If a human being is produced via cloning, no fertilization event ever happened, so fertilization cannot be the beginning of personhood.

Since my position is that personhood begins when organism-hood begins, not necessarily that personhood begins when fertilization occurs, this does not defeat my position. In ordinary circumstances, fertilization jump-starts the developmental trajectory that comprises a human organism’s life-history. However, if it becomes possible to jump-start that developmental trajectory using other artificial means (as has been done for non-human animals and may soon be feasible in humans), then the pro-lifer would recognize that a person is present wherever a living human organism is present.