Final answer:
When crossing RrYy and RRyy pea plants, 50% of the offspring will have round yellow peas due to the dominance of the round allele (R) and the inheritance of the Y allele from one parent.
Step-by-step explanation:
If you cross a RrYy (round yellow) pea plant with an RRyy (round green) pea plant, the fraction of the offspring that will have round yellow peas can be determined using a Punnett square. In this cross, R is dominant to r (where R=round and r=wrinkled) and Y is dominant to y (where Y=yellow and y=green). All offspring will receive at least one R allele for round peas from the RRyy parent and at least one r allele from the RrYy parent. Since round (R) is dominant, all offspring will have round peas. However, the offspring's pea color is dependent on the combination of Y and y alleles they inherit.
To determine the color of the peas, we must look at the alleles for color in the parents: the RrYy parent can contribute either a Y (yellow) or y (green) allele, and the RRyy parent can only contribute y (green). Using a Punnett square, you would have two boxes with Y (from RrYy) and two with y (from RrYy), all crossed with yy (from RRyy). This means that 50% of the offspring will have the genotype Rryy (round green peas) and 50% will have the genotype RrYy (round yellow peas).
Therefore, the fraction of the offspring that will have round yellow peas is 50%.