Mixed DNA Profiles Research Articles

Use of human lice in forensic entomology.

Hematophagus arthropod bloodmeals may be useful in identifying individual hosts. To examine the application of human lice as a forensic tool, that is, as evidence of physical contact between individuals, body lice from a laboratory colony and head lice, collected from the head of infested children, were studied. The DNA profile of an individual was detectable in the pooled bloodmeals of two body lice, up to 20 h postfeeding. A mixed DNA profile of two hosts was identifiable in the pooled bloodmeals of three lice, for 3 h postfeeding. By pooling the bloodmeals of three adult and three nymphal head lice, a mixed DNA profile was obtained. These results indicate that, in criminal cases where there has been close contact between assailant and victim, louse bloodmeals should not be overlooked as potentially critical evidence.

EDNA: networking software tailored for identity testing laboratories

Software (known as eDNA 2.0) has been developed for information management for identity testing labs. The software operates in a browser-based, multiuser environment compatible with intranet or internet accessibility. eDNA 2.0 manages accessioning through reporting in a user-friendly, intuitive manner. Parentage testing and forensic applications are incorporated into eDNA, and a kinship module is under development. Highlights of eDNA 2.0 include (1) accessioning templates that record information in a flexible, annotated way that can be easily tracked, (2) phenotype entry for any number of user-defined STR test batteries, (3) questioned relationships defined using dropdown menus, (4) entry of screening results for body fluids, (5) user-defined methods for dealing with mixed DNA profiles, and (6) capability to examine exclusions for possible sample errors.

The persistence of DNA under fingernails following submersion in water

In this paper, we discuss the DNA results obtained from the analysis of cellular material located beneath the fingernails of two women in separate cases of homicide. In both cases, the deceased females had been submerged in water, the first in bath water and the second in seawater, for 2 and 3 h, respectively. Mixed DNA profiles were obtained in each case indicating the presence of foreign DNA in these samples. These results indicate the valuable forensic evidence that can be obtained from samples that may be considered unlikely to provide DNA results and the high priority that should be placed on the routine DNA analysis of material from deceased's fingernails, regardless of the scenes that they are found in, including submersion in water.

A comparison of the characteristics of profiles produced with the AMPFlSTR ® SGM Plus™ multiplex system for both standard and low copy number (LCN) STR DNA analysis

DNA STR profiles have been generated from 1 ng and low copy number (LCN) templates using 28 and 34 cycles of amplification, respectively. Characteristics which facilitate the interpretation of profiles, such as heterozygous balance, allelic dropout and stutter proportions have been quantified. We demonstrate that a reduction in DNA template coupled with an increase in amplification cycle number produces an increased rate of allelic dropout out which can be correlated to the peak areas of those alleles observed. In addition, the LCN conditions increase the degree of peak area asymmetry observed from heterozygotes and the size range of stutters. Analysis of the data allows us to develop sets of guidelines appropriate for interpreting both single and mixed DNA profiles.

Natural DNA Mixtures Generated in Fraternal Twins in utero

Analysis of multiple genetic loci using short tandem repeats (STR) is widely used in human identity testing because the extensive polymorphism at these loci allows for a high degree of discrimination among individuals. We recently received a forensic case that included several pieces of evidence and reference blood samples. Upon initial testing, one of the suspects had a DNA profile that included three alleles at four of the nine loci tested (vWA, FGA, TH01, and D5S818). At each locus, two of the alleles appeared to be "major" alleles with a third "minor" allele present. The profile appeared to be a mixture of two people. Contamination of this first reference sample was suspected and a second, unopened blood specimen was requested from this individual. The DNA profile from this second reference specimen was identical to that of the original specimen at each locus. One of the evidence samples also displayed an identical mixed DNA profile matching that of the reference specimens mentioned above. The relative peak heights of the two "major" and one "minor" allele remained constant in all three samples. Additional background information revealed that the suspect had not received a bone marrow transplant or blood transfusion. However, it was disclosed that this individual is a fraternal (dizygotic) twin. We hypothesize that an exchange of blood cells between the fetuses occurred in utero and that the additional alleles present in these reference samples are derived from cells contributed by his twin sibling. No additional specimens from the suspect or his twin could be obtained for confirmation, and our hypothesis remains untested. Forensic scientists should be aware of this possibility when faced with a DNA profile in which extra alleles at multiple loci are detected.

Taking Account of Peak Areas when Interpreting Mixed DNA Profiles

This paper establishes a logical framework for taking account of peak areas when interpreting mixed DNA STR profiles. The principles apply wherever such data are available but they are illustrated here by means of data which have been collected from made up mixtures of known concentrations analyzed at short tandem repeat loci. The data have led to some modeling assumptions which are used for numerical examples. In actual casework the proportions of the various components will not be known and there is a discussion of whether they should be allowed for by integrating over a prior distribution. This is a conceptual paper, rather than a prescription for casework, and the scope for further work is outlined.