S3 Science: Heredity, DNA And Breeding (Paper 3 (challenge))

S3 Science: Heredity, DNA and Breeding (Paper 3 (challenge))

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 3 (challenge))

This paper deals with the origin of the 3 to 1 and 1 to 1 ratios, the use of test crosses, and why sexual reproduction increases variation, challenging learners with 25% from Dd by Dd, G at 30% from A at 20%, 12 gamete chromosomes from 24, and recessive counts in samples of 1200, 640 and 1600.

Direct answers　A 3 to 1 ratio comes from Tt by Tt; a test cross uses a homozygous recessive; meiosis and random fertilisation raise variation; Dd by Dd gives one quarter recessive; A at 20% gives G at 30%; 24 chromosomes give 12 in a gamete; one quarter recessive and three quarters dominant; heterozygous parents can still have one quarter recessive children.

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 3 to 1 phenotype ratio in the offspring usually comes from which cross?
FoundationWhat is the purpose of a test cross?
FoundationWhy does sexual reproduction increase variation in the offspring?
IntermediateTwo hearing parents are both Dd. What is the chance of a deaf child with dd?
IntermediateIn a DNA sample adenine A is 20%. What percentage is guanine G?
IntermediateAn organism has 24 chromosomes in its body cells. How many are in a gamete?
ChallengeA Tt by Tt cross yields 1200 plants. About how many are expected to be short?
ChallengeTwo Tt fruit flies produce 640 offspring. About how many show the recessive trait?
ChallengeA Tt by Tt cross yields 1600 plants. About how many are tall?
Error redoA student says tall parents always have tall children. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	A cross between two heterozygous individuals, such as Tt by Tt	The genotypes are 1 TT : 2 Tt : 1 tt, and the first three show the dominant trait, so phenotypes are 3 to 1.
2	To cross an unknown individual with a homozygous recessive one to find its genotype	If all offspring show the dominant trait the unknown is probably homozygous, and any recessive offspring show it is heterozygous.
3	Meiosis shuffles chromosomes and fertilisation combines gametes at random	Each gamete carries a different mix of alleles, and random fertilisation makes repeated combinations very unlikely.
4	One quarter, that is 25%	Dd by Dd gives 1 DD : 2 Dd : 1 dd, and only dd is deaf, so 1 divided by 4 gives 25%.
5	30%	A and T are equal so together they are 40%, and the remaining 60% is split evenly between G and C, giving 30% each.
6	12 chromosomes	Meiosis halves the chromosome number, and 24 divided by 2 gives 12.
7	About 300 plants	Short is the recessive trait shown only by tt, which is one quarter, and 1200 divided by 4 gives 300.
8	About 160 flies	The recessive trait is one quarter of the offspring, and 640 times 0.25 gives 160.
9	About 1200 plants	Tall plants are three quarters, and 1600 times 0.75 gives 1200, leaving 400 short plants.
10	If both parents are heterozygous Tt, one quarter of the children can still be short	Height is also affected by environment such as nutrition, so genes set a tendency rather than a certainty.

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.