Matching DNA samples is neither simple, nor speedy

The L.A. County Medical Examiner/Coroner's Office logo
The L.A. County Medical Examiner/Coroner's Office logo

Jennifer Clark, who receives palliative care for her battle with terminal cancer, says she doesn’t have much time left.

Her sunny way of answering the phone is a testament to her optimism in the face of adversity.

But for the past six months, she’s waited by the phone for the call she’s dreaded.

She had to know for sure if the human remains found in the Santa Clara River wash in December were those of her son Christopher, who was reported missing in July 2018.

Chris Clark

On Wednesday, her anguish of not knowing ended when an investigator with the Los Angeles County Department of Medical Examiner-Coroner called her.

Sadly, she was told, the remains found in December were those of her son.

The pain suffered over six months struggling with his disappearance was matched by an equally painful six months worrying that a skull discovered in the Santa Clara River wash may be his.

The DNA-matching process, comparing one person’s DNA with those of another, is not a simple process that concludes as quickly as it seems on popular TV shows like “CSI.”

DNA specimens

Dr. W. Reef Hardy is the DNA technical leader at the Los Angeles County Department of Medical Examiner-Coroner.

Every year, investigators at the coroner’s office submit between 40 and 80 specimens for human identification made through DNA comparison, he wrote Thursday, explaining the process.

That’s almost one every week.

About 95 percent of the cases are what medical examiners refer to as “Soft Does,” meaning those for which the presumed identity of the dead person is known, he said.

The remaining 5 percent of cases are “Hard Does,” meaning they have no reliable leads to the person’s identity.

In each of these cases, identification is made by looking at the degree of similarity or sharing that exists between the genetic markers present in the DNA extracted from reference samples versus those obtained from the human remains — blood, bone, nails, teeth, muscle.

In the case of Christopher Clark, the human remains uncovered in the Santa Clara River Wash on Dec. 5 included a skull and femur, or leg bone.

Human Genomics Unit

The actual work of comparing DNA samples is done by a special lab at the coroner’s office called the Human Genomics Unit.

In May 2018, the special unit received national accreditation from the ANSI National Accreditation Board, recognizing the work it does.

Unit staffers have helped identity human remains for “soft does,” Hardy pointed out in his explanation of the DNA analysis, specifically in cases where direct or indirect reference samples for the presumed decedent can be obtained from his/her family members.

Direct references, such as a toothbrush, razor or another intimate item used by the decedent, are the most useful as the genetic markers obtained from these samples should match at each position to the decedent’s genetic markers.

DNA testing through kinship analysis, however, can also establish identification using indirect reference samples obtained from individuals who are biologically related to the decedent and which exhibit some sharing of these markers.

Christopher Clark’s next of kin is his mother, Jennifer.

Before he was identified through indirect reference samples — thanks to his mother’s obtained DNA reference sample — Christopher Clark was referred to as soft “Doe 24.”

In the Clark case, his mother supplied investigators with indirect reference samples of her own DNA. She also gave them samples of her son’s teeth, which were still among his belongings. These samples were then compared to DNA garnered from the human remains.

The extent of this sharing of DNA depends upon the degree of relationship, Hardy said.

Degrees of DNA sharing

First-degree relatives, which are parents, children and full siblings of the decedent, share 50 percent of their DNA, while second-degree relatives, such as grandparents, grandchildren, uncles, aunts, nephews, nieces and half-siblings, share 25 percent of their DNA, on average.

Reference samples from first and second cousins who share 12.5 percent or less of their DNA are not considered useful for identification in most government labs due to the much larger number of genetic markers required for identification.

In some cases, however, paternal and maternal lineage markers may be enlisted to help support the degree of relation for individuals whose ancestors are more distantly related.

By next year, DNA experts hope to be able to match people reported missing in Los Angeles County and across the state with the DNA samples of human remains collected by the coroner’s Human Genomics Unit.

In the meantime, identifying Hard Does will not be possible until 2020, when the HGU is expected to get access to the National Missing Person DNA Database, which is part of the Combined DNA Index System, or CODIS database.  

Such access, he said, will allow DNA profiles of human remains determined by the Human Genomics Unit to be uploaded and searched against all direct and indirect reference profiles on file for all missing persons registered as missing from across the country.

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