ok I _think_ I ‘get’ mtDNA (mother’s lines).
The “special code” for mtDNA that passes down unchanged from an ancient mother can’t pass on from a FATHER in the line, although plenty of the genetic code continues passing on, just not the marker.
So I _think_ that explains why I saw “H3k1a” from my “Mother’s Father’s Mother” whereas from my mother (and presumably my grandmother, grandgrandmother, gg- down the unbroken maternal line from my mother up), is J2a1a1a. Ok.
So CONSEQUENTLY, if I can find the mtDNA from ANY RELATIVE whose MOTHER’S LINE eventually merges into my own, I can _use that code_ to find earliest female ancestor, EVEN THOUGH the ‘special mtDNA code” no longer passes on. It remains valid because the genealogy up to that father where it stopped is valid.