So I was watching The Dark Knight Returns again and after his fight with Joker I noticed he was dripping blood eveeeeeerywhere. Why hasn't anyone ever collected a sample and tried to find out who he was that way? I haven't read all the comics so I am unsure if this has been answered.
There are two possible uses for DNA testing that are relevant here: determining the identity of an individual who left a sample, and determining whether two different, given samples came from a single individual or two different ones.
More to the point, finding the identity of someone who somehow left a sample (for example, some blood after a fight) involves comparing against a large number of previously (or later) collected samples which are tied to known identities of people. In other words, the two cases degenerate to one: compare a number of samples, looking for matches. DNA does not encode an identity, though specific markers can be used to compare different samples with each other which can lead to an identity.
If, for example, the police arrive at the scene of a fight they might actually collect a sample (it might be shown, or not). When running this through their databases, they would presumably come up empty, or with only matches against previously unidentified samples from similar scenes. This would be rather useless for establishing Batman's identity, as it would at best tell the police that the same individual was at both locations, and might tell them about certain traits of the individual in question which also doesn't help them much in establishing an identity.
This also assumes that the databases do not get tampered with, which in the specific case of Bruce Wayne (who, after all, isn't exactly money-poor) as Himarm points out might not be the case. A thick wad of cash can be a tempting offer for anyone, and no system is completely secure. Note that this does not need to involve bribing a police officer.
Assuming that Batman/Bruce Wayne (or some other superhero who goes by a secret identity) otherwise follows the law or does not commit more than minor crimes (nothing to warrant collecting the DNA of the individual accused or convicted), it could be that such checks are done (the in-universe crime scene investigation protocols match those of our present-day world), but there exists no matching known-identity sample and thus an identity cannot be established.
There is also, as pointed out by PointlessSpike in a comment, the matter of whether it would even be desirable to expose the identity of someone who consistently (and often) aids law enforcement. But unless and until a match is found with a known-identity DNA sample, that is a question of academic value only.
Technically speaking, with current methods of DNA sequencing, we can identify some very specific traits of the individual whose DNA has been sampled. I believe we can tell eye colour and hair colour, and certainly the person's susceptibility to a variety of genetic diseases (which has actually been the goal of a lot of DNA sequencing research).
However, you're still not going to get very far by determining that the subject is caucasian, about 6 feet tall, brown hair, brown eyes, and maybe a genetic oddity like having thumbs that bend backwards far more than most people.
To be completely honest though, Bruce Wayne is hiding in plain sight. How many other people in Gotham are there with the insane wealth necessary to be Batman, plus being young enough, strong enough, and motivated enough to accomplish the things he does? Most of the very wealthy have a tendency towards being older, balder, and rounder than Wayne. A good many of the rest spend 100+ hours a week at their jobs, providing more than enough alibi to eliminate them from the running. I'd bet that there'd be less than six people who can even qualify for the job, and any detective in the city could figure him out in a matter of days. It couldn't even be explained by corruption why they don't, because apparently most of them are working for the mob anyway, and if there's a group of people more motivated to identifying Batman, I don't know who it is.
But this is one of those plot holes we all overlook, because we'd rather believe otherwise.
DNA doesn't encode somebody's name. When you collect it, if you want to identify it, you have to compare it against some other sample and, if it matches, and if you know whose the other sample is, then you have an identity.
Bruce Wayne presumably doesn't go around leaving his DNA at crime scenes ... at least, not as Bruce Wayne. So with only Batman's DNA, how is the police supposed to divine that it in fact belongs to Bruce Wayne? :)
Batman is a master detective and normally has a answer to every possible eventuality long before it comes up. So it is highly likely that he has already thought of this possibility that someone with access to a police or federal database would be given his DNA to run.
So the possibilities I think Bruce Wayne would have thought of would be:
- Be arrested or have a reason for Bruce Wayne to submit DNA to the police or federal agency. Submit a false sample of some random person with a similar genetic background. Since you can see specific genetic signatures for hair color, ancestry and things like that; Bruce Wayne would need to submit the DNA of someone of similar ancestry, hair color, height, and a really big chin.
Then, when someone acquires Batman's DNA, they run it against whatever database and it is not in any way a match to Bruce Wayne.
- The other possibility is Batman intentionally allows villains and police to get a hold of his DNA on many occasions but the samples the villains and police get a hold of are for many prominent and random people, including well known criminals. That way if someone ever gets a hold on Bruce Wayne's DNA, no one will ever believe it.
Bruce Wayne had never broken any laws and as such his DNA is not on file. A DNA test of batman's blood would return no results. They would not even try to run a DNA test due to this fact. They would know that batman would be a good person and as such would not be in any law database. This is why they don't even try. This is also why they will run DNA tests on the bad guys as there is a very good chance they are going to be in a database for some other crime.