DNA Technology in Forensic Science (1992) / Chapter Skim
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1 INTRODUCTION
1 INTRODUCTION
Pages 27-50
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From page 27... ...
, which are associated with tissue types. The genetically determined person-toperson variation revealed by such typing was used mainly to include or exclude suspects, that is, to determine whether a person showed a combination of genetically determined characteristics consistent with having been the source of an evidence sample in a criminal case or having been the father of a child in a paternity case.
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From page 28... ...
(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)
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The dermatoglyphic fingerprints shown in Figure B-a (Twin A) and Figure B-b (Twin B)
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,( a. FIOURES B-a and H-b Fingerprints of identical twins are distinguishable.
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Samples in Unes A-E are from di~rent whke males; samples B and E are from identical twins.
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· Because DNA can be used to derive medical and other personal information, questions of confidentiality and privacy have assumed greater importance in DNA typing than in the use of non-DNA tests. By the summer of 1989, because of questions concerning DNA typing raised in connection with some well-publicized criminal cases, the scientific and legal communities had called for an examination of the issues by the National Research Council of the National Academy of Sciences.5 7 ~ As a response, the Committee on DNA Technology in Forensic Science was formed, and its first meeting was held in January 1990.
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From page 33... ...
Each autosome or X or Y chromosome is composed of a long DNA molecule constructed as a double helix (Figure 1-11. Each component strand of the double helix is a chain of nucleotides of four types designated by the names of the bases adenine (A)
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. Diagram indicates heterozygosity at a "restriction site": chromosome on left has sequence CCGG that is recognized and cut by specific restriction enzyme, whereas chromosome on right has one-nucleotide difference that results in sequence CTGG, which is not recognized and cut by enzyme.
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The two main types of variation single-nucleotide differences and VNTRs are both potentially recognizable by change in the lengths of fragments that result when DNA is cut with a restriction enzyme. Variation in the lengths of fragments can result from a change in the cluster of four, five, or six nucleotides that is the specific cutting site of the particular restriction enzyme (Figure 1-2~.
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From page 36... ...
. The tools of DNA typing include restriction enzymes, electrophoresis, probes, and the polymerase chain reaction.~~~3 Restriction Fragment Length Polymorphisms In the RFLP approach shown in Figure 1-4, DNA is subjected to controlled fragmentation with restriction enzymes that cut double-stranded DNA at sequence-specific positions.
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From page 37... ...
, neutralized, and transferred from the gel to a nylon membrane, to which they are fixed; this facilitates detection of specific RFLPs and VNTRs. RFLPs that are defined by specific sequences are detected by hybridization with a probe, a short segment of single-stranded DNA tagged with a group such as radioactive phosphorus, that is used to detect a particular complementary DNA sequence.
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DNA was isolated from stains found on vaginal swabs, digested with restriction enzyme PstI, and hybridized to probes at genetic loci D2S44, D17S79, D14S13, and D18S27 and monomorphic locus DXZ-1. Known samples are represented in lanes labeled "victim (known)
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~ ~ ~ ~ ~ ~41~4S~2~ 7~ ~ ~ {~ 1 its ~ ~ ~ ~ ~ !
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With PCR amplification, very small samples of tissue or body fluids" theoretically even a single nucleated cell can be used to study DNA.~-~9 The PCR process (Figure 1-6) is simple; indeed, it is analogous to the process by which cells replicate their DNA.20-23 Two short oligonucleotides are hybridized to the opposite strands of a target DNA segment in positions flanking the sequence region to be amplified; the two oligonucleotides are oriented so that their 3' ends point toward each other.
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~7 cudgeled sequence A Cycle 1 ~, ^ ~ ~lIlIlITIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII~ CycIe 2 ~_^ ~ Cvcie 3 ~..................................... _^ ~ CYcies 4 - 25 UnampI111ed DNA Denature and anneal primers P[lmer extension Denature and anneal primers Palmer exienslon Denature and anneal palmers P[lme'exienslon At Ieas1105- 101d Increase in DNA FICURE 1-6 Polymerase chain renc[ion (PGR)
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From page 42... ...
An alternative method uses an array of ASO probes immobilized on a test strip.34 The test strip is immersed in a solution of labeled PCR product; the PCR product hybridizes only to its complementary probe. This procedure has been called "reverse dot blotting" or "blot dotting." A commercial kit based on the reverse dot blot principle has been released (Cetus Corporation)
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PCR-amplified target DNA is immobilized on nylon membrane, and biotinylated probe hybridizes to the target if there is no nucleotide mismatch. Avidin-horseradish peroxidase (HPR)
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But it will require improvements in automated sequencing technology and the generation of larger databases on sequence variability. POPULATION GENETICS RELEVANT TO THE INTERPRETATION OF DNA TYPING The finding that two samples of human tissue differ in their DNA patterns leads to the conclusion that the two came from different persons.
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INTROD UCTION PERCENTAGE FREQUENCIES ~ 25-30 \ 20-25 15-20 10-15 5- 10 O- 5 A /,'. 45 FIGURE 1-8 Distribution of gene for blood group B in Europe.
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indicating frequency of genotypes in diallelic RFLP system. Probe Allele 1 ~ 14~}~ Allele 2 ~ 1 1 1 Alleles ~I I I I Allele4 ~I I I I I Allele 5 ~r I I I I I ~ ~ (arrows indicate sees of cutting of DNA by restriction enzyme)
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B A worked example: frequency of each of lS genotypes with allele frequencies indicated.
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The major ethnic groupings white, black, Hispanic, Asian, etc.- are each composites of many different subpopulations, which might have quite different frequencies of the alleles used in forensic DNA typing. Allele frequencies estimated from sampling of an overall ethnic group represent weighted averages.
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Construction of a genetic linkage map in man using restriction fragment length polymorphisms. Am J Hum Genet.
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Keyte J Amplification of human minisatellites by the polymerase chain reaction: towards DNA fingerprinting of single cells.
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