Editor's note: This article was originally published on May 9, 2011.
If the DNA left at a crime scene does not precisely match a profile in a criminal database, state authorities may look for similar profiles to find relatives of the perpetrator.
Children inherit one genetic marker from each parent at every location on a chromosome. In forensic DNA tests, each marker is assigned a number to designate its type. Relatives will have many markers in common.
If a crime is serious and other leads are exhausted, authorities may try to find relatives of the perpetrator by looking for DNA profiles with many of the same markers.
The search takes into account the rarity of each marker and produces a list of the 200 most likely potential relatives.
Criminalists then test the DNA of the potential relatives against the crime scene sample. They look for markers on the Y-chromosome that are shared by fathers, sons and brothers.
Once scientists identify the likely relative, investigators search records to determine who in that person's family was the right age and in the right place to have committed the crime.
After further vetting, police are given the suspect's name and obtain a DNA sample from him to check against the genetic evidence left at the scene of the crime.