Comment by natded

The parent did a partner selection before the breeding took place, unless there was rape involved; and I doubt a parent that wants a kid that turns out right goes to: "hmm today I will breed with the disabled person to make for great offspring", no they will select for a mate that they think is good and this choice is implicitly eugenic: they will not declare to be august masters of eugenics but they will look for a good partner to provide healthy offspring.