The self-fertilization of a pea plant with yellow, round seeds that is heterozygous for both the color and shape of the seeds is an example of a dihybrid cross. Yellow is the dominant allele for color. Green is the recessive allele. Round is dominant for shape, and wrinkled is recessive.
Since each trait has a dominant and a recessive allele, this cross gives the ratio common for dihybrid crosses of 9:3:3:1. This means that nine-sixteenths of the peas will have yellow, round seeds. Three-sixteenths of the peas will have green, round seeds. Three-sixteenths of the peas will have yellow, wrinkled seeds, and one-sixteenth of the peas will have green, wrinkled seeds.