Why can AB and O parents not have an O child?

An AB parent always passes IA or IB, so the child carries a dominant allele and cannot be ii.

Why are sons more at risk in X-linked inheritance?

A son has a single X from his mother, so a carrier mother gives him a one in two chance of being affected.

S5 Biology: Mendelian Genetics (Paper 3)

2026-11-01

The third paper works through real cases: reading recessive pedigrees, codominance and multiple alleles in ABO blood groups, X-linked colour blindness, and using the two thirds carrier chance to estimate the next generation risk.

Direct answer　Unaffected parents with an affected child means recessive inheritance; ABO has three alleles with IA and IB codominant; a son takes his X from his mother, so half his risk follows her carrier state; risk multiplies carrier chances by one quarter.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: S5 Biology: Mendelian Genetics
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationTwo unaffected parents have a child with albinism. What does that tell you about the allele?
FoundationTwo Aa parents have a child. What is the chance the child is affected?
FoundationWhat is the genotype of blood group AB?
IntermediateHow many alleles control the ABO blood groups?
IntermediateOne parent is AB and the other O. What blood groups can the children have?
IntermediateAmong the normal children of two Aa carriers, what fraction are themselves carriers?
ChallengeColour blindness is X-linked recessive. With a XCXc mother and a XCY father, what is the chance a son is colour blind?
ChallengeIf that couple had 200 sons, how many would be expected to be colour blind?
ChallengeTwo relatives each have a two in three chance of being carriers. What is the risk that their child is affected?
Error redoA student says a dominant disorder can skip a generation. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	It is recessive	A recessive trait can hide in heterozygous parents and only appears when both pass the allele on.
2	One quarter	Only aa is affected, one square of the four in the grid.
3	IAIB, with both alleles expressed, which is codominance	Neither IA nor IB masks the other, so the red cells carry both A and B antigens.
4	Three: IA, IB and i	Only two of the three are carried at a time, giving six genotypes and four blood groups.
5	A or B, half each; never AB or O	IAIB passes only IA or IB while O passes only i, so children are IAi or IBi.
6	Two thirds	Normal children cover one AA and two Aa, so two thirds are carriers, the figure used for the next generation.
7	One half	A son always takes his X from his mother, and one of her two X chromosomes carries the allele, so the chance is one half.
8	About 100	Two hundred times one half gives 100, and real counts will come close without matching exactly.
9	One ninth	Two thirds times two thirds times one quarter gives four thirty-sixths, that is one ninth.
10	A dominant disorder appears in every generation; skipping is a feature of recessive inheritance	A dominant condition always has an affected parent, so a skipped generation means the allele is not dominant.

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.

Book an S5 Biology tutor 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.