While federal officials said that analysis of DNA from several relatives helped confirm that it was Osama bin Laden who was killed in the military raid on Sunday, they have not yet disclosed the relationships of the family members whose DNA was used.
Officials said they collected multiple DNA samples from Bin Laden’s relatives in the years since the Sept. 11 attacks. And they said the analysis, which was performed the day Bin Laden was killed but after his body was buried at sea, confirmed his identity with 99.9 percent accuracy.
Some scientific experts said on Monday that if results really were so accurate, at least one of the sources was likely to have been a close relative, like a child or parent with whom he shared half his genes.
“That would be most likely,” said Frederic Zenhausern, director of the Center for Applied NanoBioscience and Medicine at University of Arizona.
DNA matching usually involves obtaining genetic material from a blood sample or cheek swab.
The vast majority of a person’s DNA sequence will be the same as every other person’s. So a test, which can be done in a few hours if needed, typically focuses on a small number of locations on the genome, usually 13 to 16 spots. These spots are located on what is sometimes referred to as junk DNA, areas of genetic material that do not contain instructions for building brain, bone and muscle.
A DNA analysis looks for patterns of two or more nucleotides, the chemicals that form DNA. These strings of nucleotides are called short tandem repeats. The closer the relative, the more the pattern of repeats matches. In an identical twin, they should be the same. A parent and child should share half the number of repeats. In siblings, the combinations can vary; in half-siblings, they can vary even more.
“With a sibling, there is only a likelihood that you’ll share some DNA with them,” said Mitchell Holland, a forensic scientist at Penn State University and former head of the Armed Forces DNA Identification Laboratory. “With a half-sibling, that complicates things even farther.”
Bin Laden did not have any full siblings. He did have more than 50 half-siblings, some of whom have close ties to the United States and had long ago distanced themselves from him. Dr. Zenhausern said using a half-sibling’s DNA could still yield a reasonably high chance of identification, more than 90 percent. And collecting DNA from several half-siblings would increase the likelihood of making a match.
“If you have that, you can attempt to reconstruct the parents,” Dr. Holland said. “You know the numbers the proof is coming from. But the math is relatively complex.”
If two people share a rare DNA pattern, it makes it more likely that they are related. And testing can also be taken to a more detailed level — for example, by looking at the Y-chromosome, which only males have.
Recently, some law enforcement agencies have begun using relatives’ DNA to help identify suspects in criminal cases. In 2008, California changed state law to allow use of familial DNA, relying on it last year to arrest Lonnie D. Franklin Jr., accused of being the “Grim Sleeper,” a serial killer of 11 women. A match between DNA found at the crime scene and DNA from Mr. Franklin’s son, who was arrested for unrelated weapons charges, led police to Mr. Franklin.
ABC News reported on Sunday that to identify Bin Laden, officials used DNA from the brain of a half-sister who had died at Massachusetts General Hospital in Boston. A spokeswoman for Massachusetts General said the hospital had not been able to verify that report.
“We have not been able to confirm any of this with anybody here,” said the spokeswoman, Kristen Stanton. She added, “I don’t know the name of this sister — whether it was Bin Laden or something else — but nobody we can find knows if someone who was Bin Laden’s sister was ever here.”
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