"It’s an extremely rare allele of a blood grouping. Very rare classification. Forget OAB and Rhesus and all that stuff. This is a one in a billion job. Well, not quite, but almost. Now, it so happens that this allele is tucked away on a chromosome segment very close to one of the major gene foci implicated in metatyping. The medical evidence suggests that the allele is incompatible with non-elven metatypes. Trolls, dwarfs, orks can’t be born with it. There’s a lethality effect. The fetus won’t go to term.