(HT only) Students should be able to construct a genetic cross by Punnett…

The idea

A Punnett square is a grid that shows every possible combination of alleles from two parents. Write the gametes of one parent across the top and the other down the side, then fill each cell by combining the row and column alleles. For example, a cross between two heterozygous purple-flowered plants (Pp × Pp) gives cells PP, Pp, Pp and pp. The theory of probability then predicts the chance of each outcome: one cell in four is PP, so the probability of a purple-flowered offspring is 3/4 or 75%, and of a white-flowered offspring is 1/4 or 25%. Probabilities describe chance, not certainty, so predictions are about expected proportions in large numbers of offspring.

What earns the marks

• Identify the parental genotypes and the alleles carried by each gamete before drawing the grid.
• Draw a correctly sized Punnett square with one parent's gametes along the top and the other parent's gametes down the side.
• Combine the alleles in each cell, writing the dominant allele first by convention, for example Pp rather than pP.
• State the phenotype associated with each genotype, distinguishing homozygous dominant, heterozygous and homozygous recessive.
• Express the predicted outcome as a probability, fraction, decimal or percentage, for example 1/4, 0.25 or 25%.
• Explain that the prediction is a statistical expectation for many offspring, not a guarantee for a single offspring.

Common mistakes and how to put them right

• Writing gametes as pairs of alleles instead of single alleles.

  Put it right by remembering that gametes carry one allele of each gene.

• Filling the grid by copying parental genotypes rather than combining one allele from each parent.

  Put it right by taking one allele from the row and one from the column for every cell.

• Treating a 3:1 predicted ratio as certain for four offspring.

  Put it right by explaining that probability predicts expected proportions over many offspring, not the exact result of a small family.

Exam tips

• Draw the Punnett square clearly with a ruler and label the parental gametes so the examiner can follow your reasoning.
• Always finish by linking the genotype ratio to a phenotype ratio and then to a probability, because the prediction is the point of the cross.
• Use the wording of the question to decide whether the answer should be a ratio, a fraction or a percentage, and give the probability in the requested form.