This Article considers the advent of genetic genealogy, used by law enforcement in capturing the Golden State Killer suspect and in other cold cases. In these investigations, police used genetic information obtained from the open source genealogy site, GEDmatch, to build vast family trees spanning the entire country and several generations in order to locate suspects whose DNA matched that left at a crime scene. This Article analyzes the Fourth Amendment implications of government use of such powerful technology to explore such sensitive information as DNA. The conclusion the Supreme Court could reach, should it be called upon to examine the privacy issues involved in such intrusions, would vary depending on which avenue of Fourth Amendment analysis it chose to pursue. Maryland v. King, Court precedent on government collection of DNA, is so narrow that it provides little guidance on the issues presented by genetic genealogy. Instead, the Court could consult its recent ruling in Carpenter v. United States, which limited the third party doctrine that had previously nullified privacy expectations in shared information. If it relied on Carpenter, the Court would likely prohibit government downloads from genealogy sites without a warrant. Further, the Court could view individuals’ uploads of genetic information onto open source genealogy sites as amounting to consent to view the DNA shared with all relatives. The Court might therefore apply its third party consent precedent, which, in relying on widely shared societal expectations, would likely prevent warrantless collection of genetic information from genealogy sites. The Court could, however, view police visits to genealogy sites as government searches that occurred after private intrusions. If the Court chose this approach, it could rule that law enforcement is free to collect the DNA information because it is only viewing information already exposed by private parties. Finally, the Court could see law enforcement’s use of genetic genealogy as an issue of standing, as recently analyzed in Byrd v. United States. Application of Byrd’s property rights definition of standing would likely enable the government to admit genetic evidence since suspects lack the power to exclude others from open source sites. Thus, although some Fourth Amendment doctrines would forbid warrantless collection of DNA information, the government could likely rely on either antecedent private search or standing precedent to successfully use genetic genealogy evidence.