Genetic typing helps to narrow the field of suspects but cannot specify an individual. Forensic analysis of evidence is used to determine genetic markers for the victim, the evidence, and the possible assailants. Four genetic markers are available in semen, blood, and vaginal fluid: ABO blood group antigens, peptidase A, phosphoglucomutase (PGM) and deoxyribonucleic acid (DNA). -U
Of the general population, 80 percent secretes ABO antigens into other body fluids such as saliva. Secretor status is determined by testing for ABO blood type and then testing saliva for the presence of these antigens. Foreign antigens found on a victim may be from the assailant. Peptidase A and PGM are present in semen regardless of secretor status.11 Peptidase A subtypes are common in black populations; typing of peptidase A is done when the assailant may have been black. 4 Peptidase A activity decreases rapidly after intercourse and can rarely be detected after 3 h. PGM has a number of subtypes. By combining ABO type and PGM subtypes, two individuals can be differentiated from each other 90 percent of the time.11 PGM is rarely detected beyond 6 h. Semen stains found on a victim can be analyzed for these enzymes as well.
Deoxyribonucleic acid typing can be used to characterize an individual's unique genetic material and distinguish two individuals statistically. There is a 1 in 30 billion chance that two unrelated individuals will have the same DNA fingerprint.9 This typing can be performed on very small samples of evidence, as well as on old or decomposing samples.
All evidence collected from a victim should be carefully labeled with the victim's name, the type and source of the evidence, the date and time of collection, and the name of the person who collected the evidence. Following instructions for proper collection and storage of evidence is critical to producing optimal laboratory results.-i1
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