Background

DNA analysis is a cornerstone of sexual assault investigations. In some cases, forensic experts face challenging casework due to critical samples containing low-template DNA or degraded DNA. These conditions often result in complex DNA mixtures involving allele drop-in and drop-out, as well as variable contributor ratios. Such complexity complicates the attribution of genetic evidence to a potential perpetrator.

Case Description

A woman reported a violent sexual assault involving digital penetration by a single assailant. Collected samples included vaginal lavage, swabs (vaginal, vulvar, hand, and thigh), and clothing from the victim, as well as unknown samples collected from the suspect’s hands.

Methods

Autosomal and Y-chromosome STRs analysis were performed. DNA mixture interpretation was conducted using LRmixStudio™ and EuroForMix. Likelihood ratios (LRs) were calculated to quantify the strength of the genetic evidence.

Results

STR analysis revealed only the suspect's profile in his samples and only the victim's profile in hers, except for a complex DNA mixture of at least three contributors detected in a swab from the victim’s thigh.

This mixture was evaluated by using both EuroForMix and LRmixStudio™. The LR obtained with EuroForMix was seven orders of magnitude higher than that obtained with LRmixStudio™, due to its quantitative continuous model, which accounts for peak heights, stutter, drop-in/drop-out, and degradation.

Y-chromosomal haplotype analysis from the vulvar swab reinforced the autosomal findings.

The suspect’s profile linked him to two additional sexual assaults through the National Police DNA Database.

Conclusions

The detection of the aggressor's genetic profile on the victim’s thigh confirmed the victim’s account. The interpretation of the DNA mixture highligths both the value and complexity of probabilistic genotyping: while EuroForMix and LRmixStudio™ supported the suspect’s inclusion, although EuroForMix provided stronger support due to its use of to its use of the full electropherogram data. Continuous-model software such as EuroForMix enhances the interpretation of complex DNA mixtures, providing robust evidence to forensic investigations and judicial proceedings.

The genetic evidence enabled the conviction of the suspect, and his DNA profile also led to the resolution of two prior sexual assault cases. These results highlight the combined effectiveness of probabilistic interpretation, lineage markers, and database searches, while emphasizing the need for standardized validation and careful interpretation of LRs. Overall, this case highlights the pivotal role of forensic genetics in criminal investigations.

Authors

• Maria Jesus Sanz-Martin (General Commissary of Scientific Police, Spanish Forensic Police, Spain)
• Maria Jesus Sanz-Martin (University Institute for Research in Police Sciences (IUICP), University of Alcalá (UAH), Spain)
• Jenyfer Fernandez-Perez (General Commissary of Scientific Police, Spanish Forensic Police, Spain)
• Jenyfer Fernández-Pérez (University Institute for Research in Police Sciences (IUICP), University of Alcalá (UAH), Spain)
• Adoración Martin-Lopez (General Commissary of Scientific Police, Spanish Forensic Police, Spain)
• Gema Sanchez-Purificacion (General Commissary of Scientific Police, Spanish Forensic Police, Spain)
• Gema Sanchez-Purificacion (University Institute for Research in Police Sciences (IUICP), University of Alcalá (UAH), Spain)
• Marina Gamonal (General Commissary of Scientific Police, Spanish Forensic Police, Spain)
• Marina Gamonal (University Institute for Research in Police Sciences (IUICP), University of Alcalá (UAH), Spain)
• Isabel Martínez de Yuso (General Commissary of Scientific Police, Spanish Forensic Police, Spain)
• Isabel Martínez de Yuso (University Institute for Research in Police Sciences (IUICP), University of Alcalá (UAH), Spain)