At ISHI35 last month, Julie Conover Sikorsky, a Forensic Scientist Supervisor at the Palm Beach County Sheriff’s Office, spoke about an unusual case she recently encountered.
During the routine processing of a sexual assault kit, the victim’s standard was discovered to be a two-person mixture.
As if DNA interpretation wasn’t challenging enough, it looked like the lab may be working on the case of a chimera. The situation forced analysts to think outside the box to ensure accuracy and transparency—and ultimately justice.
Case background
On Feb. 23, 2023, a 21-year-old woman reported being sexually battered by her ex-boyfriend in the back of her car. The victim’s father located her afterward using the Find My Friends iPhone app when he had a feeling something was wrong.
An extensive sexual assault kit was taken, with the forensic nurse noting that the victim was menstruating at the time and had bleeding from the anal cavity.
Both the victim’s sexual assault kit (SAK) and the suspect’s physical evidence recovery kit (PERK) were sent to the lab. In April, the SAK was y-screened, and male DNA was indicated on 11 samples.
In July, analysts triaged the samples based on the Y-screening results and forwarded 2 samples from the SAK and 1 sample from the PERK, as well as the suspect and victim reference standards.
This is where the complication started—the victim’s standard was discovered to be a 2-person mixture. Only 3 loci had more than 2 allelles.
“The first thing we think of is maybe it was contaminated, but the other thing we think of is maybe its related because there’s not a lot of mixture there,” said Sikorsky.
The team stopped interpretation and requested a new standard. In the meantime, they ruled out both contamination before and during lab processing.
The second standard arrived from the victim and it was the same 2-person mixture.
Medical history and chimerism
Sikorsky said she recalled a presentation at ISHI 2021 from the Washoe County Sheriff's Office (Nevada) about a staff member going through a bone marrow transplant and tracking his transition and what happened to his DNA and bodily fluids in that time. Sikorsky also had a current staff member going through the University of New Haven’s Forensic Investigative Genetic Genealogy program who was working on a Master’s Thesis regarding chimeras.
At this point, all the evidence pointed toward the possibility that the victim had a medical procedure that caused chimerism. So, Sikorsky asked the detective to inquire about any transplant-related events in the victim’s medical history.
“It seems easy for us to do this, but this is a victim who has been traumatized,” she said. “It’s not easy.”
The victim did however confirm she had a bone marrow transplant from her biological sister. The sister’s standard could not be visually excluded as a contributor to the victim’s mixed reference profile. Essentially, it is now the victim’s true DNA profile.
DNA interpretation
The team threw around ideas before deciding to attempt to deduce a single source profile for the victim using replicate analysis in STRmix.
“We essentially treated the victim’s mixed reference sample as an unknown. And we used the DNA profile from replicate runs of the victim’s standard as the evidence and we conditioned on the sister’s profile. We generated a single source profile from that,” said Sikorsky.
While it was a good approach that could be applied to future cases with interpretation involving related and unrelated donors, there were more complications. The suspect could not be visually excluded as a contributor to the SAK profiles, so the team needed to determine if the deduced victim profile and donor (sister) were visually included or excluded.
This is where the analysts thought outside the box. Both the vaginal swab and the peri-anal/anal swab contained all 3 contributors. The front of neck swab contained two contributors—with the victim’s deduced profile excluded and donor profile conditioned.
“In that area (of the body), the donor’s DNA had completely taken over the cellular population,” explained Sikorsky.
The penile swab was a different story. The number of contributors was two again, with the suspect conditioned on. The original mixed victim standard could not be visually excluded. Then, when the donor and deduced victim were compared individually, they were excluded due to drop out. In fact, the degradation index was 24.99—Sikorsky said the lab usually flags at 2.
Knowing the victim was included, the team decided to reamplify to 1.2 ng initially, and again to 2 ng. There was less genetic information and even more allelic drop out.
However, there was a saving grace—at one genetic marker, vWA, there was one more allele. And that allele had “crazy” imbalance, which allowed analysts to call it 3 contributors.
“I asked the analyst, ‘Is this a standard-driven interpretation?’ She said, ‘No, we already knew this. If you look at the data, it is already supported,’” recalled Sikorsky.
So, while it was less information overall, STRmix considered drop out because the number of contributors was determined to be 3. The forensic analysts were then able to calculate the compound likelihood ratio for the penile swab.
That is how the DNA of 3 people showed up in a rape kit involving only one suspect and one victim. And that is how the forensic analysts thought outside the box to ensure the victim’s greatest chance at justice in the coming weeks/months/years.