Assume that in a given plant, T represents the dominant trait for tallness over dwarfism (t) and P represents the dominant trait for the purple flower over the white flower (p). If a pure-breeding plant with the genotype TTPP is crossed with the pure-breeding plant with the genotype ttpp and produced the genotype TtPp, how many gametes are formed by the F1 plants when self-crossed in the F2 generation?