S3 Science: Heredity, DNA And Breeding (Paper 4 (error Clinic))

S3 Science: Heredity, DNA and Breeding (Paper 4 (error clinic))

2026-11-14

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Tutor Prep Guide · chromosomes and genes, the DNA double helix and base pairing, dominance and recessiveness, genotype and phenotype, homozygous and heterozygous, sex determination, meiosis and sexual reproduction, heredity and environment, mutation, selective breeding and genetic modification

S3 Science: Heredity, DNA and Breeding (Paper 4 (error clinic))

This paper corrects ten common heredity errors one by one, covering gene location, gamete chromosome number, the belief that recessive alleles are not inherited, the phenotype of Tt, the chance of a boy, base pairing, monohybrid ratios, base percentages, body cell against gamete, and cloning against diversity.

Direct answers　Genes are on chromosomes; gametes have 23; recessive alleles are still inherited; Tt is tall; a boy has a 50% chance; A pairs with T and C with G; short plants are one quarter of a Tt by Tt cross; A at 30% gives G at 20%; a body cell has 46 against 23 in a gamete; cloning lowers diversity.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	20
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: S3 Science: Heredity, DNA and Breeding
Level	S3

02　Questions (10 items)

FoundationA student answers that genes are in the cytoplasm. What is the correct answer?
FoundationA student says a human gamete has 46 chromosomes. What is wrong?
FoundationA student says recessive alleles are not inherited. What is wrong?
IntermediateA student says a Tt pea plant is short. Why is that wrong?
IntermediateA student says the chance of a boy is three quarters. What is correct?
IntermediateA student says A pairs with G in DNA. What is the correct pairing?
ChallengeA student works out a Tt by Tt cross of 1000 plants and says 500 are short. What is correct?
ChallengeSeeing that A is 30%, a student answers that G is 30% too. What is correct?
ChallengeA student copies 46 chromosomes of a body cell as the gamete number. By how many is that wrong?
Error redoA student says cloning increases genetic diversity in a population. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Genes are on the chromosomes inside the nucleus	Chromosomes in the nucleus are made of DNA with protein, and genes are lengths of that DNA.
2	A gamete has 23, half the body cell number	Meiosis halves the number and fertilisation restores 46, so 46 minus 23 gives 23.
3	Recessive alleles are inherited normally but only show when homozygous as tt	A parent carrying t can pass it on, and the child can pass it further, so a recessive allele never disappears.
4	T is dominant, so one T is enough for a tall stem	Only tt, with two recessive alleles, gives a short stem.
5	One half, that is 50%	X and Y sperm are equally numerous so the sexes are equally likely; three quarters belongs to a monohybrid cross and has been confused.
6	A pairs with T and C pairs with G	A purine pairs with a pyrimidine, with two hydrogen bonds between A and T and three between C and G.
7	250 plants	Short is recessive and only one quarter, and 1000 times 0.25 gives 250; 500 comes from wrongly using a 1 to 1 ratio.
8	20%	A equals T, so together they take 60%, and the remaining 40% is shared equally by G and C at 20% each.
9	A gamete has 23, which is 23 fewer than a body cell	Subtracting 23 from 46 gives 23, and halving is the defining result of meiosis.
10	Cloning produces genetically identical individuals, so diversity falls	Diversity comes mainly from mutation and the reshuffling of sexual reproduction, and cloning involves neither.

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.

Separate the three levels first: chromosomes carry genes, genes are made of DNA, and DNA carries information by base pairing; for ratio questions write Tt by Tt as 1 TT : 2 Tt : 1 tt and then check whether the question asks for phenotype or genotype.Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.