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RIGHT?! THAT MAKES NO SENSE

The way they explain the flaws in the blood affinity test is a little incorrect. Actually is based on blood TYPE. Baby has a certain blood type and has antibodies against the other types. Which means that blood will mix with the same blood type and won't mix with incompatible blood types. The issues with this are obvious.

(a) This means that blood will mix with the blood of ANYONE who has a compatible blood type -- so yes, this often includes the father since we inherit our blood type from our parents, but also includes any stranger off the street who might also be Type A. Resulting in false positives. In this case here, it's a good gamble that, since they're biologically related, the uncle would have the same blood type as his nephew resulting in the blood mixing.

(b) The other problem with this test is that the REVERSE could also be true, because two people with a blood type of A and B can have a child with blood type O (O-type can come from having two parents who have the AO or BO genotype -- so child takes one O from each of them resulting in OO-genotype) which WON'T mix with Type A or Type B blood; resulting in false negative.

Basically it's a really bad test.

Thank you! That makes much more sense!
Either way, I wonder how many lives were ruined because of those false results (￣∇￣")

Yeah the amount of problems with that test are endless but the worst thing is that they mixed everything in water. Like, of course the blood wont coagulate right? Anyway I think thats what the story wanted to point out