The Evaluation of Forensic DNA Evidence (1996) / Chapter Skim
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1 Introduction
1 Introduction
Pages 47-59
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From page 47... ...
From an 1892 review in The Athenaeum of Finger Prints, by Sir Francis Gallon DNA technology makes possible the study of human variability at the most basic level the level of genetic material, DNA. Previous methods using blood groups and proteins have analyzed gene products, rather than DNA itself.
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From page 48... ...
, affirmed the value of DNA typing for forensic analysis and hailed it as a major advance in the field of criminal investigation. In an introductory statement, the committee wrote: We recommend that the use of DNA analysis for forensic purposes, including the resolution of both criminal and civil cases, be continued while improvements and changes suggested in this report are being made.
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From page 49... ...
THE COMMITTEE'S TASK The committee's task statement reads: The committee will perform a study updating the previous NRC report, DNA Technology in Forensic Science. The study will emphasize statistical and population genetics issues in the use of DNA evidence.
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From page 50... ...
Those three sets of questions are related. All fall within the committee's task of analyzing "statistical and population genetics issues in the use of DNA evidence," and of reviewing "major alternative approaches to statistical evaluation of DNA evidence." To help answer the questions, we discuss the current state of scientific knowledge of forensic DNA-typing methods (Chapter 2)
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From page 51... ...
Furthermore, a number of convicted persons, some of whom have spent as long as 10 years in prison, have been exculpated by DNA testing.2 Because cases in which a suspect is excluded by nonmatching DNA almost never come to court, experts from testing laboratories usually testify for the prosecution. In exceptional cases, the prosecution, relying on other evidence, Scores of convicted felons are petitioning courts to allow tests to be performed on preserved samples, and more than seventeen of those exonerated by post-conviction DNA testing have been released.
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From page 52... ...
The ceiling principles have been strongly criticized by many statisticians, forensic scientists, and population geneticists (Cohen 1992; Weir 1992a, 1993a; Balazs 1993; Devlin, Risch, and Roeder 1993, 1994; Morton, Collins, and Balazs 1993; Collins and Morton 1994; Morton 1994) , and supported by others (Lempert 1993; Lander and Budowle 1994~.
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From page 53... ...
The standard ANTE system entails data that are subject to imprecision of measurement, so that very similar DNA patterns cannot be reliably distinguished; we discuss these problems in Chapter 5. Furthermore, most current VNTR methods require radioactive materials, and the procedures are slow; it can take six weeks or more for a complete analysis.
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From page 54... ...
Nevertheless, the two applications are different in important respects. Paternity testing involves analysis of the genetic relations of child, mother, and putative father; crime investigations usually involve the genetically simpler question of whether two DNA samples came from the same person.
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From page 55... ...
It might also be misleading, for example, if this gross underestimate led a person to think that he could drive from Los Angeles to New York on one tankful of gasoline. Extreme differences arise if one expert relies solely on direct counts of genetic types in the database and uses no population genetics theory whereas the other makes assumptions grounded in theory.
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From page 56... ...
VERY SMALL PROBABILITIES If a testing laboratory uses genetic markers at four or five VNTR loci, the probability that two unrelated persons have identical DNA profiles might well be calculated to be one in millions, or billions, or even less. The smaller the probability, the stronger is the argument that the DNA samples came from the same person.
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From page 57... ...
stated that "an expert should given the relatively small number of loci used and the available population data avoid assertions in court that a particular genotype is unique in the population." Yet, what meaning should be attached to a profile frequency that is considerably less than the reciprocal of the world population? Given a person with a profile the frequency of which is estimated at only one-tenth the reciprocal of the world population, the probability that no one else in the world has this profile is about 9/10.
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From page 58... ...
THE NATURE OF OUR RECOMMENDATIONS To deal with uncertainties about population structure, the 1992 NRC report recommended a ceiling principle and an interim ceiling principle. We replace those ad hoc recommendations with the explicit assumption that population substructure exists and recommend formulae that take it into account.
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From page 59... ...
Such prescriptions inevitably involve considerations beyond scientific soundness. Nevertheless, we recognize the connection between our scientific assessments and the efforts of the legal system to develop rules for using forensic DNA analyses; we describe the relationship between our conclusions about scientific issues and the admissibility and weight of DNA evidence in Chapter 6.
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