S5 Biology: Genetics And Mendelian Inheritance (Paper 2 (intermediate))

S5 Biology: Genetics and Mendelian Inheritance (Paper 2 (intermediate))

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 2 (intermediate))

This paper adds incomplete dominance, codominant blood groups and probability work, requiring all four blood-group genotypes and extending single-child probability to several children.

Direct answers　Marks are lost by treating codominance as blending or by forgetting to multiply independent events. List parental gametes first, then write the offspring genotypes.

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)

FoundationWith incomplete dominance, what is the F1 phenotype from red RR crossed with white rr?
FoundationA couple IA IB x IA i has 400 children. How many are expected to be AB?
FoundationWhat is the difference between an allele and a gene?
IntermediateSelf-crossed pink F1 gives 800 plants. How many are expected to be red?
IntermediateOut of 1000 seeds from Tt x Tt, how many are expected to be heterozygous Tt?
IntermediateA test cross Rr x rr gives 240 plants. How many are expected to be round?
ChallengeTwo Tt parents have three children. What is the probability that all three show the recessive trait?
ChallengeThe colour-blind allele is X-linked. A carrier mother XCXc and a normal father XCY have four sons. How many are expected to be colour blind?
ChallengeWhat is the probability that parents of type A (IA i) and type B (IB i) have a type O child?
Error redoError clinic: a student claims the recessive allele is lost when it disappears in F1. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	All pink flowers.	The heterozygote Rr shows an intermediate phenotype.
2	100 children.	The four genotype classes are IAIA, IAi, IAIB and IBi, so AB takes one quarter.
3	An allele is one version of a gene, sitting at the same locus on homologous chromosomes.	Two versions can occupy that locus, one from each parent.
4	200 plants.	Rr x Rr gives 1 RR : 2 Rr : 1 rr, so red takes one quarter.
5	500 seeds.	Heterozygotes take one half, and 1000 divided by 2 is 500.
6	120 plants.	A test cross gives 1 : 1, so half are round.
7	One in sixty-four, that is 1/64.	Each child has probability one quarter, and 4 x 4 x 4 gives 64.
8	Two sons.	Sons take X from the mother, and half of those carry Xc.
9	One quarter, that is 25 percent.	Each parent can pass i, and the ii combination appears once.
10	The allele is still present but masked, and it reappears in F2.	A recessive allele is not expressed in a heterozygote yet is still inherited.

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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