Abstract: The usual way for new cells to come into being is by division of old cells. So the zygote, which is a—new—single cell formed from two, the sperm and ovum, is an exception. Textbooks of human genetics usually say that this new cell is beginning of a new human individual. What this indicates is that they suddenly forget about identical twins.
Comment: This paper can be particularly useful in teaching in two contexts: (1) ethical issues at the beginning of life; and (2) metaphysics of personal identity.