Why DNA databases may not be a good idea - but we should to them anyway.

New York State is passing a new law that DNA from anyone convicted of almost any crime will be required to give a DNA sample.  This will double the state's DNA database. (Source)

I am conflicted about this.   On the one hand, it is likely to put more criminals in jail, and possible (no guarantee) of protecting innocent people.   But there are several valid arguments against it.  I will use fingerprints for comparison purposes.

1. Targeted vs scatter shot DNA tests.  The current system is fairly targeted.  What they are talking about is a scatter shot method.  As most people know, DNA is not perfect.  They don't say "THE BLOOD WAS HIS."  Instead they say "There is 1 in 30 million chance that the blood is anyone but his."  (See Reason #4 below).   That's fine if you use a targeted approach:  you start with five suspects and you compare their DNA and find one and only one of them is a DNA match (with a 1 in 30 million chance of a false match).  It's not fine when you don't have a suspect and check the DNA against all 3 million people currently in the system (scatter shot).   Then the odds become 3 million in 30 million.  By shear chance you get a guy in Buffalo, with no connection to the victim, who was once convicted of drunk driving and is now a suspect in a murder that happened in East Hampton (over 400 miles away).  Hope he has an alibi.  Even then, chances are he is going to have to pay for a lawyer and a lot of travel.  This can't happen with a full fingerprint, and the cops are experienced enough to take partial fingerprints with a grain of salt.
2. DNA contains evidence of intensely private things you don't want made public.  Sometimes you don't want to know it yourselves.  Are you adopted?   Did your mother cheat on your father?  Did your father cheat on your mother and you have a half brother you didn't know about?  Some diseases that don't have treatments are detectable by DNA.  Some people don't want to know that information, and once it gets into a database, it can sneak out.  Particularly in a small town where the cops are nosy busybodies - and who may know the suspects.  Fingerprints don't have these issues, they are not connected to diseases nor do they run in families.
3. Fingerprints are MORE accurate than DNA.  But most people falsely believe that fingerprints are less accurate..  Yes, you heard me, fingerprints are more trustworthy.  There has not been a single case of someone being found with the same fingerprints as someone else.  Moreover, every single person has ten fingerprints, and they don't match itself or other people.  With seven billion people in the world, that's a pretty good sample size of 70 billion, individual, non-matching fingerprints.  That is NOT the case for DNA.  Even assuming we use a full genome decoding, people have twin brothers and cloning is just around the corner.  Of the 7 billion, approximately 10 million are identical twins (monzygotic), or about 2 out of a thousand people.   (Source)
4. Most fingerprints found are partials.   When you say partial print, the jury and the judge all realize that it is not 100% accurate.   EVERY SINGLE DNA TEST IS A PARTIAL - BUT THEY DON'T ADMIT IT.   They never decode your entire genome, they test only a tiny fraction of it. You have 3 billion base pairs they could test, but in the US, only test 13 loci, each of which may have as little as 4 base pairs. (Source).  Up until recently they did not have the technology to decode your entire DNA, and even now it is very expensive and takes months, if not years to do.
5. It is easier to fake DNA, but harder to convince a jury the DNA is fake.  If your want to fake DNA you get your hands on a sample and spill it over the place.  Most men are perfectly willing to give pretty women a DNA sample (forgive me if I don't explain why).  To fake a fingerprint, you have to get a sample, then use some rather complex scanning equipment to create a rubber mold - it's like something out of a spy movie, not something most people can do.  Or you have to trick the target into touching something small and mobile - which a good defense attorney will bring up ("Note sirs, the only place my client's fingerprints were found was on a bicycle handle. How exactly did the bicycle get into the changing room in Macy?")
6. The prosecutors do not just let you out if DNA proves you are innocent.  Prosecutors come up with strange crime theories ("He must have had a partner - even though I never argued that to a jury."), and often refuse to let people even get a DNA test, let alone get out of jail if proven innocent by the DNA test.  (Source)
7. It's not just the arrested person whose DNA privacy gets invaded.  They also do "family matches' and because you got arrested, your sister's DNA is now detectable.  ("Sir we got a family match with John Terrel, a man convicted of check fraud.  The DNA found in this brothel is similar but not identical.  It must belong to a half sister of his.  Shall we go ask him where his sister is?)

All that said, I do think DNA is a good crime detection device.   I would probably pass a similar law, with some minor changes.  You need to protect against the problems I have mentioned.

Specifically, I would require that everyone, even convicted criminals have the right to test DNA related to a case.   If the DNA does not support the conviction, then they will, required by law, get a trial to determine if the DNA would have been sufficient to get them off.   In such a case, the prosecutor should be forbidden from arguing any theory of the crime not presented in the original trial unless they have evidence to support it.  That is, if they want to claim he had a partner, when they did not do so originally, they must find evidence of said partner (besides the DNA).

In addition, I would want all DNA experts to be required to refer to DNA matches as partial DNA matches unless the entire genome was decoded and matched.   You can't get away with imprecise technical language just because it's common practice.

Finally, I would require that if someone was not a suspect before the DNA match was done, then the prosecutor must admit that and explain how many people they tested the DNA against to find him.  That is you can't say "One in 200 million chance of false match" if you compared the DNA to one million suspects.  You have to say "One million in 200 million - or only 1 in 200."