The Evaluation of Forensic DNA Evidence (1996) / Chapter Skim
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Pages 9-46
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From page 9... ...
These newer molecular techniques permit the study of human variability at the most basic level, that of the genetic material itself, DNA. Standard techniques of population genetics and statistics can be used to interpret the results of forensic DNA typing.
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From page 10... ...
In the meantime, the justification for an inference that two identical DNA profiles come from the same person rests on probability calculations that employ principles of population genetics. Such calculations are, of course, subject to uncertainty.
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According to the FBI, about a third of those named as the primary suspect in rape cases are excluded by DNA evidence. Cases in which DNA analysis provides evidence of innocence ordinarily do not reach the courts and are therefore less widely known.
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From page 12... ...
Prior to cell division, the double strand splits into two single strands, each containing a single base at each position. There are free-floating bases in the cell nucleus, and these attach to each single strand according to the A-T, G-C pairing rule.
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From page 13... ...
The total number of base pairs in a set of 23 chromosomes is about 3 billion. A gene is a stretch of DNA, ranging from a few thousand to tens of thousands of base pairs, that produces a specific product, usually a protein.
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FORENSIC DNA IDENTIFICATION ~NTRs One group of DNA loci that are used extensively in forensic analysis are those containing Variable Numbers of Tandem Repeats (YNTRs)
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DNA Profiling Genetic types at VNTR loci are determined by a technique called VNTR profiling. Briefly, the technique is as follows (Figure 0.31.
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Each of the different DNA samples to be analyzed is placed in a different well. Additional wells receive various known DNA samples to serve as controls and fragmentsize indicators.
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If the two DNA samples, E and S came from the same individual, the two bands in each lane will be in the same, or nearly the same, positions; if the DNA came from different persons, they will usually be in quite different positions.
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If the bands do not match, that is the end of the story: the DNA samples did not come from the same individual. If the DNA patterns do match, or appear to match, the analysis is carried farther, as described in the next section.
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The bottom diagram shows the match window of a fragment along with the fixed bin. The match window overlaps bins 10 and 11.
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(The 1992 NRC report recommends taking the sum of the frequencies of all overlapped bins, but empirical studies have shown that taking the largest value more closely approximates the more accurate hoating-bin method.) Frequency estimates for very rare alleles have a larger relative uncertainty than do those for more common alleles, because the relative uncertainty is largely TABLE O.1 Bin (Allele)
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From page 21... ...
This involves merging all bins with an absolute number fewer than five genes into adjacent bins, so that no bin has fewer than five members. We endorse this practice, not only for fixed bins but also for floating bins, and not only for VNTRs but also for rare alleles in other systems.
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Illinois white population, (B) Georgia white population, (C)
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From page 23... ...
If the contaminating DNA is present at a level comparable to the target DNA, its amplification can confound the interpretation of typing results, possibly leading to an erroneous conclusion. A second disadvantage is that most PCR loci have fewer alleles than VNTRs.
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From page 24... ...
It is difficult to arrive at a meaningful and accurate estimate of the risk of such laboratory errors. For one thing, in this rapidly evolving technology, it is the current practice and not the past record of a laboratory that is relevant, and that necessarily means smaller numbers and consequent statistical uncertainty.
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POPULATION GENETICS If the DNA profile from the evidence sample and that of the suspect match, they may have come from the same person. Alternatively, they might represent a coincidental match between two persons who happen to share the profile.
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However, the A~A~ genotype can also be produced, with equal frequency, by A2 eggs fertilized by A, sperm, so the total frequency of ALAS genotypes is twice the product of the allele frequencies, or 1/125. Therefore, the frequencies of the genotypes are: Homozygote A~A~: (1/10~9 = 1/100, Heterozygote A~A2: 2~1/10~1/25)
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From page 27... ...
Even moderate-sized DNA databases (drawn from samples of several hundred persons) are subject to statistical uncertainty, and in smaller ones, the uncertainty is greater.
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The main reason for departures from random-mating proportions in forensic DNA markers is population structure due to incomplete mixing of ancestral stocks. Suppose that we estimate genotype frequencies in a subgroup by applying the product rule to allele frequencies based on overall population averages.
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From page 29... ...
The measured value of ~ is usually considerably less than 0.01 for forensic markers in the United States, so we recommend 0.01 as a conservative value, except for very small, isolated populations of interrelated people, where 0.03 may be more appropriate. Persons from the Same Subpopulation Usually, the subgroup to which the suspect belongs is irrelevant, since we want to calculate the probability of a match on the assumption that the suspect is innocent and the evidence DNA was left by someone else.
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From page 30... ...
Clearly, this calculation is more conservative than the simple product rule. SOME STATISTICAL CONSIDERATIONS The Reference Database Ideally, the reference data set, from which profile frequencies are calculated, would be a simple random sample or a scientifically structured random sample from the relevant population.
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From page 31... ...
Match Probability, Likelihood Ratio, and Two Fallacies Forensic calculations are conventionally presented in one of two ways: the probability of a random match (called the match probability) , calculated from the frequencies of DNA markers in the database; and the likelihood ratio (LR)
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From page 32... ...
odds that the suspect and evidence DNA came from the same person are the prior odds multiplied by the likelihood ratio (LR) : Posterior odds = Prior odds X LR.
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Uncertainty About Estimated Frequencies Match probabilities are estimated from a database, and such calculations are subject to uncertainties. The accuracy of the estimate will depend on the genetic model, the actual allele frequencies, and the size of the database.
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We conclude that, when several loci are used, the probability of a coincidental match is very small and that properly calculated match probabilities are correct within a factor of about 10 either way. If the calculated probability of a random match between the suspect and evidence DNA is 1/~100 million)
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From page 35... ...
They were intended to place a lower limit on the size of the profile frequency by setting threshold values for allele frequencies used in calculations. The ceiling principle calls for sampling 100 persons from each of 15-20 genetically homogeneous populations spanning the racial and ethnic diversity of groups represented in the United States.
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From page 36... ...
Admissibility of DNA Evidence (Chapter 2) DNA analysis is one of the greatest technical achievements for criminal investigation since the discovery of fingerprints.
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The best protection that an innocent suspect has against an error that could lead to a false conviction is the opportunity for an independent retest. Recommendation 3.3: Whenever feasible, forensic samples should be divided into two or more parts at the earliest practicable stage and the unused parts retained to permit additional tests.
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From page 38... ...
For VNTRs, using the 2p rule for single bands and HW for double bands is generally conservative for an individual locus. For multiple loci, departures from linkage equilibrium are not great enough to cause errors comparable to those from uncertainty of allele frequencies estimated from databases.
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From page 39... ...
Database Searches If the suspect is identified through a DNA database search, the interpretation of the match probability and likelihood ratio given in Chapter 4 should be modified.
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From page 40... ...
If one wishes to describe the impact of the DNA evidence under the hypothesis that the source of the evidence sample is someone in the database, then the likelihood ratio should be divided by N As databases become more extensive, another problem may arise.
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From page 41... ...
We hope that our review of the research will contribute to this process. Our conclusions and recommendations for reducing the risk of laboratory error, for applying human population genetics to DNA profiles, and for handling uncertainties in estimates of profile frequencies and match probabilities might affect the application of the rules for the discovery and admission of evidence in court.
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From page 42... ...
The DNA is first cut into small segments by an enzyme, Hae III. The fragments from the evidence sample (E)
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From page 43... ...
This operation is ordinarily done by a computer programmed to scan the autorad and measure the sizes of the bands. E S1 S2 1~ 3000 2000 1 000 T T , 3000 2000 1 000 FIGURE 0.7 Diagram of a hypothetical autorad for evidence DNA (E)
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From page 44... ...
, which will be used to find the frequency of this marker in a relevant database of DNA marker frequencies. This is the measurement E plus and minus No of TABLE 0.3 Match Windows and Frequencies for Several VNTR Markers in an Illustrative Example Locus Band Size 5~O Match Window Bin(s)
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From page 45... ...
The next step is to compute the probability that a randomly chosen person has the same profile as the evidence sample, E For this, we use the product rule with the 2p rule for the single band.
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From page 46... ...
The allele frequency is determined directly from the database, and the calculations of match probabilities and likelihood ratios are exactly the same as those just illustrated.
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