Punnett Squares

A Punnett square predicts the probability of specific genotype and phenotype combinations from a genetic cross, based on how each parent’s alleles can combine. For a classic pea plant cross — purple flower color (P, dominant) versus white (p, recessive) — a cross between two heterozygous Pp parents predicts a 1 PP : 2 Pp : 1 pp genotype ratio, and since P is dominant, a 3:1 purple-to-white phenotype ratio. Because P is dominant, both PP and Pp genotypes display the identical purple phenotype — you can’t tell which one an individual has just by looking at it.

But a Punnett square only predicts a probability — it doesn’t guarantee any single offspring’s outcome. Breed just a handful of offspring in the simulator above and the observed ratio can look nothing like the prediction; breed dozens or hundreds and the observed ratio should converge much closer to what the Punnett square predicted. That gap — and its narrowing — is the actual statistical lesson a static diagram can’t show you: real inheritance is a random process, and probabilities only become reliable predictors of the average outcome once you’re looking at enough trials.

Try a few different crosses in the simulator: a PP × pp cross should produce exclusively Pp offspring, since one parent can only contribute a P gamete and the other only a p, with no other combination possible. A PP × Pp cross should produce 100% purple offspring, since every possible resulting genotype carries at least one dominant P allele.