"Dexter" Fridge Logic

So I've been roped into watching Dexter recently (not that I'm suffering or anything), and I noticed something that triggered my geek reflex. This kind of thing might involve spoilers, so anyone who hasn't watched Dexter yet and wants to might do well to skip this post and watch some more Halloween videos.

I'm still on season one (yeah, yeah... lots of catching up to do), and I think it was episode 8 that I was watching (maybe 9) tonight, when we had one of the flashbacks.

As a child, Dexter was injured while playing. He needed a blood transfusion, but he has the rarest of blood types: AB-. The hospital was stymied by the lack of a suitable donor, and only Dexter's foster father was able to identify a blood relative who would have the right blood type.

Two problems:

1. It's not that hard to find a donor for a recipient with type AB-. Any Rh-negative donor will do, because AB types can also receive A, B, or O types without any problems.
2. The episode implies that the donor is Dexter's biological father, but if Dexter is type AB-, then his father must be either A- or B-, not AB-, so even if Dexter needs an exact match for some reason, his biological father couldn't be an exact match.

Whew! I've got that bit of geekery off my chest. Back to Halloween videos.

Edit: I suppose Dexter's biological father could be AB-, since he could donate either the A or the B if his wife donated the other one. My bad. Point #1 still stands.

Further Edit: A little research indicates that less than 10% of the population of Florida has Rh-negative blood, so finding a relative might still be fairly important.