IGCSE Biology / Genes and inheritance
Punnett squares revision notes
Edexcel International GCSE Biology (4BI1), chapter 18: Genes and inheritance, specification point 3.23. Free lesson.
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A Punnett square shows the possible offspring genotypes from a genetic cross. It helps you work out genotype and phenotype ratios, and a test cross can reveal a genotype that cannot be seen from appearance alone.
How to set out a Punnett square
Start with the parents’ genotypes, then show the alleles each can pass on in their gametes. Gametes are haploid, which means they carry one allele of each gene. Put one allele in each edge box. The pairs of letters inside the square are the possible offspring genotypes. The edges show single alleles, not whole genotypes. Each inner box combines one allele from each parent.
What Bb × Bb produces
In the standard monohybrid cross, both parents are heterozygous Bb. Each can pass on B or b. B is dominant, so offspring with BB or Bb have brown eyes. Only bb offspring have blue eyes. The genotype ratio is 1 BB : 2 Bb : 1 bb. The phenotype ratio is 3 brown : 1 blue.
How a test cross reveals a genotype
A brown-eyed person could be BB or Bb, so appearance alone does not show their genotype. Cross them with a homozygous recessive individual, bb, who can pass on only b. If the unknown parent is Bb, some offspring will be bb and have blue eyes. A blue-eyed offspring proves that the brown-eyed parent carried b, so they were heterozygous. If the unknown parent is Bb, the offspring ratio is 1 brown : 1 blue. If the unknown parent is BB, all offspring are brown.
How to avoid the common setup mistake
Do not put Bb in an edge box. Each edge box represents one gamete, and a gamete carries only one allele of the gene. Putting a whole genotype on an edge means the square no longer represents fertilisation correctly. Write single alleles on the edges. Write two alleles together in each inner box to show an offspring genotype.
What to check before giving a ratio
A correct square can still earn no marks for the ratio if you answer the wrong question. Read whether you are asked for the offspring genotype ratio or phenotype ratio, then give the matching result. For Bb × Bb, genotype ratio: 1 BB : 2 Bb : 1 bb. For Bb × Bb, phenotype ratio: 3 brown : 1 blue. For a test cross with bb, a 1 brown : 1 blue result indicates the unknown parent is Bb.