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Learn5090Paper 1 · Multiple ChoiceInheritanceMonohybrid inheritance
Inheritance · Subtopic 2 of 3

Monohybrid Inheritance

The genotype is the pair of alleles an organism has (e.g. Bb); the phenotype is the characteristic shown (brown eyes). A dominant allele (capital letter) shows whenever present; a recessive allele (lowercase) shows only when there are two (bb). Homozygous means two identical alleles (BB or bb); heterozygous, two different (Bb). A Punnett square predicts offspring: Bb × Bb gives 1 BB : 2 Bb : 1 bb, a 3 : 1 phenotype ratio. A test cross with a homozygous recessive reveals an unknown genotype. In codominance both alleles show (blood group AB).

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 45 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Use genetic terms correctly.
  • Draw genetic diagrams and Punnett squares.
  • Calculate ratios and probabilities.
  • Interpret pedigree diagrams; describe test crosses and codominance.
Pause and think

Two brown-eyed parents have a blue-eyed child. How is that possible?

Section 2 of 14

Core idea

Start hereHidden alleles

Two healthy parents can have a child with cystic fibrosis. Each carried one copy of a recessive allele, invisible in their own phenotype.

The big idea

Each parent passes one allele of each gene. Punnett squares predict the probabilities of offspring genotypes and phenotypes.

1. A full genetic diagram

Worked exampleCystic fibrosis carriers

Two carriers (Ff) have a child.

Goal
Probability the child has cystic fibrosis.
Strategy
Gametes, Punnett square, ratio.
  1. gametes: F, f and F, f\text{gametes: F, f and F, f}
    Each parent gives one allele.
  2. FF, Ff, Ff, ff\text{FF, Ff, Ff, ff}
    Punnett square.
  3. P(ff)=14P(\text{ff})=\tfrac14
    One of four outcomes.
Answer25% chance per child.
✓ CheckRatio 3 unaffected : 1 affected ✓.
Test cross

Cross with homozygous recessive to reveal the genotype.

Key takeawayCodominance Both alleles are expressed: IAIBI^AI^B gives blood group AB. IOI^O is recessive to both.
Pause and think

Can two blood group O parents have a group A child?

Summary

  • Bb × Bb → 3 : 1.
  • Bb × bb → 1 : 1.
  • Codominance; test cross; pedigrees.
Key words
Homozygous
Two identical alleles.
Heterozygous
Two different alleles.
Codominance
Both alleles expressed in the phenotype.
HistoryMendel's peas. Gregor Mendel, a monk, crossed about 28,000 pea plants in the 1850s–60s and discovered the 3 : 1 ratio. His work was ignored until 1900, 16 years after his death.
Fun factTongue rolling myth. Tongue rolling is often taught as a simple dominant trait, but identical twins can differ in it, so it isn't controlled by a single gene.
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