In order to obtain the F1 generation Mendel pollinated a pure-breeding tall plant with a pure breeding dwarf plant. But for getting the F2 generation, he simply self-pollinated the tall F1 plants. Why?

To obtain the F1 generation Mendel pollinated a pure-breeding tall plant with a pure breeding dwarf plant because a true breeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations. On doing so he obtained the F_{1 �}generation with phenotype as tall and genotype to be Tt.

For getting the F2 generation, he simply self-pollinated the tall F1 plants because already the two alleles say T from one parent from pollen and t from the other parent through egg are united to produce zygote having one T allele and one t allele. Now to interpret the possible genotype and phenotype of progenies, Mendel self pollinated the F₁ plant of genotype Tt. When fertilisation takes place, the pollen grains of genotype T have 50% chance to pollinate eggs of the genotype t, as well as of genotype t. Also the pollen grains of genotype t have 50% chance to pollinate eggs of the genotype T, as well as of genotype t. As a result of random fertilisation the resultant zygote can be of genotypes TT, Tt, or tt.