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Jake's right on the homosexuality example used being not the best, and the question on whether there is a gentic predisopsition for this is an interesting one (I think there is).
If an argument was used that screening could be done only for any characteristic that might impede in procreation, it is, as China Doll, said, impossible to quantify. A girl born with one arm, could be argued to have reduced her ability to attract a mate.
Any any case, it's kind of scary to imagine a future where parents could create their absoulte designer baby, and what that might be...