how many number of genotypes and phenotypes are there in a dihybrid cross, if so how do you find out the number?

• Law of independent assortment was proposed by Mendel based on the results of dihybrid crosses.  Dihybrid crosses involve two traits. It is essential to recognize dihybrid (two-trait) crosses in order to solve them. Dihybrid crosses are predictions of how two traits will show up in the offspring produced by a mating between two parent organisms. It is the result of two characters segregating simultaneously.
• For example, a cross between a pure breeding plant with two characters (Yellow coloured and round shaped)and another pure breeding plant with two characters( Green coloured with wrinkled seeds). The F1 offspring are therefore all RrYy( round and yellow).In F2 plants, the alleles segregate independently. Thus, all four possible di allelic combinations occur in equal proportions, and each has a probability of 1/4. The phenotypes and phenotypic ratios of the 16 genotype can be determined by constructing a Punnett square for a dihybrid cross.

Selfing   F2                   YyRr                                    X                              YyRr

F2 Generation

The progeny contains 

1. Phenotypic ratio:   9:3:3:1
2. Genotypic ratio:    4:2:2.2:2:1:1:1:1. 

A characteristic 9:3:3:1 phenotypic ratio:

 Round-yellow: Wrinkled-yellow : Round-green : wrinkled-green peas.

A characteristic 1:2:1:2:4:2:1:2:1 genotypic ratio: 

Of the nine genotypes can be grouped into four phenotypes, for example 1 YYRR + 2 YYRr + 2 YyRR + 4 YyRr = 9Y-R- round, yellow peas.

By understanding this concept, we can conclude that, two pairs of contrasting characters, the factors of each pair of characters segregate independently and each gamete has one factor for each of the two traits.