The y-chromosome DNA test is designed to show only which father-to-son lineage a person belongs to. By comparing a man's y-chromosome test results to those of another man, one can see very quickly if they belong to the same lineage and have a common ancestor, that is, a father, grandfather, or great grandfather, and so on, somewhere back along the line. So if your Dad or one of his sons was alive, that individual could be tested to see which Bryan, Bryant, or other male line (yes, that happens sometimes) he belongs to. I did that with my Bush line, and I persuaded two of my Mom' s cousins (one first cousin and one second cousin) to do that in my Garner and Young lines. All these results were very useful in distinguishing my line from others in the same geographic area using the same names. I had tried for decades to solve these problems without DNA and it just could not be done. So your aunt's sons will give you the lineage of the father of her sons, not her own father.
Now, that is not the end of the story. There is mitochondrial DNA handed down mother to child (a son gets his mother's mitochondrial DNA but cannot pass it on - the kids get their Mom's mitochondrial DNA - so only the girls can pass it on.). Although comparable, this is less useful than y-chromosome DNA because as soon as you reach the brick wall on finding out a woman's surname you are done. You can't test all the possible great grandmas in the township 100 years ago to see which ones match.
So that leaves autosomal DNA tests like those done at 23andme (which include the y-chromosome and mitochondrial DNA results). The autosomal test uses the entire genome to find areas of change that show common relationships but may derive from common ancestors so far back in the family tree that one does not know about them and cannot immediately link to them. That is why two things are necessary to make good use of autosomal results: first, the two individuals making the comparison must each have enough depth in their charts to make the comparison useful. Otherwise the probable area of linkage will not be apparent. it can be in any surname. The size of the matching segment gives some idea of whether the match is "recent" or "distant," but it is often much farther back than you imagined even with a good size segment. Second, you need to make the comparison with at least two other people. This is called triangulation. If all three of you, from different starting positions, end up at the same place, then you have a vastly improved chance of getting the right answers. Lots of people take the 23andme test, which is good, but only a few of them share data, which makes the test problematical for genealogical purposes. But some people only want to know their genetic composition and health risks, and an even larger group are unprepared to do the genealogy work necessary to make the DNA results useful. Maybe they naively believed that if they took the DNA test, all would be revealed. But that is obviously not the case. It is also the reason that sharing DNA results does not pose and risk to the person doing the sharing. The raw data is not published or shared by 23andme, although it is accessible to the user if he wants it. I encourage you to do the 23andme test yourself, but it is only a complement to traditional genealogy, not a substitute.
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