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DNA evidence is one of the most powerful tools available to law enforcement and the court system. With modern techniques, forensic science can test very small amounts of DNA and identify who that sample came from. Because it is so reliable, evidence of “who” deposited DNA in a place or on an item is virtually irrefutable. DNA can reliably answer, “Who?”.

However, current DNA science has its limits. Importantly, forensics cannot tell when someone left a DNA sample on an object or clothing. Also, where there is DNA from more than one person, forensic science cannot tell who used an object last – even if there is more DNA from one person than the other. DNA cannot answer “When?” or “How?”.

“When” DNA Central Issue in Browne House Appeal

In a current Browne House appeal, a defendant was convicted of being the masked man in a robbery. The mask was left at the scene of the crime, and when it was tested the mask (a used spray sock) turned up DNA from two people, one of whom was the defendant.

The DNA sample was too small to identify the second person and the defendant was charged with the crime. Although the defendant had a plausible reason for why his DNA was on the item – it was a spray sock of the type he and his fellow painters regularly used and discarded at work – he was nevertheless convicted on some less than reliable DNA testimony.

What Impacts How Much DNA is Left Behind?

With DNA from two people on an item, it might seem reasonable to conclude that whoever used or wore the item last would leave more DNA. Not so, according to current forensic science. We can’t tell who wore or used an item last based on how much DNA is left behind. There are too many factors at work.

If you wear a piece of clothing, how much DNA you leave behind depends on many unrelated factors. For example, someone who wears a particular t-shirt on a regular basis is going to leave more DNA on it than someone who borrows the shirt and wears it once but last in time. This is true even if the owner of the t-shirt washed it before lending it out.

More important than frequency or recency of use, how much DNA we leave behind is influenced by our own individual biology. Some people are “good shedders” while others are “bad shedders” of DNA. Apparently, some of us naturally leave more genetic material behind than others. A “good shedder” is may leave more DNA on a piece of clothing than a “bad shedder,” even if the bad shedder wore the item last.

DNA Testimony Can Be Treacherous

Testimony about “source DNA” – who’s DNA that is – is common in criminal trials. However, if testimony goes beyond the “who” and into the “when” or “how” it may not be supported by current science. Such is the situation of a current appellate client who saw a forensic expert testify that if an unknown third party wore a mask last, during a robbery, they would have left behind more – an amount from which they could an identification – DNA.

The Defendant was convicted in large part based on that faulty forensic testimony. For that reason, the Defendant now asks for a new trial.

Where to learn more:

If DNA evidence is an issue in your case, be sure to think about the “how” and “when” it got there, not just “who” it’s from. More importantly, be ready to protest if forensic experts get ahead of the science.

  1. For an excellent resource of forensic science check out the journal Forensic Science International;
  2. While we don’t recommend acting “pro se” (i.e., without a lawyer), if you are doing your own appeal check out our sample brief outline for formatting tips; and
  3. If you have a criminal or civil appeal in Alabama, contact the lawyers of Browne House Law to see if we can help.