level. Indeed, the probability can, in principle, be made so low that DNA typing becomes not simply a method for exclusion or inclusion, but a means of absolute identification.

The potential applicability of DNA typing to forensic samples was demonstrated during the mid-1980s by laboratories in the United Kingdom, United States, and Canada. Their work established that DNA was present in forensic samples in sufficient quantity for testing (see Table 1.1) and that it survived in a state that allowed it to be typed. In the publications in 1985 by Jeffreys and colleagues,2,3 the term ''DNA fingerprint" carried the connotation of absolute identification. The mass-media coverage that accompanied the publications fixed in the general public's minds the idea that DNA typing could be used for absolute identification. Thus, the traditional forensic paradigm of genetic testing as a tool for exclusion was in a linguistic stroke changed to a paradigm of identification. (See Box 1 for a contrasting of dermatoglyphic fingerprints with "DNA fingerprints.")

Forensic DNA typing, first used in casework in 1985 in the United Kingdom, was initiated in the United States in late 1986 by commercial laboratories and in 1988 by the Federal Bureau of Investigation (FBI) and is now being used by dozens of state and local crime laboratories. Because of its great potential benefits for criminal and civil justice, but also because of the possibilities for its misuse or abuse, forensic DNA typing has been subjected to special scrutiny. Important questions have been asked about reliability, validity, and confidentiality:4-6

TABLE 1.1 DNA Content of Biological Samples

Type of Sample

Amount of DNAa

Blood

20,000-40,000 ng/ml

stain 1 cm2 in area

ca. 200 ng

stain 1 mm2 in area

ca. 2 ng

Semen

150,000-300,000 ng/ml

postcoital vaginal swab

0-3,000 ng

Hair:

 

plucked

1-750 ng/hair

shed

1-12 ng/hair

Saliva

1,000-10,000 ng/ml

Urine

1-20 ng/ml

a The amount of DNA is given in nanograms (ng); 1 ng = one-billionth of a gram (10-9 g).



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