What is the commonest genetics mistake?

Applying the 3:1 ratio before writing the parental genotypes; test crosses and homozygous parents then break it.

Do units matter in magnification questions?

Yes: mm and micrometres differ by a factor of 1000, so a missed conversion is out by three orders of magnitude.

Secondary Biology: Inheritance and Mendel (Paper 3)

2026-10-31

Ten original questions on XY sex determination, the chance of a daughter, short stem counts from a Tt cross, AB and O blood groups, colour blindness in sons, and the fallacy that a fourth child must be a boy.

Direct answer　The father decides sex and each fertilisation is independent; a recessive trait shows only when homozygous.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: Secondary Biology: Inheritance and Mendel
Level	4 basic, 3 advanced, 2 challenge and 1 redo question

02　Questions (10 items)

FoundationWhat pair of sex chromosomes does a human male have?
FoundationWhich sex chromosome is carried by a human egg cell?
FoundationWhat is the chance that a child of any couple is a girl?
FoundationWhen does a recessive allele show in the phenotype?
IntermediateTwo Tt pea plants are crossed and produce 1200 seeds. How many are expected to give short stems?
IntermediateAn AB father and an O mother have a child. Which blood groups are possible?
IntermediateA carrier mother (XCXc) and a father with normal colour vision (XCY) have a son. What is his chance of being colour blind?
ChallengeWhat is a gene mutation, and which mutations can be passed to the next generation?
ChallengeBoth parents are Aa. Out of 600 offspring, how many aa are expected, and can the real number differ?
Error redoSomeone says that after three daughters the fourth child must be a son. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	XY.	The mother gives an X and the father gives X or Y, so the father decides the sex.
2	X.	A female is XX, so every egg carries an X after meiosis.
3	One half.	Half the sperm carry X and half carry Y, so the chance is one half each time.
4	When both alleles are recessive, that is, when the individual is homozygous recessive.	One dominant allele present is enough to hide the recessive trait.
5	300 seeds (1200 multiplied by one quarter).	A Tt by Tt cross gives three tall to one short.
6	Group A or group B; groups AB and O are not possible.	The father gives A or B and the mother gives i, so the child is IAi or IBi.
7	One chance in two, because half of her X chromosomes carry the allele.	A son takes his X from his mother, and half of her X chromosomes carry the allele.
8	A permanent change in the structure of a gene; only mutations in reproductive cells are inherited.	A mutation in a body cell affects only that individual and is not passed on.
9	150 are expected (600 multiplied by one quarter), though the real number can differ because fertilisation is random.	Probability gives an expectation, and small samples vary more.
10	Each fertilisation is independent, so the chance of a girl stays one half and earlier births do not affect the next one.	Adding up the chances of independent events is the gambler fallacy.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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