Final answer:
A Punnett square is a chart used to predict genotypes and phenotypes from a genetic cross, with the example of yellow and green pea seeds where all offspring from a 'YY' and 'yy' cross are 'Yy' with yellow seeds.
Step-by-step explanation:
A Punnett square is a graphical device used to predict the results of a genetic cross. This chart is named after British geneticist Reginald Punnett. To demonstrate how it works with a monohybrid cross, we can use the example of true-breeding pea plants with yellow and green seeds. The yellow seed color, being dominant, is represented by 'YY', while the recessive green is denoted as 'yy'. When setting up a Punnett square, one lists all possible combinations of parental alleles on the top and side, representing how alleles segregate during meiosis into gametes.
By filling in the grid with the combinations of egg and sperm, we can predict the potential genotypes of the offspring. For instance, crossing 'YY' with 'yy' would result in all offspring having the genotype 'Yy', displaying the phenotype of yellow seeds. Because each combination is equally possible, the Punnett square can show genotypic and phenotypic ratios, illustrating the power of this tool to predict inheritance patterns.