If you give your DNA to one of those Geanology sites could law enforcement try and subpoena that DNA to use against you if they don't have probable cause for a warrant for your DNA or would the issue of a warrant be exactly the same whether they get the DNA from you personally or some other source?
From my open-source research it seems that law enforcement did not use any coercive powers, such as a subpoena or warrant, in the following case but "covertly" uploaded a suspect's DNA profile in order to identify familial matches for further investigation. So it does show that (in California at least) evidence may be recovered from privately run genealogical databases without the suspect's knowledge or consent depending on which site is used.
- Joseph James DeAngelo, Jr. Known as The Golden State Killer, Original Night Stalker (amongst others)
Thirty-two years after the ending of his killing spree, and 45 years after the beginning of his crimes, DeAngelo was finally arrested on April 25, 2018 ... DeAngelo had been identified four months earlier as the main suspect, when DNA from an ONS [Original Night Stalker] rape case was uploaded to the personal genomics website GEDmatch. With help from a genealogist, Paul Holes (a sheriff investigator who worked on the EAR-ONS cold cases) and an FBI lawyer constructed a family tree based on GEDmatch's results and eventually narrowed the list to DeAngelo. After a DNA sample was surreptitiously collected from the door handle of DeAngelo's car, it was matched with samples related to the Golden State Killer's crimes. Source
However, according to the UK's Biometrics and Forensics Ethics Group (BFEG):
This process of uploading DNA from a crime scene in the ‘Golden State killer’ case to GEDmatch violated the terms and conditions of use. These terms stated that the person submitting the DNA had to declare that; it was their own DNA; or they were the legal guardian of the DNA donor; or they were otherwise authorised.
But I cannot see if this was ever challenged by DeAngelo's defence.
The BFEG goes on to say that different sites and different jurisdictions have different requirements:
23andMe, AncestryDNA and MyHeritage do not allow law enforcement use of their databases without a warrant. FamilyTreeDNA offers an ‘opt-out from law enforcement matching’ possibility, and all European users are automatically opted out in line with the EU General Data Protection Regulation (GDPR). Contributors to GEDmatch, which allows law enforcement use of ‘public’ profiles with permission in serious cases, must actively opt in to law enforcement matching.