Scientist have no idea how often Chimeras occur.
Perhaps being a Chimera--perhaps having mixed-up genes--makes someone more inclined to criminal behavior, and so a very high percentage of criminals might be Chimeras.
In such case, it's no wonder their DNA doesn't match the crime scene!
In the last year or so I read two murder mysteries where part of the plot was someone changing their blood type by having a bone marrow replacement for leukemia. (Can't remember the titles or authors)