Because exceptions and aberrations are just that: exceptions and aberrations.
Well that is hardly going to help the pro-choice crowd: the monozygote twins are TWO persons, so abortion would kill 2 instead of 1
Most human chimeras (HC) are usually result of IVF (so not something natural), although there are some natural occurring HCs.
However this is hardly an argument that stands up against pro-life arguments.
Aberrations occur in nature, some are worse than others. The point is that we have no right to kill even if the embryo is defective, just like we do not have right to kill when a human being is defective, either mentally or physically.
Expendable because it’s just a clump of cells with merely potential.
The point also is that an embryo is not a ‘potential person’ but rather a ‘person in e early stage of existence’ like a millimeter high sapling is a ‘early stage of a might oak’. It has the potential of being a ‘full grown person’, but it’s not some kind of abstract concept.
If nature takes its own course, and there is no ‘disturbance’, the embryo will in fact become a person.
Sure the embryo might not become a full grown person at all for natural reason (e.g. diseases)… but then babies and kids can also die (and died a lot in the past!). Then it’s all within human nature however. It’s not a willful act to destroy life.
Pro-choice people are just arbitrarily chosing what a person is and what is not one, at their own convenience.
… and some are nowadays arguing newborn babies are not persons or that mentally handicapped are not persons…