Full Idea
It is suggested that conception cannot be the boundary for the beginning of a genetic person, because monozygotic twins can split at any time during the first two weeks.
Gist of Idea
Conception isn't the fixed boundary for a person's beginning, because twins are possible within two weeks
Source
Jonathan Glover (Causing Death and Saving Lives [1977], §9.3)
Book Reference
Glover,Jonathan: 'Causing Death and Saving Lives' [Penguin 1982], p.123
A Reaction
Interesting, but not convincing. If I suddenly learned that I could fission into twins tomorrow, I would be no less of a single person today.