Science, people. You should study it!

[hellziggy]

So I'm watching the episode of Sarah Connor Chronicles from two weeks ago. John's uncle needs a blood transfusion. His blood type is AB-. Sarah is O-, universal donor. John says "Test my blood" 'cuz he figures he's got a good chance of being a match since it is his dad's brother. And it turns out that John has AB- blood too, just like his uncle.
O-kay...
If we use actual science, then there is no way Sarah can be his mother because she has two O blood genes and to be AB John would have to have gotten either an A or a B from her.
This isn't rocket science people! If your story has a plot point contingent on blood type you should probably do 5 minutes of research!