Who this is for: Cambridge IGCSE Coordinated Science (0654) students who want monohybrid inheritance — alleles, dominance and genetic crosses — to become reliable marks instead of guesswork on Punnett squares.
What query it owns: how to understand and revise monohybrid inheritance in Cambridge IGCSE Coordinated Science.
Why this is safe: this page owns the monohybrid-inheritance revision-guide angle, while Tutopiya’s Monohybrid Inheritance subtopic page owns the learning resource and the free Monohybrid Inheritance quiz owns the practice.

Monohybrid inheritance is the inheritance of a single characteristic controlled by one gene with two alleles. Cambridge IGCSE Coordinated Science (0654) tests whether you can define key terms, set up Punnett squares correctly, and predict genotype and phenotype ratios. This guide covers the syllabus definitions, worked cross examples, and the question types that appear every year.

Key takeaways

• An allele is a version of a gene; dominant alleles are expressed even if only one copy is present; recessive alleles need two copies.
• Genotype = genetic makeup (e.g. Tt); phenotype = observable characteristic (e.g. tall).
• Homozygous = two identical alleles (TT or tt); heterozygous = two different alleles (Tt).
• A monohybrid cross involves one characteristic and parents that are homozygous or heterozygous for that gene.
• Punnett squares must show parent gametes and offspring genotypes with correct ratios.

What is monohybrid inheritance in Cambridge IGCSE Coordinated Science?

Monohybrid inheritance describes how one characteristic is passed from parents to offspring through alleles on homologous chromosomes. Each parent contributes one allele per gene via gametes. Dominant alleles mask recessive ones in heterozygotes. A classic cross — tall (T) vs short (t) in pea plants — shows a 3:1 phenotype ratio when both parents are heterozygous (Tt × Tt).

You can read the full explanation, worked crosses and notes on Tutopiya’s Monohybrid Inheritance subtopic page before you attempt questions.

The core ideas you must master

Term	Definition	Exam use
Gene	A length of DNA coding for a characteristic	”Define gene”
Allele	Alternative form of a gene	”State the alleles”
Dominant	Expressed with one or two copies	”Identify the dominant allele”
Recessive	Expressed only with two copies	”Explain why offspring are short”
Homozygous	Two identical alleles	”State the genotype”
Heterozygous	Two different alleles	”State the genotype”

Punnett square outcomes — key ratios

Cross	Genotype ratio	Phenotype ratio
Tt × Tt	1 TT : 2 Tt : 1 tt	3 dominant : 1 recessive
TT × tt	All Tt	All dominant phenotype
Tt × tt	1 Tt : 1 tt	1 dominant : 1 recessive
TT × Tt	1 TT : 1 Tt	All dominant phenotype

Monohybrid inheritance in past-paper wording

Command word	What the question wants	Typical stem
Define	Precise syllabus definition	”Define the term allele.”
State	Short factual answer	”State the genotype of a heterozygous plant.”
Complete	Fill in Punnett square or ratios	”Complete the genetic cross.”
Explain	Cause and effect	”Explain why all offspring show the dominant phenotype.”
Predict	Outcome from a cross	”Predict the phenotype ratio.”

Worked exam-style stems

1. “Define the term dominant allele.” A dominant allele is always expressed in the phenotype when present, even if only one copy is inherited. Reward: expressed + one copy sufficient.
2. “Two heterozygous tall plants are crossed. Predict the phenotype ratio of the offspring.” Genotypes: 1 TT : 2 Tt : 1 tt → phenotypes: 3 tall : 1 short. Reward: correct ratio with working or Punnett square.
3. “Explain why a homozygous recessive individual always shows the recessive phenotype.” Both alleles are recessive, so no dominant allele is present to mask the recessive characteristic. Reward: both recessive alleles + no dominant masking.

Practise on the Monohybrid Inheritance quiz.

How monohybrid inheritance connects to the syllabus

Monohybrid inheritance builds on Chromosomes and Genes and Cell Division (gametes carry one allele each). It links to Variation and Selection. The Cambridge IGCSE Coordinated Science resource hub links every Inheritance subtopic.

Common mistakes students make

• Confusing genotype with phenotype.
• Writing dominant allele as the phenotype (e.g. “tall” instead of “T”).
• Forgetting that gametes carry one allele each in Punnett squares.
• Stating a 3:1 genotype ratio when the question asks for phenotype ratio.
• Using different letters for the same gene in one cross (e.g. T and D).

When you need more support

If Punnett square questions keep costing marks, work through the Monohybrid Inheritance quiz, then get help from a Cambridge IGCSE Coordinated Science tutor.

Frequently asked questions

Is monohybrid inheritance hard in Cambridge IGCSE Coordinated Science? The concepts are logical, but marks are lost on Punnett square setup and genotype vs phenotype confusion.

What is a monohybrid cross? A genetic cross involving one characteristic controlled by alleles of a single gene.

What phenotype ratio appears when two heterozygotes are crossed? Typically 3 dominant : 1 recessive (e.g. 3 tall : 1 short).

How do I revise monohybrid inheritance effectively? Learn definitions, practise Punnett squares from memory, check ratios, then take the quiz.

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