Mendel crossed a true-breeding tall pea plant (TT) with a true-breeding dwarf pea plant (tt). All F₁ plants were tall, but when the F₁ plants were self-pollinated, dwarf plants reappeared in F₂, exactly as dwarf as the original dwarf parent, with no blending. Which Mendelian law is based on this recovery of the parental characters unchanged in F₂, and explains it by the separation of the two alleles into different gametes?