The US government has said that DNA samples from family members of Osama bin Laden were used to positively identify his body with “99.9 percent” certainty. How would this work, and how can the government be so certain?
Modern forensic techniques analyze short tandem repeats — pieces of DNA that are copied different numbers of times in different individuals on the chromosomes that contain an individual’s genetic code. Christie Wilcox blogs about how polymerase chain reaction and genetic sequencing are used to count the number of tandem repeats in a genome. It’s then easy to compare the tandem repeat profile between two individuals to find out whether their DNA profiles match.
Chad Nusbaum of the Broad Institute of MIT and Harvard in Cambridge, Mass., estimates that this process would take five or six hours, or even faster in a lab using proprietary technology dedicated to forensic identification. This would have given military analysts time to perform a DNA analysis after the U.S. raid on Monday morning in Pakistan, and deliver results to President Obama by the time of his announcement on Sunday night in Washington, DC.
The equipment necessary for such a DNA analysis could have been located on a ship or at a base near the site of the raid, for instance, at Bagram Airfield in Afghanistan.
But how can the government be so certain that the man it has killed is bin Laden?
The strongest possible match would be made by comparing a sample taken from the putative bin Laden body to previously collected samples of his own DNA.
“My guess is maybe they had samples of bin Laden’s that could have been around for a while,” says Robert Shaler, who directed the DNA identification efforts for victims of the September 11, 2001 terrorist attacks on the World Trade Center in New York City. Shaler points out that his team was provided with DNA profiles of some of the hijackers involved in the 9/11 attacks to identify their remains in the World Trade Center rubble, and that a sample of bin Laden’s own DNA could have been collected by military or intelligence agents from places that bin Laden had lived or visited.
Samples also could have been collected from any of bin Laden’s living siblings or children, either with or without their knowledge, by obtaining discarded material bearing their DNA, such as beverage cups or dental floss, says David Kaye, a lawyer at Pennsylvania State University.
Bin Laden reportedly has no full siblings, as he was the only child born to his mother and father. However, his mother remarried after she and bin Laden’s father divorced, so bin Laden reportedly has three half-brothers and one half-sister born to her, as well as dozens of half-brothers and half-sisters also born to his father. He also is said to have fathered as many as 26 children.
“Bin Laden’s relatives might have cooperated, or even if they didn’t, agencies might have obtained DNA samples surreptitiously — it’s a well known practice in law enforcement now,” says Kaye, who last year published a book, The Double Helix and the Law of Evidence, on genetic forensic evidence in the criminal justice system.
For instance, DNA collected from a discarded slice of pizza was used to identify the man charged last year with the so-called “Grim Sleeper” murders in California. DNA taken from the alleged killer’s son was also used in that case.
The strength of a match made between bin Laden’s putative DNA and that of his relatives would depend on how many related DNA samples are available for comparison, and how closely they are related to bin Laden, says Allison Williams Dobson, a lawyer and molecular biologist who works with the Center for Genomics and Society at the University of North Carolina Chapel Hill.
“The more siblings and children you have samples from, the easier and more certain it would be,” Williams Dobson says.
It is likely that sequencing of DNA from mitochondria — organelles in the cell that are passed down from mother to child — could have been used to confirm that the man killed in Afghanistan was a sibling to bin Laden’s half-sisters and brothers, or was the son of bin Laden’s mother, who is still living. Since mitochondria are passed down from mother to child, they can establish with near certainty that two individuals are siblings, says Mary-Claire King of the University of Washington, who pioneered the use of mitochondrial DNA to identify victims of mass killings and homicides in the 1990s.
“Mitochondrial DNA sequencing would be the most certain way, by far, of identifying two people as full siblings,” King said.
Short tandem repeat analysis could also be conducted on the y-chromosome, which is passed down from father to son. Such an analysis could be used to link the putative bin Laden to any of his known half-brothers or sons, further strengthening the evidence that it was indeed he who was killed in the Navy raid in Pakistan.
If it is true that bin Laden was the only son born to his mother and father, a combination of y chromosome and mitochondrial DNA markers could yield a strong match to bin Laden himself, because none of his half-brothers would have inherited the same combination of y chromosome and mitochondrial DNA markers that he has.
Short tandem repeat analysis could also have been conducted on nuclear chromosomal DNA from many bin Laden siblings and children, further cementing the case that the killed man is Osama bin Laden himself.
“If they have the right samples, the identification is virtually certain,” Shaler says.
In addition, news reports have also mentioned other identification techniques, such as facial analysis, that would complement the DNA analysis.
Whatever DNA evidence does exist, Williams Dobson points out, the U.S. government must feel very confident about it.
“They must be quite sure it is him, because if they are wrong, we will have a video from him circulating pretty soon, showing that he is actually alive,” she says.