It just occurred to me that in the matter of detective fiction, during the early days of mysteries, the private detective was often on the forefront of forensic analysis of evidence. Sherlock Holmes ran all sorts of chemical analyses and Lord Peter Wimsey dusted for fingerprints. Holmes was famous for examining things with his magnifying glass and Lord Peter would send all sorts of samples off to chemists he knew for analysis. With plenty of exceptions, the police tended to content themselves with taking witness statements, seeing who got the most money in the victim’s will, and jumping to conclusions.
The forensic habits of modern detectives seem, by contrast, muted. Again, I’m sure that there are plenty of exceptions, but I think that modern police have acquired a reputation for having forensic teams which are very professional and thorough, and moreover have access to forensic labs which have extraordinarily expensive equipment.
I’m not sure why this should spoil the fun; there are professionals who made all of the gadgets which MacGyver made during his adventures, yet it was always interesting when MacGyver made them.
I suspect that the answer actually lies in the realm of genre, rather than structure. There is an entire genre of mystery called the “police procedural”. In it the story isn’t really so much of a mystery as merely following the police on the twists and turns as new evidence shows up, much of it forensic in nature. If one still watched broadcast television, I believe one could watch a different show each night in which the police sequence DNA to identify people.
If one really likes that sort of thing, one can get far more of it from police procedurals. As a result, there’s less fun in including it in mystery novels.
There’s another element, which is that DNA sequencing is a bit too much like magic to really fit into a detective story. Granted, it’s not all that conceptually different from fingerprints, but I don’t think that fingerprints really lasted long as a denouement—if they ever were much of one. Fingerprints because a standard part of police procedure in the west in the very early 1900s, so in the 1910s a detective dusting for them had at least the element of novelty to it.
They’re not really interesting, however. In the structure of a story, a fingerprint or DNA sample which proves who the murderer was is not really any different from a witness who was found at the end of the book. If that’s the solution, the detective did not really solve anything, he merely found someone who knew the answer and asked.
As such in modern detective stories, fingerprints and DNA evidence become like cell phones in horror movies—a nuisance which the author must spend a little effort to explain away. In modern horror movies there tends to be a scene where the main character either doesn’t have cell reception or his phone has run out of battery. In mystery novels our culprits must wear gloves and possibly scrub the floor with bleach.