A Punnett square combines the possible gametes of two parents. For a single gene, a heterozygous Aa parent produces A or a gametes in equal expected proportions. Crossing Aa with Aa gives AA, Aa, Aa and aa as four equally likely combinations in this model. The expected genotype ratio is 1:2:1. With complete dominance, three combinations show the dominant phenotype and one shows the recessive phenotype.
Probabilities apply to each independent offspring; they do not guarantee exact counts in a small family. A recessive phenotype identifies aa in this model, but a dominant phenotype could be AA or Aa. State the inheritance assumptions: real traits may involve more genes, incomplete dominance, sex linkage or environment. Use only the inheritance model supplied.
Example 1
Cross Aa × aa with A completely dominant. Find the recessive-phenotype probability.
The first parent supplies A or a; the second supplies only a. Outcomes are Aa and aa in equal proportions, so recessive probability =21=50%.
Example 2
Cross AA × aa. Find all offspring genotypes.
The AA parent supplies only A and aa only a. Every combination is Aa, so the expected genotype is 100% heterozygous.
Watch out: It is the probability for each independent offspring. Small families can differ from the expected ratio.