Punnett Squares

Part of Genetic Inheritance ยท Section 2 of 9

Key FactsUnit: Inheritance & EvolutionGCSE

A Punnett square shows possible offspring from two parents:

Example: Eye colour (Brown = B, Blue = b)

Parent 1: Bb (heterozygous brown)

Parent 2: Bb (heterozygous brown)

B b
B BB Bb
b Bb bb

Result: 3 Brown (BB, Bb, Bb) : 1 Blue (bb) = 3:1 ratio

Probability of blue eyes: 1 in 4 = 25%

Visual: Punnett Squares and Genetic Crosses

Punnett square diagram showing key genetic terms, genotype vs phenotype, examples of Bb x Bb and Bb x bb crosses with 3:1 and 1:1 ratios, and step-by-step guide to drawing Punnett squares

Remember: Bb x Bb = 3:1 phenotype ratio | Bb x bb = 1:1 ratio | Dominant allele (capital) masks recessive (lowercase)

This key facts covers Punnett Squares within Genetic Inheritance for GCSE Biology. Genetic inheritance patterns, alleles, and inheritance diagrams It is section 2 of 9 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.

Practice questions for Genetic Inheritance

What is the term for an allele that is always expressed when present?

  • A. Recessive allele
  • B. Dominant allele
  • C. Homozygous genotype
  • D. Recessive phenotype
1 markfoundation

What is the purpose of a Punnett square in genetic inheritance?

3 marksstandard

Quick recall flashcards

What is a recessive allele?
An allele that is only expressed when two copies are present (homozygous recessive). Represented by a lower-case letter (e.g. b). It is 'hidden' by a dominant allele.
What is a dominant allele?
An allele that is always expressed in the phenotype, even if only one copy is present. Represented by a capital letter (e.g. B). Think of it as the 'bossy' allele.

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Key Terms for Genetics

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How It Works: Dominant and Recessive Alleles

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