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Detailed notes on Inheritance for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Chromosomes carry genes — segments of DNA that code for proteins. Cambridge tests the hierarchy: DNA → gene → chromosome → nucleus. Know the definitions of gene, allele, genotype, phenotype, dominant, recessive, homozygous, heterozygous.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
DNA is a double helix of two complementary strands, with bases paired by specific hydrogen bonds.
DNA (deoxyribonucleic acid) is the molecule that carries genetic information.
Structure: Double helix — two strands twisted around each other, held together by hydrogen bonds between bases.
Nucleotide components (the repeating units):
Complementary base pairing (always):
Hierarchy from small to large:
Base pair → Nucleotide → Gene (sequence of bases) → DNA strand → Chromosome → Nucleus
Genes: a gene is a specific sequence of bases on DNA that codes for the production of a specific protein (including enzymes). Different sequences of three bases (codons) code for different amino acids — the genetic code.
Human genome: ~3 billion base pairs; ~20,000 genes; 46 chromosomes (23 pairs) in diploid cells.
These definitions must be learned precisely — examiners mark for exact wording.
| Term | Definition |
|---|---|
| Gene | A section of DNA that codes for a specific protein (and hence a specific characteristic) |
| Allele | An alternative form of a gene; same locus (position) on homologous chromosomes |
| Genotype | The genetic make-up of an organism — which alleles it possesses |
| Phenotype | The observable characteristics of an organism — product of genotype AND environment |
| Dominant | Allele that is expressed in the phenotype when only ONE copy is present (heterozygous or homozygous) |
| Recessive | Allele that is only expressed in the phenotype when TWO copies are present (homozygous) |
| Homozygous | Having two IDENTICAL alleles for a particular gene (e.g. BB or bb) |
| Heterozygous | Having two DIFFERENT alleles for a particular gene (e.g. Bb) |
| Codominant | Both alleles expressed equally in the phenotype when both are present |
Convention: Dominant allele = capital letter; recessive = lowercase (e.g. B = brown eyes dominant; b = blue eyes recessive).
Codominance example: Blood group AB — both IA and IB alleles expressed (neither dominates the other). The phenotype shows features of both.
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 4: 'Define the term allele' (1 mark — must include reference to alternative form AND same gene/locus). 'State the difference between genotype and phenotype' (2 marks). MCQ tests base-pairing rules (which base pairs with which), and the definitions of dominant/recessive from specific cross results. These definitions appear in almost every Paper 4 genetics question.
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (B20); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on chromosomes and genes, written exactly the way a tutor would explain them at the board.
Question
State what is meant by the term 'allele'.
Step-by-step solution
Step 1
An allele is an alternative form of a particular gene. Multiple alleles can exist for one gene, but a diploid organism has two alleles of each gene (one on each homologous chromosome).
Answer
An allele is an alternative form of a gene; it occupies the same locus (position) on homologous chromosomes.
Examiner tip
The phrase 'alternative form' or 'different version' is required. Simply writing 'a type of gene' is insufficient.
Question
A plant has genotype Tt. State its phenotype if T (tall) is dominant to t (dwarf).
Step-by-step solution
Step 1
The plant is heterozygous — it has one dominant allele (T) and one recessive allele (t).
Step 2
The dominant allele (T) is always expressed when present, masking the recessive allele (t).
Answer
The plant is tall. The dominant allele T is expressed, so the plant shows the tall phenotype regardless of having one recessive t allele.
Question
Distinguish between genotype and phenotype.
Step-by-step solution
Step 1
Genotype: the complete set of alleles an organism possesses for a particular gene (or all genes). E.g. Tt, BB, Aa.
Step 2
Phenotype: the observable characteristics of an organism resulting from both its genotype and the effect of the environment. E.g. tall, blue eyes, brown fur.
Answer
Genotype: the alleles an organism possesses (e.g. Tt). Phenotype: the observable characteristics of an organism resulting from its genotype (and environment), e.g. tall.
Question
Explain why a person with one recessive allele for a characteristic does not show that characteristic.
Step-by-step solution
Step 1
A heterozygous person (e.g. Aa) has one dominant allele (A) and one recessive allele (a).
Step 2
The dominant allele masks (suppresses) the expression of the recessive allele. The organism shows only the dominant phenotype.
Step 3
The recessive phenotype is only expressed when the organism is homozygous recessive (aa) — having two copies of the recessive allele and no dominant allele to mask it.
Answer
The dominant allele masks the expression of the recessive allele in a heterozygote. The recessive allele is only expressed when the organism is homozygous recessive (two copies of the recessive allele, no dominant allele present).
Definitions to memorise and the exact keywords mark schemes credit for chromosomes and genes answers — sharpened from recent examiner reports for the 2026 0654 sitting.
A section of DNA that codes for the production of a specific protein; the basic unit of heredity.
An alternative form of a gene. Alleles occupy the same locus on homologous chromosomes. A diploid organism has two alleles of each gene.
The alleles possessed by an organism for a specific gene or for all its genes.
The observable characteristics of an organism resulting from the interaction of its genotype with the environment.
An allele that is expressed in the phenotype whenever it is present (even as a single copy in a heterozygote). Denoted by a capital letter.
An allele whose effect is masked by the dominant allele in a heterozygote; it is only expressed when two copies are present (homozygous recessive).
Having two identical alleles for a particular gene (e.g. AA or aa).
Having two different alleles for a particular gene (e.g. Aa). The dominant allele is expressed.
The traps other students keep falling into on chromosomes and genes questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
0654 Examiner Report 2023
Why it happens
Both terms refer to sections of DNA, and their distinction is subtle.
How to avoid it
A gene is a locus on a chromosome that codes for a trait. An allele is one specific version of that gene. Multiple alleles can exist for one gene.
Why it happens
Students confuse 'dominant' (expressed) with 'frequent'.
How to avoid it
Dominant refers only to the pattern of expression — a dominant allele is expressed when present. It says nothing about frequency. Recessive alleles can be MORE common than dominant ones in a population.
Why it happens
Students learn genotype determines phenotype and overlook environmental influence.
How to avoid it
Phenotype is determined by genotype AND environment. E.g. identical twins (same genotype) can differ in height due to nutrition. Always include 'and environmental factors' in phenotype definitions.
Why it happens
Students know gametes have 23 chromosomes and confuse this with body cells.
How to avoid it
Human body (somatic) cells are diploid: 46 chromosomes (23 pairs). Only gametes are haploid (23 chromosomes), produced by meiosis.
The things students keep getting wrong in this sub-topic, answered.