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FAQ: YDNA tests versus Autosomal Tests
Posted by smithsworldwide in Smith DNA on June 1, 2020 Views:(212Replies (0)
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Wally Smith posted this on the Smiths DNA Project Activity Feed at FamilyTreeDNA. He, sadly, passed away over a year ago. Still love his answer so reposting it here for all to read.

I get this question often. OK, I’ll give you the basic overview. YDNA is passed only from father to son. It is the best way to locate potential paternal ancestors through STR matching. The Str markers mutate at a rate which allows for sorting out paternal ancestors in the genealogical time frame, say 5-10 generations back. MTDNA is passed only from mother to daughter therefore it is of no use in tracing one’s paternal line and is of limited use in finding a maternal ancestor. You would get hundreds of matches with people who have a common maternal ancestor 20.00 years ago or more. Autosomal DNA is passed down randomly from both parents, their parents and parent’s parents and so on back through history. The only way to tell which side a match may have passed through is to test many relatives with known relationships so you can compare and sort them out. There are 23 chromosomes in all humans. 1-22 are autosomal DNA, 23 is special, in that it is the one where you get your XDNA. A female has no YDNA because the father passes his XDNA that he got from his mother to daughters only. She gets X from both parents. The son gets X from mother and Y form father. So in summary, Yes, you may get some benefit by testing your autosomal DNA, but the only way to really prove a paternal match is to test a known male descendant of the direct line male to male from the earliest known ancestor to the male testee and hope for matches with someone who has a well documented tree and go from there.
-Wally Smith