S5 Biology: Genetics And Mendelian Inheritance (Paper 4 (error Clinic))

S5 Biology: Genetics and Mendelian Inheritance (Paper 4 (error clinic))

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 4 (error clinic))

This paper corrects ten common errors in Mendelian genetics one by one, covering inverted ratios, heterozygote phenotype, the lost recessive allele, blood-group combinations, the direction of X-linked transmission and the partner used in a test cross.

Direct answers　In error-clinic work, state the correct answer and then the reason. Fix dominance first in ratio questions, list gametes first in blood-group questions, and trace the sex chromosome in pedigree questions.

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)

FoundationError clinic: the F2 ratio 3 : 1 was written as three short to one tall. What should it be?
FoundationError clinic: Tt was described as short. What is the correct phenotype?
FoundationError clinic: the recessive allele was said to vanish in F1. What is correct?
IntermediateError clinic: parents IA i and IB i were said to be unable to have a type O child. What is the probability?
IntermediateError clinic: a colour-blind father was said to pass the allele to his sons. What is correct?
IntermediateError clinic: a test cross was said to use a homozygous dominant partner. What is correct?
ChallengeError clinic: from Rr x Rr, 400 of 800 seeds were said to be wrinkled. What is correct?
ChallengeError clinic: AB and O parents were said to be able to have an AB child. What is the probability?
ChallengeError clinic: two tall plants were assumed to share one genotype. What is correct?
Error redoError clinic: Tt x tt was said to give a 3 : 1 ratio. What ratio and count are correct?

03　Answers and one-line notes

#	Answer	One-line note
1	Three tall to one short.	The dominant class is the larger one in F2.
2	Tt is tall.	T is dominant, so one dominant allele is enough.
3	The recessive allele remains present but is not expressed.	A heterozygote carries the recessive allele, which reappears in F2.
4	They can, with probability one quarter, that is 25 percent.	Each parent can pass i, and ii appears once in four combinations.
5	No, a father gives sons only Y; the X-linked allele goes to his daughters.	Sons receive X from the mother, so a carrier mother is needed for a colour-blind son.
6	Use a homozygous recessive partner such as tt.	The recessive parent masks nothing, so offspring phenotypes reveal the unknown gametes.
7	200 seeds.	Wrinkled seeds take one quarter, and 800 divided by 4 is 200.
8	Zero percent, a type AB child is impossible.	The O parent carries only i, so no child can receive a second codominant allele.
9	Not necessarily, they may be TT or Tt.	A heterozygote and a dominant homozygote look the same, so a test cross is needed.
10	1 : 1, so about 50 of 100 plants are short.	A heterozygote crossed with a recessive homozygote gives two genotype classes in equal numbers.

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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