Dihybrid cross involves the inheritance of two pairs of contrasting characteristics (or contrasting traits) at the same time. The two pairs of contrasting characteristics chosen by Mendel were shape and color of seeds: roundyellow seeds and wrinkled-green seeds.
Mendel crossed pea plants having round-yellow seeds with pea plants having wrinkled-green seeds.
Mendel made the following observation:
(i) Mendel first crossed pure breeding pea plants having round-yellow seeds with pure breeding pea plants having wrinkled-green seeds and found that only round-yellow seeds were produced in the first generation (F1 ). No wrinkled-green seeds were obtained in the F1 generation. From this it was concluded that round shape and yellow color of the seeds were dominant traits over the wrinkled shape and green color of the seeds.
(ii) When the hybrids of F1 generation pea plants having round-yellow seeds were cross-bred by self pollination, then four types of seeds having different combinations of shape and color were obtained in second generation or F1 generation. They were round yellow, round-green, wrinkled-yellow and wrinkled-green seeds.
The ratio of each phenotype (or appearance)of seeds in the F2 generation is 9 : 3 : 3 : 1. This is known as the Dihybrid ratio.
Monohybrid cross is a genetic cross that involves a single pair of gene or trait. In this parents differ by single trait.
Eg: Height.
Dihybrid cross is a genetic cross that involves two pairs of genes, which are responsible for two trait. In this, parents have two different independent trait.
Eg: flower colour, stem length.
