When Mendel's first generation pea plants were allowed to self-pollinate, the ratio of tall (dominant) to short (recessive) plants in the second generation was) plants in the second generation was

In second generation, ratio of tall (dominant) to short (recessive) plants = 3:1.

Explanation:

When Mendel crossed two pure breeding parents for plant height, in filial 1 (F1) generation he got all the plants with tall height. But when he further allowed to self-pollinate F1 generation plants, he got tall to short ratio as 3:1 in F2 generation.

The crosses are depicted as under:

Parental:       TT    x   tt

                    /   \       /   \

F1 :             Tt   Tt   Tt   Tt

When F1 were self pollinated as depicted below he got 3:1 ratio for tall and short height:

            Tt    x  Tt

           /  \       /  \

F2:    TT  Tt   Tt   tt  

The plants with genotype TT, Tt, Tt will be tall heighted while the ones with tt genotype will be short heighted.