As the sensitivity of DNA analysis increases, scientists are able to develop profiles from ever-smaller samples of DNA. This has lead to testing of a wider array of samples collected from crime scenes, including window panes, bullets, hats and other clothing, cigarette butts, and many other items.
Attorneys sometimes ask me about the likelihood of obtaining a useful DNA profile from certain items of evidence. A study from six European forensic laboratories may give some idea of how likely it is to find a DNA profile on items commonly found at crime scenes. This article contains an interesting chart that lists the item and the likelihood of finding a full profile, usable partial profile, possibly usable partial profile, or no profile. It seems best to use this type of information as a rough guide, but it is interesting nonetheless.
It is important to keep in mind that as labs are able to analyze smaller amounts of DNA, the possibility of developing partial profiles and complex DNA mixtures increases. Where very small amounts of DNA are involved, the sample may have been deposited by secondary transfer. Here’s an interesting article on secondary transfer and “touch” DNA.
Attorneys should be aware that forensic laboratories may have case submission guidelines that limit the number and type of items that may be tested by the lab. To understand why and how an item was tested (or not), it is important to read the lab’s policies and guidelines. Here is a link to the North Carolina State Crime Lab’s Evidence Guide. There is information about Touch DNA testing on p. 52.