S5 Biology: Genetics And Mendelian Inheritance (Paper 1 (basics))

S5 Biology: Genetics and Mendelian Inheritance (Paper 1 (basics))

2026-11-14

Tutor Prep Guide · dominant and recessive alleles, genotype and phenotype, monohybrid crosses, Punnett squares, F1 and F2 ratios, test crosses, incomplete dominance and codominance, ABO blood groups, X-linked traits, pedigree analysis and probability calculations

S5 Biology: Genetics and Mendelian Inheritance (Paper 1 (basics))

This paper starts from the basic monohybrid cross: write the gametes of each parent, complete a Punnett square, read the genotype and phenotype ratios of F1 and F2, and turn ratios into actual plant and seed counts.

Direct answers　Beginners often invert the dominant ratio or forget that Tt shows only the dominant trait. Write the parental genotypes first, pair the gametes cell by cell, and state the ratio last.

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: S5 Biology: Genetics and Mendelian Inheritance
Level	S3

02　Questions (10 items)

FoundationWhat phenotype ratio is expected in the offspring of a self-crossed heterozygous tall pea plant Tt?
FoundationWhat is the phenotype of the F1 from a cross between pure-bred tall TT and pure-bred short tt?
FoundationHow do dominant and recessive alleles differ in expression inside a heterozygote?
IntermediateIn 200 offspring from Tt x tt, how many are expected to be short?
IntermediateRound R is dominant to wrinkled r. Out of 800 seeds from Rr x Rr, how many are expected to be wrinkled?
IntermediateHow many kinds of gametes does a plant of genotype Tt produce in meiosis?
ChallengeA single-gene trait gives 1200 F2 plants. How many are expected to be tall?
ChallengeA test cross pairs an individual of unknown genotype with which kind of individual?
ChallengeA couple with blood groups IA i and IB i can have children of how many blood groups?
Error redoError clinic: a student wrote the F2 ratio 3 : 1 as three short to one tall. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	Tall : short = 3 : 1.	TT : Tt : tt appears in 1 : 2 : 1, and only tt is short.
2	All are tall, and all have genotype Tt.	T is dominant over t, so the heterozygote Tt is tall.
3	The dominant allele is expressed while the recessive one is masked.	A heterozygote shows only the dominant trait; the recessive trait appears only when homozygous.
4	100 plants.	Tt x tt gives 1 Tt : 1 tt, so half are short.
5	200 seeds.	Wrinkled seeds take one quarter, and 800 divided by 4 is 200.
6	Two kinds, one carrying T and one carrying t.	Homologous chromosomes separate so each gamete receives only one allele.
7	900 plants.	The dominant class takes three quarters, and 1200 times 3 over 4 is 900.
8	A homozygous recessive individual such as tt or rr.	The recessive parent masks nothing, so offspring phenotypes reveal the unknown gametes.
9	Four: A, B, AB and O.	IA and IB are codominant and i is recessive, so all four combinations are possible.
10	Three tall to one short.	The dominant class is the larger one in F2, so it is written first.

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.

Sort reproduction questions by place and order first: the testes make sperm, the ovaries make eggs, fertilisation happens in the oviduct and implantation in the uterus, while the placenta and umbilical cord carry nutrients and oxygen; for cycle questions draw the days on one timeline and remember the fertile window runs about three days before and one day after ovulation.Tutor Registration →

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