Personal Identification.


There are three basic fingerprint patterns: Loops, Arches and Whorls.  Everyone falls into one of these three patterns (diagram).  Within these patterns are what we call minutia points.  There are about thirty different types of minutae points, and no two people have the same types of minutae in the same number in the same places on their fingertips.  This is why our fingerprints are totally unique.  


Your fingerprint patterns are hereditary.  They are formed before you are born, while you are still in the womb, they never change through out your lifetime, and they are even around for awhile after you die.  So, why are fingerprints so good for identification purposes?  They are totally unique, and they never change.  

Your fingerprints are formed underneath your skin in a layer called dermal papilae.  As long as that layer of papilae is there, your fingerprints will always come back, even after scarring or burning.  

Gloves don't necessarily help you from leaving fingerprints.  Surgical gloves were made to keep surgeons from infecting their patients.  You can actually leave prints through surgical gloves.  Surgical gloves were made to keep sterile conditions during operations.  They have to fit like a second skin for surgeons to be able to pick up their instruments. They fit so tightly that fingerprints 'pass through' the latex membrane.  They can also be turned inside out to yield fingerprints from the inside surfaces.  Leather gloves can be treated in the same manner.  Also, leather gloves can leave a print that is unique to that glove and no other (leather comes from cow skin, which is just as random as human skin).  Even cloth gloves, such as mittens, can leave a distinctive print that can be traced back to the mitten that made it.

Prints are left on a surface because we are constantly secreting water and body oils and other compounds through our pores.  This material is left on the surface we touch in the form of a fingerprint.  

Different surfaces require different techniques for developing prints.  In the movies, you usually see detectives with brushes.  They are powder processing the prints.  Minute particles of powder cling to the print residue as the brush passes over it.  The print is then lifted with tape.  Another process involves fuming.  Vapors of iodine and superglue (bonds in seconds) will coalesce inside the print residue to reveal a latent print.  

Try this at home:  You will need a zip lock baggie, a tube of superglue (i've found the gel version works a little better) and a can of soda or drinking glass.  Place the can or glass in the zip lock baggie.  Lay the baggie flat on the counter.  Squeeze out a generous amount of superglue into the baggie (don't glue the glass to the baggie).  Seal up the baggie.  If you can, blow some hot, moist air into the baggie, like when you're trying to fog up a window with your breath.  Seal it tight.  The vapors from the superglue will build up in the tiny zip lock atmosphere and creep up into the prints on the glass or can.  There, they will crystallize and, after awhile, you should see starchy white fingerprints develop on the glass or can.  

There are special processes that develop prints on paper, wood and cardboard.  Fingerprints can be developed on objects that have been in water.  Prints can be developed off of skin (such as from the neck of a strangulation victim).  There are very few surfaces on which a print cannot be developed.

Computers have revolutionized the techniques used to match fingerprints.  Until recently, the old standard was the Henry Classification System; a cumbersome sequence of letters and numbers broken down into several levels of classification.  It could take weeks, sometimes months to compare a suspect fingerprint to a department's print files.  The advent of digital technology has changed all of that. Prints can be image scanned directly into a computer, doing away with ink and fingerprint cards.  Prints can be compared at a rate of 400,000 per second.  You couldn't do that in your lifetime.

It's called AFIS.  Automated Fingerprint Identification System.  Departments will input all the prints from arrests and all of the print cards they already have on file to create an historical record.  They also input all of the prints from any unsolved crimes, in the hope that a hit might come up from a routine arrest.  Local departments are linking their systems into a national database.  The FBI wants national standards and a fully functional national network in place by the year 2000.  With a national network, you could get busted in New York and have a print hit come up from a crime you committed in California.  


A quick primer on DNA: DNA is constructed like a ladder; a ladder that has been grabbed at both ends and twisted, creating the double helix shape.  The rails of the ladder are phosphate and sugar groups.  They link together (sugar+phosphate+sugar+phosphate) to form the backbone.  So far it's not so tough.  There are four bases that form the rungs of the ladder: Cytocene, Guanine, Thiamine and Adnine.  They are always in pairs and always complement each other; Cytocene is always paired with Guanine and Thiamine is always paired with Adnine.  Each base forms half of the rung, meeting in the middle.  Now, think of that ladder as a zipper.  When DNA replicates, is unzips down the middle, separating the base pairs like a zipper.  The complementary bases now attach to the opened segments to make new DNA.  


One DNA analysis technique looks at junk DNA.  Everybodys' DNA is pretty similar.  Everyone who has blue eyes has pretty much the same code for blue eyes. Everyone who has brown hair has pretty much the same code for brown hair.  But these coding sequences are separated by 'junk' DNA.  This 'junk' DNA is non-coding and only serves to separate the coding sequences.  These 'junk' DNA sequences are totally random and totally unique to an individual.  The process is extremely technical, but that is the concept, and it's really not that hard to understand.  

Now, the way this works in a criminal case is thusly:  The examining labs have samples of DNA, taken from a representative population group.  These are entered into a database, to which the questioned DNA (that being compared for analysis in a case) is compared for frequency among the population group.  That is DNA testing at its most basic.  

Other Sub-disciplines.

There are numerous other areas of study in the field of personal identification.

Forensic Anthropology seeks to identify remains, such as bones.  From a skeleton, you can tell if the subject is human, its sex, the age of the subject when death occurred, stature, how long the remains have been in their current state and the cause of death.  

Facial Reconstruction gets in the media once in awhile.  The photo imposition technique involves imposing a photo of the skull over a photo of the face in question.  This is examined in the Romanov book in the 'suggested reading' link.  Three dimensional reconstruction technique involves adding depth markers for tissue thickness onto a skull.  The depths are an average taken from cadavers.  Clay is placed over the skull, to match the depth of the markers, to simulate skin and muscle.  The features are then smoothed out to make a 'face.'  This has worked in a number of missing persons cases, but is considered more art than science.  

Hair Comparisons are another biggie in forensic science.  Hair can be determined to be human or animal.  The body area from which a questioned hair came can be identified.  Race can sometimes be determined.  Disease conditions can be determined. And, of course, a hair found at a crime scene can be matched to the person that left it there.  It's a relatively simple comparison, involving side by side examination of the suspect and known hairs, similar to bullet matching.


About the Author.

The author is a corporate/legal/forensic investigator and holds a B.A. in Criminal Justice   and an M.A. in Forensic Science, specializing in counter-terrorism and questioned document examination.  He has worked with local police, at the federal level, and at the international level specializing in investigative work and executive protection.  The author currenlty works in the private sector as Manager of Investigations for a corporate investigative firm.