NCJ Number
218825
Date Published
October 2000
Length
13 pages
Annotation
This article presents a general formula for calculating likelihood ratios for DNA mixtures based on Recommendation 4.1 of the Second National Research Council Report (NRC-II).
Abstract
For a variety of crimes, such as rape, evidence samples often contain material from more than one person. Recommendation 4.1 is the most popular procedure for single-source samples, and many laboratories have adopted it because of its simplicity. This article proposes a general formula for calculating match probabilities for mixed samples under the population genetics model underlying Recommendation 4.1. The authors consider mixture problems with indistinguishable contributors. In some cases, when one of the contributors is known, the genetic profile of the unknown contributor may be inferred. The peak area or height can also be used to enhance or improve interpretation by distinguishing the major and minor contributors. The proposed method considers all possible contributing genotypes. This yields a conservative conclusion that gives the benefit of any doubt to the defendant. The authors present a theorem for calculating the match probabilities for mixed-stains based on Recommendation 4.1. The theorem is applied to a Hong Kong rape case in which a mixture of two DNA profiles was found, as well as to the O.J. Simpson murder case, in which blood evidence found in a vehicle owned by Simpson contained a mixture of Simpson's DNA profile and that of one of the murder victims (Ron Goldman). The authors advise that besides the likelihood ratio method proposed in this article, the probability of exclusion (PE) can also be used for DNA mixture analysis. The PE provides an estimate of the portion of the population that has a genotype composed of at least one allele not observed in the mixed profile; however, the PE does not use all of the available genetic data. 18 references