[prolepunk]

There was an actual real-life crime episode involving that, look up Andrei Chikatilo, the police and a real trouble catching him because of mismatching blood types.

I'd give you a wikipedia link, but apparently I can't use google cache.

[andreyvit]

Actually, English wikipedia does not seem to mention that, and the Russian one says that not only Chikatilo did not have mismatched AB types of blood and sperm, but also that the condition has been proven to be genetically impossible (citing a broken link as a source for this statement, though).