It depends on what the initial genotypes were, but lets use Mendel's experiment to work this out. Lets say that the genotypes for the yellow seeds are YY and GG for the green ones for the parental generation. You have to show two letters, with each letter representing an allele from each parent. In the case of the parental generation, both seeds either inherit only Y alleles are G alleles from each parent. If you cross these two seeds, all progeny in the F1 generation will have a genotype of YG, but the phenotype (the physical characteristic) of all the progeny in the F1 generation wil be yellow coloring because the Y allele is autosomal dominant. Now, if you cross breed the F1 progeny, which are all carrying one allele for Y and one allele for G, the phenotypes and genotypes will be different in the F2 progeny then they were in the F1 progeny. The genotypes for the F2 progeny will be produced in the following ratio: YY, 2YG, GG or 1:2:1. Since Y is an autosomal dominant, the phenotypes that the seedlings show will also be different from the F1 in the F2 generation as followed: 3 yellow and 1 green or 3:1. Notice that 2 of the seedlings produced a genotype of YG, but since the y allele is autosomal dominant, those seedling will have a yellow color to them and not green. The only way that you can produce a seedling with green coloring in the F2 generation is that the seedlings have to inherit two G alleles from their parents. This is why you will not observe any green seedlings in the F1 generation because only one parent had alleles for green in the P1 generation, but you will observe it in the F2 generation because both parents carried at least one allele for green in the F1 generation.