There is a good chance that his DNA would produce, at the very least, a 4C with enough shared DNA to identify a MRCA (most recent common ancestor) from which a family tree could be built using traditional genealogy methods. In fact, this is how I identified my biological father this year! Although, once I saw a picture, we really didn't need the DNA.
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I just re-listened to the 2nd episode of Sworn Statement that discussed DNA. They talked about the differences between the traditional crime lab DNA samples taken from a crime scene, which are directly matched to a specific individual, compared to the commercial testing companies doing familial DNA testing, such as Ancestry and 23andMe. The two are completely different tests and cannot be compared. However, unless I completely missed it, they didn't go into why genetic genealogy couldn't be employed yet for this case, legally.
I could have sworn someone explained that they were unable to submit MH's DNA due to the circumstances of his death not being considered a criminal investigation. Does anyone recall if they discuss that in Ep. 1 or 3? Did I just make that up?