Final answer:
A Punnett square is a tool used in genetics to predict the genotypic and phenotypic ratios of offspring resulting from a cross between two parents, by taking into account the combinations of alleles and the rules of probability.
Step-by-step explanation:
A Punnett square is used to determine the expected genotypic and phenotypic ratios of offspring from a genetic cross between two parents. When demonstrating a monohybrid cross, such as that between true-breeding pea plants with yellow and green seeds, the Punnett square visualizes all the possible combinations of alleles. For instance, with yellow seeds being the dominant trait (allele Y) and green seeds being recessive (allele y), a cross between a YY and a yy parent would result in all Yy offspring, predicting only yellow seeds in the first generation. This device makes it easier to see the statistical probabilities of inheriting certain traits.
To further illustrate, if we cross a white-eyed male fruit fly with a female that is heterozygous for red eye color, we would use a Punnett square to lay out the possible gene combinations and determine the ratio of offspring that will have the red or white eye color trait. By following the rules of probability and the pattern of inheritance (dominant and recessive traits), we can predict the phenotypic ratio of the offspring.