Final answer:
In a dihybrid cross with one dominant allele of either of two genes necessary to produce a phenotype, the expected phenotypic ratio is 9:3:3:1. This ratio can be collapsed into two 3:1 ratios for each gene independently. Using the product rule, three quarters of the offspring will have one phenotype and one quarter will have the other phenotype.
Step-by-step explanation:
Because of independent assortment and dominance, the 9:3:3:1 dihybrid phenotypic ratio can be collapsed into two 3:1 ratios, characteristic of any monohybrid cross that follows a dominant and recessive pattern. Ignoring seed color and considering only seed texture in the above dihybrid cross, we would expect that three quarters of the F₂ generation offspring would be round, and one quarter would be wrinkled. Similarly, isolating only seed color, we would assume that three quarters of the F₂ offspring would be yellow and one quarter would be green. The sorting of alleles for texture and color are independent events, so we can apply the product rule.
Therefore, the proportion of round and yellow F₂ offspring is expected to be (3/4) × (3/4) = 9/16, and the proportion of wrinkled and green offspring is expected to be (1/4) × (1/4) = 1/16. These proportions are identical to those obtained using a Punnett square. Round, green and wrinkled, yellow offspring can also be calculated using the product rule, as each of these genotypes includes one dominant and one recessive phenotype. Therefore, the proportion of each is calculated as (3/4) × (1/4) = 3/16.