A Segment-ology TIDBIT
To understand how it works, we need to think like the ThruLines algorithm does. I’m pretty sure the algorithm has three parts: 1) start with two DNA Matches (you and someone else); 2) find a Common Ancestor in their Trees*; 3) find and report the path from the CA down to you and the path down to your Match.
The TL algorithm has no way to check to see if the ancestral lines back to the CA are correct or not (witness the TL reports that report incorrect genealogy). It doesn’t know if the lines are even biologically correct or not – it relies on what you and your Match have entered into your respective Trees. TL also does not check spelling of the names of the descendants from the CA – you could list your grandfather as Rumplestiltskin JONES, or Bio Father, and that is what TL will report.
Also note that even if all the information in a TL report is correct, there is no guarantee that the identified CA is the Ancestor who passed down the shared DNA segment. As reported here, I found at least 10% had a shared DNA segment that did not come from the CA.
Edit 20190801 *sometimes other “3rd-party” Trees are used to complete the linkage from a Common Ancestor down to you and/or your Match.
[22AO] Segment-ology: How Does ThruLines Work TIDBIT by Jim Bartlett 20190801