Final answer:
The phenotype of an offspring with a Yy genotype would be yellow seeds, as yellow is dominant over green. When crossing true-breeding green-seeded plants with yellow-seeded plants, all F1 offspring will display the dominant yellow seed color, resulting in 100% yellow seeds.
Step-by-step explanation:
When performing a cross involving seed color in garden pea plants where yellow seed color is dominant over green, the phenotype of an offspring that is heterozygous (Yy) will express the dominant trait. This means that the offspring with a Yy genotype will have a yellow seed. Therefore, if you cross true-breeding parents that produce green seeds (yy) with those that produce yellow seeds (YY), all the F1 offspring would be Yy, displaying the dominant yellow seed color.
A cross between true-breeding green (yy) and true-breeding yellow (YY) parent plants will result in offspring that are all heterozygous Yy, which will have a phenotype of yellow seeds. Hence, the expected F1 offspring phenotype from this cross would be 100 percent yellow seeds (b.).