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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key point
Follow six steps: parental phenotypes → genotypes → gametes → Punnett square → offspring genotypes → phenotype ratio. Use capital for dominant, lower case for recessive. Show all working for full marks.
A Punnett square is a grid used to predict the genotypes and phenotypes of offspring from a genetic cross. Getting full marks requires showing parental phenotypes, genotypes, gametes, the Punnett square itself, and the offspring ratio. Here is the step-by-step method.
The six-step method
Write the parental phenotypes: e.g. "Tall × Short"
Write the parental genotypes: e.g. "Tt × tt"
Identify the gametes: each parent produces gametes with one allele from each pair. Tt produces T and t gametes; tt produces t and t gametes.
Draw the Punnett square: put one parent's gametes across the top and the other's down the side. Fill in each box by combining the alleles.
Write the offspring genotypes: list all possible genotypes from the grid
Write the offspring phenotypes and ratio: determine the phenotype for each genotype and state the ratio (e.g. 1 tall: 1 short)
Using the wrong letters (capital for dominant, lower case for recessive, same letter for both alleles of a gene)
Not stating phenotypes alongside genotypes in the answer
Claiming a 3:1 ratio is exact: it is a probability, not a guarantee
Frequently asked questions
How many marks is a genetic cross worth?
Typically 4–6 marks. One mark for parental genotypes, one for correct gametes, one for a correct Punnett square, one for offspring genotypes, one for the ratio, and sometimes one for showing the method clearly.
Put the guide into practice
Questions and practical resources
Use a fresh question to check whether the idea can be applied without prompts.