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 F1 generation shows hybrids with dominant phenotypic effect. When F1 heterozygous plants are self-ferilized to produceF2 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 F2 generation comes out to be 9 : 3 : 3 : 1, while the genotypic dihybrid ratio is 1 : 2 : 2 : 4 : 1 : 2: 1 : 2 : 1.