no ratio to calculate.

An unknown genotype you are asked to identify → test cross with the homozygous recessive, because it can only give a recessive gamete, so nothing can be masked. Any recessive offspring proves heterozygous; no recessive offspring only suggests homozygous dominant.

A third phenotype that is neither parent → codominance, letters C^R and C^W, ratio 1 : 2 : 1 for both genotypes and phenotypes.

Blood groups → ABO: alleles I^A, I^B, I^O, with I^A and I^B codominant and I^O recessive to both. AB and O each have one possible genotype — start there.

A feature affecting mostly males, or the word carrier → sex linkage, symbols X^B, X^b and Y. The Y carries no allele for the gene, which is why a single X^b is enough in a male and why there is no such thing as a male carrier.

Reading a Pedigree Without Guessing ▶

Two unaffected parents with an affected child proves the allele is recessive, and makes both parents heterozygous. Two affected parents with an unaffected child proves it is dominant.

Write the certain genotypes first: everyone showing the recessive phenotype is homozygous recessive. Then anyone showing the dominant phenotype who has a recessive-phenotype parent or child must be heterozygous. Everyone else is “BB or Bb”, and writing that is a full-credit answer.

To test for sex linkage: many more affected males than females, the condition skipping a generation through unaffected mothers, and no affected father passing it to a son. An affected female with an unaffected father rules sex linkage out in a single sentence.

❌ “Why Is This Wrong?” Exercises

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Six answers of the kind that read fluently and score badly. Find the fault before you reveal it.

Exercise 1: “State two differences between mitosis and meiosis. [2]”

Student’s Answer“Mitosis makes two cells and meiosis makes four cells. Also mitosis happens in the body and meiosis happens in the reproductive organs.”

The FlawThe first difference is correct and earns its mark. The second is a difference in location, not in the process, and it is a consequence of the real difference rather than the difference itself — mark schemes rarely credit it. What has been left out is the pair of facts the question was really testing: the chromosome number, and whether the daughter cells are genetically identical.

Correct Answer“Mitosis produces two cells and meiosis produces four [1]. Mitosis maintains the chromosome number (diploid to diploid) while meiosis halves it (diploid to haploid) [1].” A third, equally creditworthy difference: mitosis gives genetically identical cells and meiosis gives genetically different ones.

Key RuleWhen a question asks for differences, give differences in the process, not in where it happens. And always have the chromosome-number difference and the identical-or-different difference at the front of your mind — they are the two the mark scheme prints.

Exercise 2: “Explain why a black rabbit might be BB or Bb, but a brown rabbit can only be bb. [3]”

Student’s Answer“Because black is the stronger colour, so it shows even if there is only one black allele. Brown is weaker so it needs two of them to be able to show through.”

The FlawThe biology is broadly right and the vocabulary destroys it. “Stronger” and “weaker” are on the refusal list, and “show through” suggests the brown allele is straining against something. Nothing is competing. The answer also never uses the words dominant, recessive or expressed, which are the three the marker is looking for.

Correct Answer“Black is caused by a dominant allele, which is expressed whenever it is present, so BB and Bb both give a black rabbit [1]. Brown is caused by a recessive allele, which is only expressed when no dominant allele is present [1], so a brown rabbit must have two brown alleles, bb [1].”

Key RuleRewrite “stronger” as “expressed when present” every single time. It is the same length and it is the difference between three marks and one.

Exercise 3: “Two heterozygous black rabbits are crossed. Use a genetic diagram to predict the ratio of the offspring. [4]”

Student’s Answer“Bb crossed with Bb gives 3 : 1, so three quarters will be black and one quarter brown.”

The FlawThe final ratio is correct and almost all the marks are gone. The question said use a genetic diagram, which is an instruction: the parental phenotypes, the parental genotypes, the circled gametes, the Punnett square and the offspring genotypes and phenotypes are each worth credit, and none of them is here. A bare correct ratio typically collects one mark of the four.

Correct AnswerWrite all seven lines. Parental phenotypes: black × black. Parental genotypes: Bb × Bb. Gametes: B and b from each parent, circled. Punnett square with those four gametes on the outside. Offspring genotypes: 1 BB : 2 Bb : 1 bb. Offspring phenotypes: black, black, black, brown. Ratio: 3 black : 1 brown.

Key RuleThe layout is the answer in this topic. If you can only remember one rule from this whole page, make it that one.

Exercise 4: “Explain why red-green colour blindness is more common in males than in females. [3]”

Student’s Answer“Because the allele for colour blindness is on the Y chromosome, and only males have a Y chromosome, so only males can get it.”

The FlawThis is confidently wrong in a way that also contradicts the data, because females do get red-green colour blindness — just far less often. The allele is on the X, not the Y, and the explanation has to be about how many X chromosomes each sex has, not about the Y carrying the allele.

Correct Answer“The allele is carried on the X chromosome [1]. A male is X^bY and has only one X, so a single recessive allele is expressed — there is no second X carrying a dominant allele to mask it, and the Y carries no allele for this gene [1]. A female has two X chromosomes, so she needs two copies, X^bX^b, to be colour blind, which is much less likely [1].”

Key RuleCheck your answer against the observation. If your explanation makes a female colour-blind impossible, it is wrong, because such women exist. An explanation must fit all of the data, not just the headline.

Exercise 5: “A woman of blood group AB and a man of blood group O have a child of group A. Explain how. [3]”

Student’s Answer“The A from the mother was dominant over the O from the father, and the B was recessive, so the child came out as group A.”

The FlawOne phrase is right — I^A is dominant to I^O — and the rest invents a rule that does not exist. I^B is not recessive; it is codominant with I^A, and the reason the child is not group AB is not that B lost a contest but that the child never received an I^B allele at all. The answer has confused which alleles were inherited with how they interact.

Correct Answer“The mother is I^AI^B and the father is I^OI^O [1]. The mother passes on either I^A or I^B, and the father can only pass on I^O [1]. This child received I^A from the mother and I^O from the father, giving I^AI^O, and since I^O is recessive to I^A the child is group A [1].”

Key RuleSeparate the two questions every time: which alleles did this individual inherit, and then how do those two interact? Most ABO errors come from answering the second question about alleles the person never had.

Exercise 6: “A cross between two heterozygous plants gave 61 tall and 19 dwarf offspring. Explain what this shows. [3]”

Student’s Answer“It shows a ratio of 61 : 19, which is not 3 : 1, so something else must be affecting the result, possibly the environment.”

The FlawThe student has done no arithmetic and then explained away the result they did not calculate. 61 divided by 19 is about 3.2, which is approximately 3 : 1 — exactly what two heterozygous parents predict. Reaching for the environment to explain ordinary sampling variation is a real and common error, and it throws away all three marks.

Correct Answer“61 : 19 simplifies to approximately 3 : 1 [1], which is the ratio expected from a cross between two heterozygous parents, Tt × Tt [1]. The numbers are not exactly 60 : 20 because fertilisation is random, so the actual numbers vary by chance around the predicted ratio [1].”

Key RuleDivide by the smaller number before you say anything. And when real data are close to a predicted ratio, the explanation for the gap is random fertilisation and sample size — not a new biological mechanism.

✍️ Ultra-Detailed Practice Questions

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Ten Cambridge-style challenge questions, each drawing on more than one sub-topic. Write your answer first, then reveal the model answer and the examiner notes.

Question 1

[6 marks]

(a) Define the terms gene and allele. [2] (b) Explain what is meant by a haploid nucleus, and state how many chromosomes are in a human haploid nucleus. [2] (c) A student says that a sperm cell is “half a cell”. Explain why this is wrong. [2]

Model Answer(a) A gene is a length of DNA that codes for a protein [1]. An allele is an alternative form of a gene [1].
(b) A haploid nucleus contains a single set of chromosomes [1]; in a human that is 23 [1].
(c) Haploid and diploid describe the nucleus, not the cell [1]. A sperm is a complete cell with a membrane, cytoplasm and mitochondria; it is its nucleus that contains one set of chromosomes rather than two [1].

Examiner’s NotesPart (c) is the one that separates candidates. The whole point of the haploid and diploid vocabulary is that it describes what is in the nucleus, and a great many students carry a mental picture of gametes as half-built cells. Notice also that part (a) is worth two marks for two memorised sentences — the cheapest marks in the topic.

Question 2

[7 marks]

(a) State three roles of mitosis in a mammal. [3] (b) Explain why the cells produced by mitosis must be genetically identical for two of those roles to work. [2] (c) A student writes that stem cells divide by meiosis because they can become any kind of cell. Explain why this is wrong. [2]

Model Answer(a) Any three of: growth; repair of damaged tissues; replacement of cells; asexual reproduction [3].
(b) In repair, the new cells must be the same kind as the tissue they are replacing, which requires the same genes [1]. In growth, every new cell must carry the same genetic information as the rest of the organism, or the organism would not be a single functioning individual [1].
(c) Stem cells divide by mitosis, which produces genetically identical diploid cells [1]. Meiosis halves the chromosome number and produces gametes only — a haploid cell could not become a body cell [1].

Examiner’s NotesPart (c) rewards the chromosome-number argument rather than a flat contradiction. “No, it is mitosis” scores one at best; adding why meiosis could not do the job — because the product would be haploid — earns the second mark. That habit of explaining why the wrong answer is impossible is worth practising.

Question 3

[8 marks]

In a species of fruit fly, normal wings (N) are dominant to vestigial wings (n). (a) Use a genetic diagram to predict the offspring of a cross between two heterozygous normal-winged flies. [5] (b) The cross produced 148 normal-winged and 44 vestigial-winged flies. Explain whether these results support your prediction. [3]

Model Answer(a) Parental phenotypes: normal-winged × normal-winged [1]. Parental genotypes: Nn × Nn [1]. Gametes: N and n from each parent, circled [1]. Punnett square giving NN, Nn, Nn, nn [1]. Offspring phenotypes and ratio: 3 normal-winged : 1 vestigial-winged [1].
(b) 148 ÷ 44 is about 3.4, so the results are approximately 3 : 1 [1] and therefore do support the prediction [1]. They are not exact because fertilisation is random, so real numbers vary by chance around the expected ratio [1].

Examiner’s NotesFive marks in part (a) for five lines of layout — this is the standard shape of a Paper 4 genetics question and it is why the drill matters. In part (b), the mark for “approximately” and the mark for “random fertilisation” are separate; giving only one of them halves the score.

Question 4

[7 marks]

A farmer has a black bull and wants to know whether it is homozygous. (a) Name and describe the cross the farmer should carry out. [3] (b) State what result would prove the bull is heterozygous, and explain why. [2] (c) The farmer carries out the cross and gets 12 black calves. Explain what the farmer can and cannot conclude. [2]

Model Answer(a) A test cross [1]: cross the bull with a brown cow, which must be homozygous recessive (bb) [1], and examine the phenotypes of the calves [1].
(b) Any brown calf proves the bull is heterozygous [1], because a brown calf must be bb and so must have received a b allele from the bull as well as from the cow [1].
(c) The farmer can conclude that the bull is probably homozygous BB [1], but cannot be certain, because a heterozygous bull could produce 12 black calves by chance; more calves would make the conclusion safer [1].

Examiner’s NotesThe three parts test three different things: the method, the logic, and the limits of the evidence. Part (c) is the one to be careful with — write “probably” or “suggests”, never “proves”. Cambridge marks that distinction deliberately, because it is a genuine point about experimental evidence and not just wording.

Question 5

[8 marks]

(a) Explain what is meant by codominance. [2] (b) In cattle, C^RC^R is red, C^WC^W is white and C^RC^W is roan. Use a genetic diagram to show the offspring of a roan bull crossed with a roan cow. [4] (c) Explain why the phenotype ratio in this cross is different from the 3 : 1 ratio you get in an ordinary monohybrid cross. [2]

Model Answer(a) Both alleles in a heterozygous organism contribute to the phenotype [1]; a roan animal has red hairs and white hairs together, so both alleles are expressed [1].
(b) Parental phenotypes roan × roan; parental genotypes C^RC^W × C^RC^W [1]; gametes C^R and C^W from each, circled [1]; Punnett square giving C^RC^R, C^RC^W, C^RC^W, C^WC^W [1]; ratio 1 red : 2 roan : 1 white [1].
(c) In ordinary dominance the heterozygote looks like the homozygous dominant, so the 1 : 2 : 1 genotype ratio collapses into a 3 : 1 phenotype ratio [1]. In codominance the heterozygote has its own phenotype, so the phenotype ratio stays 1 : 2 : 1 [1].

Examiner’s NotesPart (c) is the conceptual heart of codominance and it is worth rehearsing the word collapses. Note also that the genotype ratio is 1 : 2 : 1 in both kinds of cross — what changes is only whether that ratio is still visible in the phenotypes.

Question 6

[8 marks]

A woman of blood group B and a man of blood group A have four children, of blood groups A, B, AB and O. (a) Give the genotypes of the two parents, explaining how you worked them out. [4] (b) Give the genotype of the child of group O. [1] (c) Explain why the child of group AB has a blood group that neither parent has. [3]

Model Answer(a) The mother is I^BI^O and the father is I^AI^O [2]. Both must be heterozygous, because one child is group O, which is I^OI^O [1], and that child must have received an I^O from each parent [1].
(b) I^OI^O [1].
(c) That child received I^A from the father and I^B from the mother [1]. I^A and I^B are codominant [1], so both are expressed and the phenotype is group AB, which neither parent shows because neither parent has both alleles [1].

Examiner’s NotesThe whole question is unlocked by the group O child, whose genotype is certain. Work backwards from certainty: the O child forces an I^O into both parents, and everything else follows. Part (c) needs the word codominant explicitly — describing the outcome without naming the relationship usually loses a mark.

Question 7

[9 marks]

Red-green colour blindness is caused by a recessive allele carried on the X chromosome. (a) Explain why there is no such thing as a male carrier. [2] (b) Use a genetic diagram to show the children of a carrier mother and a colour-blind father. [5] (c) State the probability that a daughter of this couple is colour blind. [2]

Model Answer(a) A carrier has the allele but does not express it, which requires a second X chromosome carrying the dominant allele to mask it [1]. A male has only one X, and his Y carries no allele for this gene, so whatever is on his X is expressed — X^bY is colour blind, not a carrier [1].
(b) Parental phenotypes: carrier female × colour-blind male [1]. Parental genotypes: X^BX^b × X^bY [1]. Gametes: X^B and X^b from the mother; X^b and Y from the father, circled [1]. Punnett square giving X^BX^b, X^bX^b, X^BY, X^bY [1]. Phenotypes: carrier daughter, colour-blind daughter, normal-vision son, colour-blind son — ratio 1 : 1 : 1 : 1 [1].
(c) 1 in 2 (50%) [1], because two of the four boxes are daughters and one of those two is X^bX^b [1].

Examiner’s NotesPart (c) is the classic denominator trap: 1 in 4 of the children is a colour-blind daughter, but the question asked about daughters, so the answer is 1 in 2. The second mark is for saying why — naming which boxes you counted. Get into the habit of writing “two of the four boxes are daughters” before you give the number.

Question 8

[8 marks]

A pedigree shows that two individuals with normal hearing have a child who is deaf, and that in the same family a deaf woman has a father with normal hearing. (a) State, with a reason, whether the allele for this form of deafness is dominant or recessive. [2] (b) State, with a reason, whether it is sex-linked. [3] (c) Give the genotypes of the two hearing parents, using D and d. [3]

Model Answer(a) Recessive [1], because two parents with normal hearing have a deaf child, and a dominant allele would have to be expressed in at least one parent [1].
(b) Not sex-linked [1]. A female showing a sex-linked recessive condition would have to be X^dX^d, receiving one recessive allele from each parent, including from her father [1]; her father would therefore have to be deaf, and he has normal hearing, so the allele cannot be on the X chromosome [1].
(c) Both are Dd [2], because each must carry a recessive allele to have passed one to their deaf child, but neither shows the condition [1].

Examiner’s NotesThis is the most examinable pedigree question there is, because it asks the two structural questions in order: is it dominant or recessive, and is it sex-linked? Part (b) needs the full argument, not just the conclusion — the affected female and her unaffected father, spelled out. Three marks, three sentences.

Question 9

[8 marks]

(a) Explain how the sex of a human is determined at fertilisation. [3] (b) A couple have four daughters and no sons. Explain why this does not mean they are more likely to have a son next. [2] (c) A student writes: “The father decides whether the baby is a boy or a girl.” Rewrite this sentence so that it would earn a mark. [3]

Model Answer(a) A female is XX and a male is XY [1]. All egg cells carry an X chromosome, while half the sperm carry an X and half carry a Y [1]. If a sperm carrying an X fertilises the egg cell the zygote is XX and develops as a female; if a sperm carrying a Y does, the zygote is XY and develops as a male [1].
(b) Each fertilisation is independent, with a 1 in 2 chance of each sex every time [1]; previous children do not change which sperm reaches the egg cell next [1].
(c) “The sex of the child depends on whether the sperm that fertilises the egg cell carries an X or a Y chromosome [2], and since half the sperm carry each, there is a 1 in 2 chance of each sex at every fertilisation [1].”

Examiner’s NotesPart (c) is a rewriting exercise, and rewriting is a good habit generally: the original sentence is not factually far off, but it describes an intention rather than a mechanism. Mark schemes want the mechanism. Note that the marks in (a) come from the three separate steps, so a single sentence saying “XX is female and XY is male” scores one of three.

Question 10

[10 marks]

This question draws the whole topic together. (a) Explain how a change in a single base in a gene could stop an enzyme working. [4] (b) Explain why a change of this kind in a cell of the skin would not be passed to the person’s children, but the same change in a cell in an ovary might be. [3] (c) Explain why a species in which every individual reproduced asexually would be at risk from a new disease. [3]

Model Answer(a) The sequence of bases in a gene determines the sequence of amino acids in the protein [1]; a changed base can change one amino acid [1]; a different sequence of amino acids gives a different shape [1]; the substrate no longer fits the active site, so the reaction is not catalysed [1].
(b) A skin cell divides by mitosis and produces more body cells, which are never passed to offspring [1]. A cell in an ovary can undergo meiosis to produce gametes [1], and a gamete carrying the change could be involved in fertilisation, so the change would be present in the zygote and in every cell of the child [1].
(c) Asexual reproduction produces offspring by mitosis, so they are genetically identical to the parent and to each other [1]; there is therefore no genetic variation in the population [1]; if the parent has no resistance to the new disease then no individual has any, so the whole population could be destroyed [1].

Examiner’s NotesA ten-mark question that runs from 17.1 to 17.2 to 17.3 and out into Topic 16, which is exactly how the longest question on a real paper is built. Part (b) is the one worth rehearsing: only changes in cells that can produce gametes can be inherited, and the reason is the route — meiosis, gamete, fertilisation, zygote, every cell of the child.