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Relaxing the assumption of unrelatedness in the numerator and denominator of likelihood ratios for DNA mixtures

NCJ Number
Forensic Science International - Genetics Volume: 51 Dated: 2021
Date Published
March 2021

This study conducted a small, preliminary examination of the potential effect of relaxing the assumption that in a DNA mixture, donors are unrelated to each other.


DNA mixtures will have multiple donors under both the prosecution and alternate propositions when assigning a likelihood ratio for forensic DNA evidence. These donors are usually assumed to be unrelated to each other. The current study considered the simple situation of a two-person mixture with no dropout and a two-person major/minor mixture with dropout of the minor contributor. No adjustment was made for subpopulation effects. Mixtures were simulated under the assumptions that the donors were siblings or that they were unrelated. Both unresolvable and major/minor mixtures were considered. The likelihood ratio was compared assuming sibship with the likelihood ratio assuming no relatedness. The LR for hypotheses assuming no relatedness was less than the LR assuming relatedness approximately 95 percent of the time when relatives were present in the mixture. (publisher abstract modified)

Date Published: March 1, 2021