Actually, DNA testing that people do at Ancestry, 23 and Me or any of the other companies is NOT thrown into a database that law enforcement can search. The only way that LE can access the DNA at those companies is by a valid search warrant which would need to be specific, LE cannot go on a fishing expedition. This is Ancestry's basic statement to LE on the issue:
Law Enforcement Requests in the United States:
Ancestry will release basic subscriber information as defined in 18 USC § 2703(c)(2) about Ancestry users to law enforcement only in response to a valid trial, grand jury or administrative subpoena.
Ancestry will release additional account information or transactional information pertaining to an account (such as search terms, but not including the contents of communications) only in response to a court order issued pursuant to 18 USC § 2703(d).
Contents of communications and any data relating to the health or DNA of an Ancestry user will be released only pursuant to a valid search warrant from a government agency with proper jurisdiction.
If we receive a valid request under U.S. law to preserve records that constitute potentially relevant evidence in legal proceedings, we will preserve, but not disclose, a temporary snapshot of the relevant account records for 90 days pending service of valid legal process as described above.And here is a link to the entire page to LE.
http://www.ancestry.com/cs/legal/lawenforcementAll of the companies have basically the same position on releasing DNA to LE.