Analysts used a number of methods to identify the body of Osama bin Laden after the raid early Monday in Pakistan. One of these was genetic kinship analysis, a method of typing DNA that has been in common use for many years, often helping to identify crime victims and settle questions about paternity. The technique was used to identify those killed during the 9/11 attacks, as well as victims of Hurricane Katrina.
Dr. Frederick Bieber, a medical geneticist at Brigham and Women’s Hospital in Boston and professor at the Harvard Medical School, explained how the process works.
Details about the tests performed on Bin Laden’s DNA are not yet available. But in general, Bieber said, people performing DNA identifications try to compare a body or body part to a known reference sample from the individual being identified — a biopsy, a blood sample, maybe even cells collected from a toothbrush. When a reference sample isn’t available -- as it doesn’t appear to have been for Bin Laden -- indirect identification is based on DNA collected from one or more close biological relatives.
Once samples from all sources are in hand, analysts isolate a bit of DNA from each sample, make lots of copies of it, and then process the copies through a machine that analyzes genetic markers -- DNA fingerprints -- that have been passed down through a subject’s family. Typically, Bieber said, DNA tests examine around 15 of these markers.
By looking at similarities and differences among samples, analysts are able to calculate how likely is it that two or more people are related. They also take into account how common or rare shared markers are in the general population. “If you share rare alleles [forms of a gene], it’s more likely you’re related,” Bieber said.
There is a very small chance that any two random people would share genetic markers. The likelihood that three unrelated people would randomly have the same markers is less. The likelihood that four would is even less, and so on. “The more relatives, the better for the math,” Bieber said.
It typically takes several hours to complete each step of the analysis process, Bieber said, though he noted that in high-profile cases like this one, law enforcement agencies might already have genetic profiles of the relatives available -- which means they’d only have to complete one additional test.
High-priority tests can be expedited, he added.
[Corrected at 1:19 a.m. May 3: An earlier version of this post incorrectly referred to the small chance of three related people having the same genetic markers. It should have said three unrelated people.]