While a Punnett square predicts expected offspring genotypes and phenotypes based on probability, it doesn't ensure that each actual genetic cross will yield the exact predicted outcome due to the random nature of inheritance, especially in small sample sizes.
A Punnett square does not guarantee that every actual genetic cross will result in the predicted outcome, due to the nature of probability. While a Punnett square provides the expected frequencies of genotypes based on the law of probability, it does not account for random events that can lead to deviation from the expected ratios in small sample sizes. In large populations, the actual results tend to be closer to the predicted outcomes, but in small populations, there can be significant variance due to chance.
This is because Punnett squares depict all possible genetic combinations from two parents' alleles as equally likely. However, the law of large numbers states that the prediction becomes more accurate as the number of events (in this case, offspring) increases. In conclusion, while a Punnett square is a useful tool to predict potential genotypes and phenotypes in offspring, it is not an absolute predictor of individual genetic crosses.