When Mendel performed his experiments, only one trait appeared in the F1 generation, (in this case, round peas) and the other trait (wrinkled peas) had seemingly disappeared

Mendel generation experiments. When Mendel performed his experiments, only one trait appeared in the F1 generation, (in this case, round peas) and the other trait (wrinkled peas) had seemingly disappeared. However, once he let the plants of the F₁ generation self-fertilize, the wrinkled peas reappeared!  If you were Mendel, how would you explain this phenomenon?