Question

how did Mendel determine that the F1 tall and parental tall in one of his experiments were genetically dissimilar

Answer 1

First he got the F1 generation in which phenotype was tall but the genotype was Tt.Then he got F2 generation in which he got phenotype 3:1 & genotype 1:2:1 .In this genotype ratio he observed 3 genotypes.Ratio =2 is the hybrid type i.e. Tt which he also observed in F1 .So he concluded that both parental & F1 tall show this same phenotypic effect due to dominant form T but actually T & t are alleles i.e. alternating forms of same gene of tallness.So they are genetically dissimilar as they produce different genotypes.