“DNA fingerprinting” is a recently-introduced biochemical procedure that uses a pattern derived from a person’s genetic material to match a suspect’s genetic material against that of a specimen from a crime scene. Proponents have claimed astronomically high odds against obtaining a match by chance alone. These odds are based on an assumption that there is independence between the different characteristics represented by a single pattern.

Which one of the following, if true, casts the most doubt on the claim of the proponents of DNA fingerprinting?

[A]. The large amount of genetic material that people share with all other people and with other animals is not included in the DNA fingerprinting procedure.
[B]. There is generally accepted theoretical basis for interpreting the patterns produced by the procedure.
[C]. In the whole population there are various different subgroups, within each of which certain sets of genetic characteristics are shared.
[D]. The skill required of laboratory technicians performing the DNA fingerprinting procedure is not extraordinary.
[E]. In the investigation of certain genetic diseases, the techniques used in DNA fingerprinting have traced the transmission of the diseases among the living members of very large families.
Answer: C

We’re asked to weaken the proponents’ argument, and conveniently enough, we’re given their assumption. Your best bet is to look for a choice that undermines that assumption, which we get in (C): If genetic characteristics can occur in sets, that breaks down the proponents’ assumption that these characteristics occur independently.

(A) is an au contraire choice. If the genetic material that all people have in common with each other and with animals is excluded from the procedure, then that makes the analysis much more specific, and, inferably, more valid.

(B) If anything, this choice strengthens the proponents claim by dismissing a possible argument against the procedure; the argument that it’s unreliable because different people could obtain different readings of the same pattern.

(D) contains a scope shift: The point here is whether the procedure is valid when done accurately.

(E) is irrelevant. The issue is whether or not DNA fingerprinting can be accurately used to match two different samples of genetic material. Any other use is a separate issue.

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