Mendel performed experiments on a garden pea plant which shows 7 pairs of contrasting characters, to study the inheritance of characters. He performed monohybrid cross and dihybrid cross to study the inheritance of some pairs of contrasting characters. Monohybrid Cross: A cross between a pure tall plant (TT) and a pure dwarf (tt) plant which involves only one trait is called the monohybrid cross. The parents with the genotype TT and tt at first form gametes T and t respectively. These gametes fuse and result in the formation of the F1 generation. The genotype of F1 generation is Tt and the phenotype is tall plants. These tall plants have both the characters and thus, the plants are called heterozygous. On selfing F1 generation, we get F2 generation. The F1 plant being heterozygous forms two types of gametes, with T and t. The union of these gametes can take place in all four possibilities. Thus, the F2 generation forms plants in genotypic ration of 1:2:1 for TT:Tt:tt and a phenotypic ration of 3:1 for tall:dwarf plants.   Dihybrid Cross: A cross between a pure round yellow (YYRR) and a pure green wrinkled (yyrr) plant which involves two traits is called a dihybrid cross. In dihybrid cross, parents homozygous for given traits produced a hybrid in F1 generation. On selfing F1 generation, offspring having characters of yellow (YY), Round (RR), green (yy) and wrinkled (rr) were produced. In the following checker board, we can see that different combinations of characters resulted in the F2 generation from a dihybrid cross. Out of the 16 combinations in the following checker board, 9 have yellow round seeds, 3 have yellow wrinkled seeds, 3 have green round seeds and 1 has green wrinkled seeds. Thus, phenotypic ratio is 9:3:3:1     These crosses helped Mendel to deduce the laws of inheritance.