Final answer:
The possible gamete combinations for the offspring with a genotype of yyRr in a di-hybrid square are YR, Yr, yR, and yr. When these gametes are arranged in a Punnett square, there are 16 equally likely genotypic combinations, resulting in a phenotypic ratio of 9 round/yellow:3 round/green:3 wrinkled/yellow:1 wrinkled/green.
Step-by-step explanation:
For the F2 generation, the law of segregation requires that each gamete receive either an R allele or an r allele along with either a Y allele or a y allele. The law of independent assortment states that a gamete into which an r allele sorted would be equally likely to contain either a Y allele or a y allele.
Thus, there are four equally likely gametes that can be formed when the YyRr heterozygote is self-crossed, as follows: YR, Yr, yR, and yr. Arranging these gametes along the top and left of a 4 x 4 Punnett square gives us 16 equally likely genotypic combinations.
From these genotypes, we infer a phenotypic ratio of 9 round/yellow:3 round/green:3 wrinkled/yellow:1 wrinkled/green. These are the offspring ratios we would expect, assuming we performed the crosses with a large enough sample size.