S3 Science: Heredity, DNA And Breeding (Paper 1 (basics))

S3 Science: Heredity, DNA and Breeding (Paper 1 (basics))

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 1 (basics))

This paper starts from chromosome number and practises the difference between body cells and gametes, the position of genes on chromosomes, dominant and recessive notation, the phenotype of Tt, how sex is determined, with basic work on the 3 to 1 ratio and probability.

Direct answers　Body cells have 46 chromosomes and gametes 23; genes sit on chromosomes; a dominant allele shows with one copy; Tt is tall; the sperm X or Y decides the sex; Tt by Tt gives three quarters tall; 400 plants give 300 tall; the chance of a girl is 50%.

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)

FoundationHow many chromosomes are there in a human body cell?
FoundationHow many chromosomes does a human sperm or egg cell carry?
FoundationWhere are genes located in a cell?
IntermediateWhat is a dominant gene?
IntermediateA pea plant has the genotype Tt. What is its phenotype?
IntermediateIs a child sex decided mainly by the father or the mother?
ChallengeTwo pea plants with genotype Tt are crossed. What fraction of the offspring is tall?
ChallengeSuch a cross produces 400 pea plants. About how many are expected to be tall?
ChallengeA couple has one child. What is the chance that it is a girl?
Error redoA student says the mother decides whether the baby is a boy or a girl. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	46 chromosomes, or 23 pairs	Body cells carry chromosomes in pairs, one set from each parent.
2	23 chromosomes each	Gametes are made by meiosis, so each carries half the body cell number, and 46 divided by 2 gives 23.
3	On the chromosomes, which lie inside the nucleus	A gene is a length of DNA carrying hereditary information, and DNA with protein makes up a chromosome.
4	A gene that shows its effect even when only one copy is present	It is written with a capital letter, such as T for tall, so Tt also shows the tall phenotype.
5	Tall stem	T is dominant over t, so a single T is enough for the tall phenotype.
6	By the father, because a sperm may carry either X or Y	The egg always carries X, so an X sperm gives a girl (XX) and a Y sperm gives a boy (XY).
7	Three quarters, that is 75%	Tt by Tt gives 1 TT : 2 Tt : 1 tt, and three of the four carry T, so 3 divided by 4 gives 0.75 or 75%.
8	About 300 plants	Tall plants are three quarters of the total, and 400 times 0.75 gives 300.
9	One half, that is 50%	Half of the sperm carry X and half carry Y, so each sex has an even chance, and 1 divided by 2 gives 50%.
10	The mother only gives an X egg, so the father sperm carrying X or Y decides the sex	Both of the mother sex chromosomes are X, so she has no Y to give and the sperm decides the sex.

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.