False
Let's break down Mendel's classic pea plant experiments to understand this:
1. P Generation (Parental Generation):
- Mendel crossed purebred tall plants (TT) with purebred short plants (tt).
- As a result, the offspring (F1 generation) were all tall (Tt), showing the dominant trait.
2. F1 Generation (First Filial Generation):
- All plants were tall because they all had the genotype Tt (one dominant allele for tallness).
3. F2 Generation (Second Filial Generation):
- Mendel then allowed the F1 generation plants to self-fertilize.
- The offspring of the F1 generation (the F2 generation) showed a phenotypic ratio of 3 tall plants to 1 short plant.
Therefore, while the F1 generation consisted entirely of tall plants, the F2 generation included both tall and short plants, disproving the statement that all plants were tall in both generations. Thus, the correct answer is False.