Familial DNA

Familial DNA is a somewhat new criminal testing technique that has raised some some controversy.  It works by searching not for a DNA match but a close match.  Close matches are likely to be related providing a clue as to who the DNA is from. This recent Colorado case illustrates the point.

In February 2008, two cars were broken into in the city. Police found blood at both scenes and ran the samples through DNA databases but couldn't find a match. Then, as part of a study being conducted by the district attorney's office, investigators used new software to see whether the DNA in the blood was close enough to potentially be from a family member of someone in the criminal DNA database.

The controversy is rooted in the notion that since the family member has done nothing wrong it shouldn't be legal to test.

"People have a reasonable expectation of privacy of their DNA," said Maryland defense attorney Stephen Mercer. "It's a basic violation of the Fourth Amendment of the Constitution"

Now the Fourth Amendment prohibits unreasonable search and seasure.  Mr Mercer's argument strikes me as not just wrong but silly. 

A person's DNA comes from their parents.  Thus the DNA of related people is similar.  Now, the DNA in a criminal database came from criminals.   It is entirely legal to have a criminal DNA database and to search it for a match.  That is the act of searching IS legal.  If someone's DNA is in the database it is NOT a Fourth Amendment violation to examine it!  Mr Mercer's argument fails on this ground.

If it is legal to search for a match, why would it be illegal to search for a near match when we know that near matches may be related to a criminal?  DNA matching is a technical technique.  That is matches are determined by matching algorithms.  Any argument about familial matches then is ultimately an argument in part about the legality of algorithms.  What would make a near match algorithm illegal if it is not illegal to examine the DNA data to begin with?  It is rather like interviewing a known criminal and asking "Did your brother do this?"  Since there is no prohibition against that why should there be a prohibition against a familial search of a legal DNA database? 

Common sense argues that there shouldn't.