Limitations of Forensic Kits in Identifying the Number of People in Blood Mix Samples of Relatives and Non-Relatives in a Pilot Study

A DNA mixture is an example of at least two individuals having the genetic material mixed together. In criminal cases related to rape or murder, several individuals have been mixed together in crime scenes through sperm, blood, and etc. Identifying the number of people in a mixed DNA sample is very effective in solving criminal cases. The purpose of this study was to investigate the limitations of identity kits for the identification of more than one person's mixed blood samples.

This pilot study was conducted at a medical genetic laboratory in Tehran, Iran in 2018-19. The 17 blood mixtures of relatives and non-relatives were combined and then amplified using an identity recognition kit containing 17 STR markers. The number of alleles in each locus was read by XL 3130 Genetic Analyzer to calculate the genotype of the samples and the number of alleles at each specific site and the number of individuals from each blood mix. Samples were analyzed using Gene Mapper 4.1 software.

Maximum four, six, six and four alleles were observed in the samples of a mixture of two, three and four unrelated and three unrelated individuals, respectively. As a result, it was not possible to accurately detect the number of individuals involved in the samples of more than three individuals using kits with a maximum of 17 markers.

One of the major limitations of conventional STR kits with up to 17 STR markers is the identification of more than three individuals in mixed samples. This limitation is much greater for mixed samples in which individuals are related.