Final answer:
The offspring of a cross between purebred yellow-seeded and green-seeded plants are all yellow because the yellow allele is dominant over the green allele.
Step-by-step explanation:
When a purebred plant that produces yellow seeds, which has the dominant trait, is crossed with a purebred plant that produces green seeds, with the recessive trait, all of the offspring exhibit yellow seeds. This is because the yellow allele is dominant to the green allele. In genetic terms, the yellow seed plants would have been homozygous dominant (YY), and the green seed plants would be homozygous recessive (yy). The offspring are all heterozygous (Yy) but display the yellow phenotype due to the dominance of the yellow allele.
Gregor Mendel's early genetic experiments with pea plants are the foundation of this conclusion. His findings showed that when true-breeding yellow plants were crossed with true-breeding green plants, all F1 offspring were yellow. Mendel's law of dominance explains this: the dominant allele is expressed in the phenotype, and the recessive allele is masked in the presence of the dominant allele.