Use a Punnett square to predict the genotype and phenotype ratios expected from a genetic cross between two parents.
A Punnett square is a simple grid tool for predicting the possible genotypes of offspring from two parents whose own genotypes are known. Each parent's two alleles are written along one edge of a 2-by-2 grid, and each box is filled in by combining the allele from its row with the allele from its column, showing one possible offspring genotype. Counting how many boxes show each genotype gives a predicted ratio for a large number of offspring.
It is important to be clear about what a Punnett square actually predicts: probabilities, not guarantees. A 3-to-1 ratio means each individual offspring has roughly a 3-in-4 chance of showing the dominant trait, not that exactly three out of every four offspring will always show it -- small numbers of real offspring can easily differ from the predicted ratio purely by chance, the same way flipping a coin four times does not always land two heads and two tails.
Mastery looks like: They can correctly fill in a Punnett square for a given cross and state the resulting genotype and phenotype ratios.
Common stumbles: Reading a predicted ratio as a guarantee for a small number of offspring; making an arithmetic slip combining alleles into the wrong box.