Let's say some viable DNA could be found for The Black Dahlia's killer.

(Or any other legendary crime case.)

Either under fingernails or under the stamps sent in the package containing some of her effects.

So my question is…could the DNA be compared to whoever is in whatever DNA databases and find a match with the killer’s descendent?

In principle it could be done (if those descendants are in your big huge database), but it’d be a lot of work for not much payoff, and much harder if you’re looking at any descendants further than the first generation.

Well, you *could * nicely ask the descendants of all the suspects for samples.

The ones who’ve written books naming their fathers as the killer would love to help prove that daddy did it.

They could sell followup books and memorabilia, get talk show appearances…

There are a few national collections of DNA samples, just as there are fingerprint databases.

IIRC all the DNA collected by law from felons is available for law enforcement to search online. IIRC there was a no too long ago L&O episode where they narrowed the suspect field to someone that way by relatives’ DNA in that database. I also recall hearing of a DNA database of military personnel to use in case they need to identify casualties.

Not sure how easy it is to identify someone from a realtively (sorry) distant family member, plus the degree of “identical” starts to fade a bit with the number of generations back to a common source. The proportion of common DNA gets diluted fairly quickly. The only definite identifiable pieces of DNA are the Y chromosome (only via direct male descendants) and the mitochondrial DNA (rings of DNA that only pass from mother to child, needing direct female ancestrial lineage). IIRC mitochondria are not typically in the DNA database?

As for age of the sample, if it were stored properly it may not have deteriorated. The standards for match in a court of law are likely a lot more strict than those for showing a possible connection for historical purposes.

Yes, it’s possible. It’s called familial DNA searching.

Wikipedia on familial DNA searching

FBI on familial searching

Yes, but most of those examples in Wikipedia are parent/child or sibling matches, which means the two share about 50% of their DNA (25% for half-siblings or uncle-nephew, grandparent, etc.).

The percent drops rapidly from there. First cousins -12.5%… At a certain level, the match won’t be noticeable unless you do a more in depth test. IANA DNA expert, I couldn’t tell you how sensitive the database data would be versus the risk of false positives.

the ability to match, as I understand it, gets better if you have two specific samples and do more in depth tests. For example, close comparison of the Y chromosome of Jefferson’s and Hennings’ male-line descendants showed some were related - but does not answer whether the culprit was Thomas Jefferson, his brother, or his father or uncle(?). Some descendants also were not - but is that because there aren’t, or because somewhere intermediate the paternity was not as advertised?

Some people don’t want to answer these questions. the degree to which “pedigree errors” occur in human lineage is a much-debated topic.

I’m periodically fascinated by the Black Dahlia case but, like the Ripper, I am certain it will never be solved at this late date. The one or two transient suspects seem most likely; move on.

However, Cracked.com had a photoplasty that, well, cracked me up. It was a well-staged evidence photo from the case, showing a strange tubular object and labeled something like “Unknown Object found near Black Dahlia scene.”

It’s a light saber handle.

Yes, that’s why I said “it’s possible” rather than “it is a failsafe method of identifying any relative regardless of distance.” If the Black Dahlia killer’s son is on a searchable database, sure, it’s possible. If we’re looking for Jack the Ripper’s great great great grandson, not so much.

It is a fascinating thought, but unless reasonably complete DNA can be extracted at this late a stage, it’s purely theoretical. Since the body was washed, I don’t know if fingernail scrapings would contain anything useful (nor do I think they would have been routinely collected in that era). Maybe - just maybe - from saliva, hair or skin flakes in envelope or stamp glue. However, I thought a lot of the letters were dismissed as hoaxes. (It’s been a while since I read up on poor LIz’s demise.)

That’s roughly how the police got the evidence to arrest the BTKserial killer. They compared DNA from under a victim’s fingernails to DNA from the suspect’s daughter’s pap smear, and got a familial match.

(To me that sounds incredibly dodgy - what happened to medical confidentiality? She wasn’t even suspected of doing anything remotely criminal, but the doctors just handed over her DNA sample? - but I don’t know the legal complexities behind it.)

This just reminds me of Patricia Cornwell’s horrible Jack the Ripper book. She claims to have gotten DNA from one of the JTR letters (which probably weren’t even written by the killer) and linked it to Sickert’s family. I can’t remember the details as all I remember was how poor the match and the science was. Since the Black Dahlia killer is unlikely to have his DNA currently registered you would probably have the best luck with using mitochondrial DNA along with samples from the maternal line of suspects.

Wikipedi says they got a warrant to test the daughter’s pap smear DNA. That gets the doctor out of having to worry about confidentiality.

I guess it similar (but a bit more invasive) to something like “we want to look at a third party’s garage to see if the suspect has been hiding evidence in there.”

Given the corruption in the 1940s LAPD and given the fact no DNA testing existed at the time any “evidence” from the Black Dahlia murder would be suspect at best. It would be rather pointless exercise as any DNA found could just likely be from the detectives investigating the case, the ambulance drivers who transported the remains or even the coroner who performed the autopsy.

FtGKid2 did the 23AndMe thing a year ago and got a match in their public DNA database for one of my 2nd cousins. So I don’t think the fall-off for matching over the generations is all that bad.

But as to the OP’s point, the LAPD lost just about everything associated with the case years ago. So no DNA.

People have done DNA tests in the Crippen case and some now believe that he didn’t kill his wife. But it’s very borderline evidence.

You’re always exactly 50% related to your parents or your children, but to your siblings, it’s only 50% on average: It could in principle be anywhere from 0% to 100% (but more likely somewhere in between). A similar principle applies to cousins. So that cousin of yours, just by luck of the meiotic dice, might happen to be more related to you than the average second cousin.