What are chromosomes and how do they relate to genes in the context of inheritance?

Chromosomes are long, thread-like structures made of DNA and proteins that are found in the nucleus of cells. They carry genetic information in the form of genes. In the context of inheritance, chromosomes are crucial because they ensure that DNA is accurately copied and distributed in the process of cell division. Each chromosome contains many genes, which are specific sequences of DNA that code for proteins and determine inherited traits. Understanding chromosomes and genes is fundamental in Cambridge IGCSE Coordinated Science as it explains how traits are passed from parents to offspring.

How do chromosomes determine the sex of an organism?

In humans and many other organisms, sex is determined by specific chromosomes known as sex chromosomes. Typically, females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The presence of the Y chromosome determines male characteristics. During reproduction, each parent contributes one sex chromosome, resulting in offspring with either XX or XY chromosomes. This concept is a key part of the Cambridge IGCSE Coordinated Science curriculum, helping students understand the genetic basis of sex determination.

What is the difference between a gene and an allele?

A gene is a segment of DNA that contains the instructions for building a specific protein, which in turn influences a particular trait. An allele is a variant form of a gene. For example, a gene for eye color may have an allele for blue eyes and another for brown eyes. Inheritance involves the combination of alleles from both parents, which can result in different traits in the offspring. This distinction is important in Cambridge IGCSE Coordinated Science as it helps students understand genetic variation and inheritance patterns.

How do mutations in genes affect inheritance?

Mutations are changes in the DNA sequence of a gene. They can occur naturally or due to environmental factors. Mutations can affect inheritance by altering the function of a gene, potentially leading to changes in the trait it controls. Some mutations may be beneficial, harmful, or neutral. In the context of Cambridge IGCSE Coordinated Science, understanding mutations is crucial as they can lead to genetic disorders or contribute to evolution by introducing new genetic variations.

Why is it important to understand the structure and function of chromosomes in genetics?

Understanding the structure and function of chromosomes is vital because they play a key role in the accurate transmission of genetic information during cell division. Chromosomes ensure that DNA is replicated and distributed correctly to daughter cells, maintaining genetic continuity. This knowledge is essential in Cambridge IGCSE Coordinated Science, as it provides the foundation for studying more complex genetic concepts, such as inheritance patterns, genetic disorders, and biotechnology applications.

Why is "Using 'gene' and 'allele' interchangeably" a common mistake in Chromosomes and Genes?

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. Source: 0654 Examiner Report 2023

Why is "Stating that dominant alleles are more common in a population" a common mistake in Chromosomes and Genes?

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 is "Stating that phenotype is determined by genotype alone" a common mistake in Chromosomes and Genes?

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 is "Stating that human body cells are haploid with 23 chromosomes" a common mistake in Chromosomes and Genes?

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.

InheritanceCambridge IGCSE Coordinated Sciences 0654

Chromosomes and Genes

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Chromosomes and Genes — Cambridge IGCSE Coordinated Science 0654

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.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

B20.1 — State the relationship between DNA, genes, and chromosomes.
B20.2 — Define gene, allele, genotype, phenotype, dominant, recessive, homozygous, heterozygous.
B20.3 — Describe the structure of DNA.

DNA Structure

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):

Deoxyribose sugar (5-carbon)
Phosphate group
Nitrogenous base — one of four: Adenine, Thymine, Cytosine, Guanine

Complementary base pairing (always):

A pairs with T (Adenine-Thymine)
C pairs with G (Cytosine-Guanine)

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.

DNA = double helix; base pairs A-T and C-G held by hydrogen bonds.
Gene = sequence of DNA bases coding for one protein.
Humans: 46 chromosomes (23 pairs); ~20,000 genes.

Key Genetic Terms — Definitions

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.

Dominant: expressed with one copy. Recessive: only expressed homozygous.
Homozygous = same alleles (BB or bb). Heterozygous = different (Bb).
Genotype = alleles carried. Phenotype = what is seen.

How it’s examined

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.

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Step-by-step worked examples — Chromosomes and Genes

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.

1State what is meant by the term 'allele'
Core• allele, gene, definition
Question
State what is meant by the term 'allele'.
Step-by-step solution
1. 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.
2Determine phenotype from a heterozygous genotype
Core• dominant, recessive, phenotype, genotype
Question
A plant has genotype Tt. State its phenotype if T (tall) is dominant to t (dwarf).
Step-by-step solution
1. Step 1
  The plant is heterozygous — it has one dominant allele (T) and one recessive allele (t).
2. 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.
3Distinguish between genotype and phenotype
Core• genotype, phenotype
Question
Distinguish between genotype and phenotype.
Step-by-step solution
1. Step 1
  Genotype: the complete set of alleles an organism possesses for a particular gene (or all genes). E.g. Tt, BB, Aa.
2. 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.
4Explain why a recessive allele is not expressed in a heterozygote
Extended• dominant, recessive, heterozygous
Question
Explain why a person with one recessive allele for a characteristic does not show that characteristic.
Step-by-step solution
1. Step 1
  A heterozygous person (e.g. Aa) has one dominant allele (A) and one recessive allele (a).
2. Step 2
  The dominant allele masks (suppresses) the expression of the recessive allele. The organism shows only the dominant phenotype.
3. 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).

Key Definitions and Keywords — Chromosomes and Genes

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.

Gene
Examiner keyword
A section of DNA that codes for the production of a specific protein; the basic unit of heredity.
Allele
Examiner keyword
An alternative form of a gene. Alleles occupy the same locus on homologous chromosomes. A diploid organism has two alleles of each gene.
Genotype
Examiner keyword
The alleles possessed by an organism for a specific gene or for all its genes.
Phenotype
Examiner keyword
The observable characteristics of an organism resulting from the interaction of its genotype with the environment.
Dominant allele
Examiner keyword
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.
Recessive allele
Examiner keyword
An allele whose effect is masked by the dominant allele in a heterozygote; it is only expressed when two copies are present (homozygous recessive).
Homozygous
Examiner keyword
Having two identical alleles for a particular gene (e.g. AA or aa).
Heterozygous
Examiner keyword
Having two different alleles for a particular gene (e.g. Aa). The dominant allele is expressed.

Common Mistakes and Misconceptions — Chromosomes and Genes

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.

✕Using 'gene' and 'allele' interchangeably
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.
✕Stating that dominant alleles are more common in a population
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.
✕Stating that phenotype is determined by genotype alone
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.
✕Stating that human body cells are haploid with 23 chromosomes
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.

Practice questions

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Chromosomes and Genes — frequently asked questions

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Chromosomes and Genes

Short Study Notes in the form of Slides

Short Study Notes — Chromosomes and Genes

Detailed Notes

What you’ll learn

How it’s examined

1State what is meant by the term 'allele'

2Determine phenotype from a heterozygous genotype

3Distinguish between genotype and phenotype

4Explain why a recessive allele is not expressed in a heterozygote

Gene

Allele

Genotype

Phenotype

Dominant allele

Recessive allele

Homozygous

Heterozygous

✕Using 'gene' and 'allele' interchangeably

✕Stating that dominant alleles are more common in a population

✕Stating that phenotype is determined by genotype alone

✕Stating that human body cells are haploid with 23 chromosomes

Practice questions

Past paper style quiz

Assess your understanding

Chromosomes and Genes — frequently asked questions