IP Biology Notes: Inheritance (Upper Sec 14)

Study guide26 Nov 2025, 00:00 Z

Free IP Biology notes on genes, alleles, Punnett squares, variation, and natural selection for Sec 3 to Sec 4.

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Use this as a free IP Biology notes chapter on inheritance for Year 3 to Year 4. It keeps the IP pacing while reinforcing the 6093 biology foundations most schools test through DBQs, diagrams, and practical explanations.

If Punnett squares, sex-linked crosses, or natural selection explanations are becoming repeated WA losses, use the IP Upper Secondary Biology tuition page for the Year 3-4 route that links inheritance content to DBQ, essay, and practical-planning support.

Status: SEAB O-Level Biology 6093 syllabus (exams from 2026) checked 2025-11-30 - scope unchanged; remains the reference for this note.

The core idea is simple: Inheritance questions track alleles from parents to offspring.

Use it as a working check: Write parental genotypes, list gametes, fill the Punnett square, then translate offspring genotypes into phenotypes and probabilities.

Then go one layer deeper: Example: a heterozygous dominant cross gives three dominant phenotypes to one recessive phenotype only as an expected ratio, not a guarantee in a small family.

What you must know

Gene is a DNA segment; allele is an alternative form. Dominant masks recessive; codominant both expressed. Homozygous = same alleles; heterozygous = different alleles. Phenotype = observable traits; genotype = allele pair.
Monohybrid crosses: use Punnett squares; F2 ratio 3:1 for heterozygous cross; test cross gives 1:1 if heterozygous. Deviations from chance due to small sample or linkage (beyond syllabus detail).
ABO blood group: multiple alleles ( $I^A, I^B, I^O$

Reviewed by

Ezekiel Tan·Academic Advisor (Biology)

Sources

SEAB GCE O-Level Biology (6093) syllabus (examinations from 2026)

Offspring pattern	What it proves about the unknown dominant parent	Why
All offspring show the dominant phenotype	Likely homozygous dominant, $TT$	The unknown parent appears to give only $T$ gametes, so every offspring receives at least one dominant allele.
About half dominant and half recessive offspring	Heterozygous, $Tt$	The recessive offspring must be $tt$ , so the unknown parent must have contributed a $t$ allele.

Question clue	First chromosome move	What it implies	Common trap
Father is affected	Sons receive his $Y$ , daughters receive his affected $X$ .	All daughters receive the affected allele from him; sons do not receive his X.	Saying an affected father passes the condition directly to all sons.
Mother is a carrier	She can pass either normal $X$ or affected $X$ .	Each son has a chance of being affected; each daughter has a chance of being a carrier.	Forgetting that sons get their only X from their mother.
Son is affected	His genotype has the affected allele on his only X.	The affected X must have come from his mother.	Looking for the father's X when the son received the father's $Y$ .
Daughter is affected	She must receive affected X chromosomes from both parents.	Father must be affected and mother must provide an affected allele.	Treating an affected daughter as possible from a carrier mother alone.

Genotype	Blood group shown	Why
$I^A I^A$ or $I^A I^O$	A	One $I^A$ allele is enough to show A when no $I^B$ allele is present.
$I^B I^B$ or $I^B I^O$	B	One $I^B$
$I^A I^B$	AB	$I^A$ and $I^B$
$I^O I^O$	O	No A or B allele is present.

Scenario clue	What to identify	Answer move	Common trap
A population already has different forms of a trait	Heritable variation already exists.	Name the advantageous phenotype or allele before explaining why it helps.	Saying the environment creates the needed allele on purpose.
A selection pressure changes survival chances	The factor causing selection, such as an antibiotic, predator, disease, drought, or pollution.	Explain why one variant survives better in that environment.	Treating the pressure as if it is "trying" to improve the species.
Some individuals survive and reproduce more	Differential reproduction.	State that the survivors pass the advantageous alleles to more offspring.	Stopping at survival without mentioning reproduction.
The trait becomes more common over generations	Allele frequency changes in the population.	Say the population changes over generations, not that one individual changes its genes during life.	Saying individuals evolve during their lifetime.

Symptom in school work	Likely cause	Fix to practise
Punnett square is correct, but the written conclusion is vague	The student has not translated genotype ratios into phenotype probabilities	End every cross with genotype ratio, phenotype ratio, and one sentence about chance.
Sex-linked answers contradict the family tree	The student fills alleles before tracking which parent gives X or Y	Write the sex chromosomes first, then attach alleles to the correct chromosome.
Natural selection answers sound memorised	The student lists keywords without a causal chain	Use the order: variation, selection pressure, survival and reproduction, allele-frequency change.
DBQ answers ignore the data	The student explains inheritance from memory instead of using the stem	Quote the evidence first, then connect it to the inheritance concept.

IP Biology Notes: Inheritance (Upper Sec 14)

What you must know

Sources

Detailed notes

Test-cross reasoning checkpoint

Sex-linked inheritance checkpoint

ABO blood group checkpoint

Natural selection causal-chain checkpoint

When inheritance becomes an IP tuition problem

Worked walkthroughs

Pitfalls and fixes

Practice drills

Quick applications

Exam cues

Next steps

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