Osama’s Sister’s Brother

DNA tests were able to confirm with a 99.9% certainty that
Osama bin Laden has been killed.

One of the key ways the U.S. government confirmed that they had killed Osama bin Laden was using genetic tests. What they did was take DNA from the corpse and compare it to known relatives of Osama bin Laden (or at the very least to his sister). From this they concluded that there was a 99.9% chance that the dead man was Osama bin Laden.

Unless they had a sample of Osama’s DNA, these kinds of tests can’t necessarily prove beyond a shadow of a doubt that the man killed was Osama. What it can do, for example, is say that the man killed was Osama’s sister’s brother. If all the other brothers are accounted for and there aren’t any unknown brothers out there, then this is him.

This shows why it is so important to have lots of relatives to test. For example, if they also tested one or more of his children, then they could conclude that the man killed was the father of Osama’s children and the brother of Osama’s sister. Unless Osama had an unknown brother who fathered his children, then the dead man in Pakistan would certainly be Osama bin Laden.

Of course they didn’t rely just on the DNA (although these results are very strong). They also used fancy biometric recognition software and eyewitness testimony to confirm that the dead man is Osama bin Laden.

Testing DNA

So how’d they do the actual DNA testing? They wouldn’t have had time to sequence all of his DNA but that’s not a big deal. No one would do that to identify a suspect anyway.

Most likely, they did a more extensive version of a run-of-the-mill paternity test. When given a high priority, these tests can easily be done in less than 24 hours.

In paternity-like tests, scientists compare the DNA of the related people at multiple spots in the genome. They look at parts of our DNA that tend to be different between unrelated people. If people share a lot of them in common in the right ways, then they are related.

These spots go by lots of different names--microsatellites, simple sequence repeats (SSRs), short tandem repeats (STRs), variable number tandem repeats (VNTRs)--but we’ll call them STRs here. STRs are bits of DNA where the sequence is repeated over and over, back to back.

Remember, DNA is made up of four chemical "letters" (or nucleotides) that are usually abbreviated as A, G, T, and C. So an STR is a string of letters that is repeated. What makes them useful here is that different people can have different numbers of these repeats.

Imagine the set of DNA letters AGTC. One person might have 10 of these in a row. And another might have 11 or 9.

Of course, lots of people will have 10 of these repeats. That is why scientists can't look at just one STR. Here in the U.S., forensic scientists have chosen 13 of them to tell people apart. Since the government only had a few relatives to compare Osama’s DNA to, they almost certainly looked at many more spots. The more spots you look at, the more solid is the evidence that two people are related.

To get enough DNA to see, scientists need to use something called the polymerase chain reaction or PCR. This technique is a really fast way to make lots of copies of a bit of someone’s DNA.

In PCR, scientists use a special protein called a "polymerase." Its job is to copy DNA. Scientists tell the protein what DNA to copy by giving it starting and ending points, called "primers."

Primers are small segments of DNA that match the part of the DNA a scientist is interested in. The protein then copies everything between the two primers.

Here the government would have premade all of the primers on each side of the STRs they wanted to test. They would have also worked out all of the reaction conditions so they could get the results in just a few hours. A single PCR reaction would look something like this:

If someone has a different number of repeats, then they'll have a different size of copied DNA. Imagine our example above. If I have 10 copies of AGTC and you have 11, then my copied piece of DNA will be 4 letters shorter.

Osama’s sister’s DNA and the DNA from the man who died in the Pakistani compound must have shared lots of these STRs. So many in fact, that there was a 99.9% chance that they were brother and sister.

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