[QUOTE=Priceguy]
The situation is this: I’m writing a role-playing scenario and trying to keep it as realistic as possible. The players find a very bloody kitchen knife and have no idea how old the blood is or whose it is, but it has spent that time in roughly room temperature. Could you find out anything meaningful from such a sample? Gender? Drugs (probably not, due to small quantity)? Approximate time since the stabbing? If they ask you to simply “tell us as much as you can”, what could you do?
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First, is there a body? Or a person missing? Because lots of times, knives are used for butchering animals.
Let’s assume it’s a murder of a human being. The blood will have dried, so it will work just fine for DNA. But let’s stop and do the obvious first: fingerprints. Blood is a great medium for preserving the assailant’s fingerprints. Very few people wear rubber gloves to a crime. Stabbings are crazy wild moments in which success is not guaranteed, violence takes over without common sense in charge, and in the vast majority of cases, the other person defends themselves before the fatal blow. In situations like that assailants just can’t guarantee there will be no prints on the knife. If they do the usual thing, cast the knife away in the moments after they’ve won, they won’t even think about their fingerprints. If they did, they would wipe the knife, which screws your scenario.
Second: DNA. Two kinds. Victim (copious) and assailant (not guaranteed, but quite possible!). Assailants cut themselves more often than you would believe. Vertical shallow cuts along the edge of the palm are classic assailant accidental self-cuts (long cuts across the palm are classic defense cuts). We’d definitely look for assailant DNA. There might be some non-bloody assailant DNA on the handle, too. Wouldn’t be much if any, but PCR does wonders.
If your story is in the present day, victim and assailant blood don’t do you much good until the police develop suspects. Because there is no central bank of DNA-typed humans to whom to compare it, and because they don’t sequence it. So it won’t tell you much, although you could do gender. If your story is in the not so far future, it could tell you ethnicity possibly, particularly if your victim or assailant came from one of those little groups that has odd quirks in their blood, but it won’t tell you race. It won’t tell you height. It won’t tell you eye color. It won’t tell you age. It won’t tell you the way the face looks. So its major use is to match up to the DNA of either a dead body (or living but wounded person), or to the DNA of a guy who swears he was never there, never seen the place, never met her in my life, Officer. And to do that, you have to find the guy first.
If you are working in the slightly farther but not so far off future, everybody born in the USA will have been karyotyped from the umbilical blood, and they’ll just stroll down to the databank plug, feed in the sequencing, and ask it to pick out your man. Of course, if he was born in Russia or Mexico or Peru, you might be out of luck.
So the answer so far is: Not much, but scrape that dried blood and hang onto it, and we’ll see whose it tells us it is.