SciFri 2.18.05

DNA evidence has been used for two decades to help convict--as well as exonerate--suspected criminals.

DNA in the courtroom: The science behind a powerful identification technique

By Deane Morrison

Published on February 18, 2005

After police showed her a photo of Ronald Cotton, North Carolina college student Jennifer Cannino was certain he was the man who had raped her. Her testimony weighed heavily when Cotton was convicted and sentenced to life in prison. Eleven years later, Cotton was released when DNA evidence pointed to another man.

This miscarriage of justice was cited by the Star Tribune on February 15 in an editorial about efforts in Minnesota to use DNA evidence and other techniques to reduce the chance of an innocent person being convicted. DNA evidence has either exonerated or helped convict countless people since 1985, when it was first developed as an identification technique. The power of DNA analysis rests on its ability to detect not whole genes, but small quirks in our DNA that identify us as surely as our faces.

To appreciate what most DNA analysis is about, a brief review of the DNA molecule is in order. It is a long, long string of chemical components called bases. They are often referred to by their initials: A, G, C, and T. These are the "letters" of the genetic alphabet, and their sequence, like the letters in this article, specifies the instructions our genes give to cells on how to conduct business. But not all DNA sequences are parts of genes; some have no apparent meaning, and some are even "turned off," like passages of a book that have been crossed out. But whether they have "meaning" or not, differences in sequences occur between individual people, and these can be used to determine a person's identity.

Now for the quirky part. Some DNA sequences are repeated in tandem. For purposes of illustration, the sequence TAG may occur several times, as TAGTAGTAGTAGTAGTAG. Or the sequence AACG may occur several times, as AACGAACGAACG.

"These are called short tandem repeats," says Doug Foster, an animal science professor who teaches about such features of DNA. "We call them STRs for short, and they are the essence of DNA profiling."

The FBI maintains a core set of 13 STRs that forensic scientists in every state analyze, says Ulland. This gives commonality to evidence across the country, so that a perpetrator who commits crimes in Maine and California can be profiled in a consistent manner.

Thousands of STRs are known. Each is a pattern where a sequence of as few as one or as many as six bases repeat, usually adding up to a stretch of up to 150 bases total. We all have them, and at the same locations. That is, if you have a stretch of TAG repeating itself on your X chromosome(s), then I have a stretch of TAG repeating itself on my X chromosomes. But unless you're my identical twin (and you're not), there is one key difference.

"The difference between people is the number of repeats," says Megan Ulland, a forensic scientist in Minnesota's Bureau of Criminal Apprehension (BCA).

That means that if your TAG repeats itself 15 times, mine might repeat 18 times. Or 12 times. Or 13. Or 15--that's not ruled out. What's ruled out is that if we compared not just TAG but all the STRs in our two genomes, they would be different sizes at most locations.

To determine if a suspect's DNA matches DNA taken from a crime scene, the BCA uses a kit that analyzes 16 STRs scattered around the human genome. Using modern biochemical wizardry, each STR is pulled out of a DNA sample and copied until there's enough of it that a machine can detect it and measure its size, which indicates how many times its particular base sequence is repeated. Then, the numbers of repeats within each STR are compared between suspects and DNA samples taken from a crime scene. A match happens only when the numbers of repeats are identical within all the STRs.

The FBI maintains a core set of 13 STRs that forensic scientists in every state analyze, says Ulland. This gives commonality to evidence across the country, so that a perpetrator who commits crimes in Maine and California can be profiled in a consistent manner.

The BCA is also evaluating a kit that would analyze only STRs found on the human Y chromosome, which is only present in men. This would be helpful in rape cases, for example, where swabs taken from victims often contain DNA from both victim and perpetrator. Y chromosome testing would eliminate false signals from the victim's DNA.

DNA profiling is also used in paternity cases. One of the most famous is that of Thomas Jefferson, who was thought to have fathered children by his slave Sally Hemings. Analysis of STRs on the Y chromosome of male descendants of both Hemings' son Eston Hemings and a paternal uncle of Jefferson (who himself had no surviving sons) showed a match, indicating that Eston Hemings was related to Jefferson. It did not prove that Jefferson himself was the father, only that a man in the Jefferson male line was.