Ok, I'm well aware, that it's stupid to complain about realism, when there is time travel and killer robots abroad, but still. In the latest episode, they suddenly needed a blood transfusion for someone. So Sarah and her son are tested if there blood type is compatible with that of the injured guy.
First the "doctor" tells Sarah, that he can't use her blood, though she's 0 negative, because the guy needs blood from his own type AB neg. So bravely the little John steps up and offers his blood to be tested. And instead of explaining to him the laws of genetic inheritance, they just tell him that AB neg is super-rare and that the odds are bad. And after the next flashback it turns out that, oh luck, he's compatible!
So, wow, that makes it physically impossible for him to be Sarah's son.
That's either a really interesting plot development or a really stupid mistake.