Ever since 1953, when the DNA double helix structure was discovered by American scientist James Watson and British scientist Francis Crick, working together at Cambridge University, the world of molecular genetics has never been the same. DNA turned out to be a polymer (a large chain of repeating molecules), and a particular type of polymer at that, called a nucleotide (sugar and phosphate in a nitrogen base), and furthermore, the nitrogen-containing base (connecting the two sides) was arranged in the form of a palindrome (a series of letters or numbers that reverse themselves; ABCDEEDCBA or 1234554321).
It therefore wasn't necessary to map the whole length of the double helix, which if stretched out would be six to nine feet long. It was only necessary to slice off and look at fragments or strands with one side of the helix where palindromes began and ended. Any one of these fragments, in theory, would be polymorphic enough (contain enough variation in form) to unlock the genetic code (genome) in its entirety.
A few single strands should define how the other strands would look because each side of the helix was an exact complement of the other side, held together by what is called base pairing, the predetermined, palindromic fashion in which base molecules bonded it all together.
DNA typing or DNA identification is perhaps a better word for it than DNA fingerprinting or DNA profiling but this varies by personal preference.
In any event, Cellmark Diagnostics has applied to register the phrase "DNA Fingerprinting" as a trademark. At first, courts were quick to accept DNA evidence almost without question (under Frye or the relevancy test). Almost every crime lab in the United States wanted to make it a routine part of their work, and a number of private labs sprung up, mostly for paternity testing.
Part of the 1994 crime bill, called the DNA Identification Act of 1994, set aside $40 million to help develop DNA capabilities in state and local labs, and also the Combined DNA Index System which is the national database for convicted offenders of sexual and violent crimes. There's a shortage of DNA Crime Laboratories in America. There are only about 120 of them, and they all have at least a year's backlog of work.
For the most part (outside of gross human error), the actual technology of DNA typing is considered unquestionably sound and reliable by the scientific community and the courts (U.S. v. Jakobetz 1992). Weak links exist mostly on the front end (crime scene sample collection) and the back end (presentation to jurors). Questions have also arisen about quality assurance standards and blind proficiency testing in laboratories.
The most important legal issues revolve around the constitutionality of compulsory DNA testing at the time of arrest and the use of DNA profiling and population databases in police investigations. These are all Fourth Amendment issues and privacy concerns. Starting in 1999, for example, the U.S. government began taking samples of blood for DNA records on 99.8% of all babies born.
Alarmed critics were quick to shout things like "bio-invasion", "nation of suspects", and "genetic criminal law". It's difficult to grasp the significance of evidentiary problems without an understanding of DNA testing techniques or the principles of population genetics. Suffice it to say that states operating under Daubert or those that rejected or modified Frye in the first place are the jurisdictions where most legal battles occur, and a proper discussion of those battles would be quite lengthy (and constantly changing).
Discovery is a pretrial process where each side (prosecution and defense) shares their lineup of witnesses, what physical evidence they have, and what trial strategies they plan to pursue. It differs from plea bargaining because charges and penalties are not discussed. Discovery is informal, and depends on the working relationship between prosecutor and defense (typically, it occurs over lunch).
By law, the only thing that must be shared by the prosecutor is anything that might shed light on the defendant's innocence (exculpatory evidence). This is called the Brady doctrine. Discovery is not a constitutional right, but a privilege. Defense should not rely upon a fishing expedition for "all Brady material", and if the process becomes uncooperative, the judge orders something called disclosure, a type of court-ordered discovery.
There's lots of other similar pretrial processes such as suppression hearings and, most importantly, Daubert hearings (for scientific evidence). DNA evidence doesn't lend itself well to informal discovery processes because even though it has the potential to be exculpatory, DNA lab reports aren't exactly the easiest thing to discuss, challenge, or replicate. The reports either say "inclusion", "exclusion", or "inconclusive". They bring an unaccustomed degree of certitude to the courts. It may very well turn out to be the technology that overpowers the criminal justice system.
It's a trial nightmare with DNA evidence. Most everyone remembers the O.J. Simpson trial where DNA testimony was the longest part of it. It's often said that juries rebel against complicated scientific evidence. DNA evidence has the potential to be exculpatory, but DNA reports, with all their certitude in terms of probability, are used less for exoneration and more for confusion. The reports sometimes express probabilities out to so many decimal places that they cover the potential population of three or four solar systems.
About the only thing you can do is attack the lab for its (lack of) quality assurance and proficiency testing, or use a "Chewbacca defense" and try to razzle-dazzle the jury about how complex and complicated the other side's evidence or probability estimates are. Forensic law is evolving in this regard, but pretty much doesn't favor the idea of fishing expeditions in the form of further DNA tests with the purpose of hopefully proving a defendant's "exclusion".
Expense is only one consideration, at least in the pretrial stage. In the postconviction stage, expense is definitely a significant matter. Every inmate in every correctional system across America it seems, right now, demands a DNA exoneration test, at state expense, to prove their wrongful conviction. They also want to see the test results to analyze for themselves. However, there's no discovery or disclosure processes at the postconviction stage.
There's a strong presumption that verdicts, especially those based on DNA evidence, are correct. The standard that appears to be emerging with the issue of postconviction exoneration (Link to Justice Department's 1999 Guidelines on DNA exoneration) is a hearing to determine if "reasonable probability" exists to vacate the verdict. .
Juries are intellectually overwhelmed by DNA evidence, and understandably so. Nothing in forensic science, except maybe mass spectrography, is as complex and complicated. Certainly, nothing has ever before been capable of producing odds like one in seventy thousand trillion (which might be the population of our solar system if all the planets were densely populated).
Except for identical twins, the chances of similarity in DNA are astronomical or infinitesimal. As the O.J. Simpson trial demonstrated, the average American citizen doesn't respond well to advanced topics in molecular genetics.
DNA typing produces what is called a random probability match — sometimes as high as one in several million, sometimes as low as one in a hundred. It is defined as the probability of a match between a sample left at the crime scene and a suspect. Different methods of calculation produce different results, but it all depends on comparing sets of bands or spots on autorads.
That is to say, there are two separate, but related issues: the statistical methodology and the matching criteria. The latter is the closest thing to a margin of error, and it's usually stated in terms of requirements that the analyst observe "no more than a 5% difference in bands on the autorad" before declaring a match.
Some experts have said that a 2.5% criteria would make a better cutoff point. What statistical methodology is used also tends to be what one is comfortable with, and this is the area where DNA typing spills over into population genetics.
Criminologists tend to be interested in karyotyping, the study of the number and type of chromosomes. A major assumption of karyotype studies is that there's an abnormality of some sort causing the criminal behavior. The abnormality most often speculated about is the XYY pattern, involving an extra male chromosome. The XYY pattern occurs in about one out of every 700 to 1,000 males. The XYY male is usually over six-foot tall, exhibits low mental functioning, suffers from acute teenage acne, and is often clumsy. XXY males also occur, about once in every 500 males, but they have smaller genitals than XYYs and tend to have breast development.
The XYY-crime link is a stronger than chance probability (for sex crimes mainly), but such people tend to be found in mental hospitals and prisons, perhaps a testimony to their mental functioning or clumsiness than their criminality. Nevertheless, in some jurisdictions, testing is done on juveniles to discover if they have the XYY genetic marker, and if so, it's used (controversially) as a predictor of risk for adult criminality.
Another area of speculation is whether or not there's anything like a criminal chromosome. This area is sometimes called mapping (for) the criminal chromosome, and you occasionally hear of a study searching for it. It is not, however, part of the Human Genome Project because that project is mainly devoted to discovering the hereditary basis of diseases and defects.
It takes a lot of work to map a chromosome as you've first got to find subjects who haven't been exposed to a lot of environmental forces, like birth or dietary problems.