Final answer:
A phenotype ratio in a Punnett square is the fraction of possible offspring with a particular observable trait, determined by the combination of parental alleles and the laws of probability.
Step-by-step explanation:
The phenotype ratio in a Punnett square refers to the fraction of possible offspring that can be observed to have a particular trait. This ratio is the result of the combination of alleles from the parents. For example, in Mendel's classic pea plant experiments, crossing a plant with yellow peas (dominant trait) and a plant with green peas (recessive trait) can result in a 3:1 phenotypic ratio in the F2 generation, where there is a 3 in 4 probability of offspring having the yellow phenotype and a 1 in 4 probability of them having the green phenotype. The Punnett square is a tool used to predict these ratios based on the genotypes of the parents, using the laws of chance and probability.