Explanation

A dihybrid cross involves two pairs of contrasting characters, e.g., yellow round seeded plant and wrinkled. Green seedes plant(both pure lines) homozygous. When a dihybrid cross is made between two pure line of homolzygous parents, then the $F_{1}$ generation shows hybrids with dominant phenotypic effect. When $F_{1}$ heterozygous plants are self-ferilized to produce$F_{2}$ generation, four types of combinations are obtained of which two are similar to parental combination and other two are new combinations. The phenotypic dihybrid ratio of these four combinations in $F_{2}$ generation comes out to be 9 : 3 : 3 : 1, while the genotypic dihybrid ratio is 1 : 2 : 2 : 4 : 1 : 2: 1 : 2 : 1.