S3 Science: Heredity, DNA And Breeding (Paper 2 (intermediate))

S3 Science: Heredity, DNA and Breeding (Paper 2 (intermediate))

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 2 (intermediate))

This paper moves on to alleles, homozygous and heterozygous, meiosis, base pairing, asexual reproduction and selective breeding, with work on deducing other bases from one percentage, the chance of three boys in a row, and the 1 to 1 ratio of Tt by tt.

Direct answers　Alleles are versions of one gene; homozygous is TT or tt and heterozygous is Tt; meiosis halves the chromosome number; A pairs with T and C with G; asexual offspring are copies; selective breeding picks traits each generation; A at 30% gives T at 30% and G at 20%; three boys have a 12.5% chance; Tt by tt gives 50% tall.

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)

FoundationWhat are alleles?
FoundationWhat is the difference between homozygous and heterozygous?
FoundationWhat is the key difference between meiosis and ordinary cell division?
IntermediateHow do the four DNA bases pair up?
IntermediateWhy are offspring of asexual reproduction almost identical to the parent?
IntermediateHow is selective breeding carried out?
ChallengeIn a DNA sample adenine A is 30% of all bases. What percentages are thymine T and guanine G?
ChallengeA couple plans three children. What is the chance that all three are boys?
ChallengeA Tt pea plant is crossed with a tt plant. What fraction of the offspring is tall?
Error redoA student says genetic modification is the same as cross breeding. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Different versions of the same gene at the same position on homologous chromosomes	T and t are both alleles for pea stem height, one inherited from each parent.
2	Homozygous has two identical alleles (TT or tt) while heterozygous has two different ones (Tt)	A homozygous plant makes one kind of gamete and a heterozygous plant makes two kinds.
3	Meiosis halves the chromosome number to form gametes	The chromosomes are copied once but the cell divides twice, so each of the four gametes has half the number.
4	A pairs with T and C pairs with G	The pairing rule is fixed, so the sequence of one strand determines the sequence of the other.
5	There is no meiosis or fertilisation, so the genetic material is the same as the parent	Cuttings and runners produce copies of the parent, and variation comes only from mutation.
6	Individuals with the desired trait are chosen to breed, generation after generation	Dairy farmers keep breeding the highest yielding cows, so average milk yield rises over generations.
7	T is 30% and G is 20%	A equals T so T is also 30%, and the remaining 40% is shared equally by C and G, giving 20% each.
8	One eighth, that is 12.5%	Each child has a one half chance of being a boy, and multiplying three times gives one eighth, or 12.5%.
9	One half, that is 50%	Tt by tt gives only Tt and tt in equal numbers, and only Tt is tall.
10	Genetic modification inserts a gene from another species directly, while cross breeding only mates related varieties	Modification transfers one known gene across species, whereas cross breeding mixes whole sets of genes, so the methods and precision differ.

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.