DNA Forensics
At least in the forensic applications commonly done today, people don't actually read out the sequence. For economic reasons, for being able to do this more quickly and more cheaply, they look instead at regions that have spelling differences that are due to repetitions of some sequence. There are repeat sequences all over the genome and, in any particular region, let's pretend this is chromosome #1, you may have three copies. I might have four copies, someone else five copies, someone else one copy - typically, an unimportant repeat that has no biological function, but we all might differ.
By taking that DNA and cutting it with an enzyme that recognizes a distinctive site here, running it out by electrophoresis to be able to separate these fragments by size, and probing it with a piece of radioactive DNA from this region, one can visualize bands corresponding to the lengths of these fragments. And each of these different chromosome configurations, each of these different spellings due to different numbers of repeats, can be visualized as different-sized bands on a ladder, much like a bar code. And so a forensics scientist, examining an evidence sample E here, might probe it first with Probe #1 for the first site of variation and see the pattern. Then he or she might probe it for the next site, the next site, and the next site, and compare it to the DNA patterns taken from two suspects, Suspect #1 and Suspect #2.
If Suspect #1 has a different DNA pattern than the evidence, the suspect is excluded from having committed - well, more exactly - from having left this evidence. It's another question of how the evidence relates to the crime. But that evidence sample of DNA cannot possibly be Suspect #1's.
Suspect #2's DNA corresponds perfectly at each of the four places of variation on the human chromosomes examined. Does this mean that Suspect #2 is indeed the person who left that evidence sample? What does it mean, to say Suspect #2 is included amongst those who left it? Of course, to know how strongly we should take this evidence, we need to know how rare that pattern would be - if it's a question of population genetics - and for that purpose databases have been assembled of how frequent these patterns are in the population. These are tricky questions, and we'll return to them very briefly.
Those are simply the ideas underlying DNA fingerprinting, as it's popularly called, or DNA typing or DNA identification, as we prefer to call it. Within five years of the notion of DNA spelling differences being used for medical purposes, there were already private companies, Selmar, Lifecodes, and others, which grew up to provide DNA typing services to law enforcement officials, and by 1989 the FBI had its own DNA typing lab in the Hoover Building in Washington. There were dramatic ads in the appropriate press, such as this one here from Selmar (showing ad). "DNA Fingerprinting Links the Criminal to the Crime," with the handcuffs here being a double helix.
For the most part, this has been a dramatic and broad success. Increasingly, in rape cases, there is no need for a victim to testify about whether a sexual act took place. There's no question, typically, about mistaken identity being the problem, because DNA from a semen sample can be used to link a suspect to that semen sample. In fact, it has been useful for excluding innocent people. The FBI says that, of many test results, that they could never exclude with standard blood markers, nearly a third of those people are exonerated immediately upon DNA testing. Many rapists, because of this, now plead guilty.
  
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Winding Your Way Through DNA Resource Book Index
