How trustworthy is yDNA testing?

[Sue Collins]

Just had a quick message from someone on a family research group I have on Facebook. She doesn't trust DNA testing nor yDNA testing. She took a class and her teacher said there were confused too many fast mutations for it to be trustworthy. Not having enough knowledge myself, I am asking you folks. Just in the use of yDNA for tracing ones paternal lines, would the mutations make that much difference? Or is it valuable to find the matches that have the same mutations?

Curious and thank you.

[John McCoy]

Before anyone answers a question about the "trustworthiness" of Y DNA testing, it is necessary to establish exactly what is meant! The answer depends on the purpose of the tests, what results have been obtained, and the genealogical context.

If, for example, you test two men who are believed to be patrilineal descendants of the same man who lived 100 years ago, and the two men turn out to belong to two different haplogroups, the conclusion that they do not share a common great-great-grandfather is certainly justified. Y DNA testing tends to be very good at ruling out shared patrilineal ancestry, because the SNP's that define the Y haplotree, and the overall STR pattern for most branches of the haplotree, will not change very much over the span of a couple centuries.

One the other hand, Y DNA by itself will not usually tell you which of several possible patrilineal great-grandfathers is yours, because the SNP's and the STR's will likely be too similar to distinguish among the various possibilities, even if all of them are available for testing.

For the problem of guessing the surname of an unknown patrilineal ancestor, some people happen to end up in a cluster of twigs on the haplotree where virtually everyone tested belongs to the same family (Buchanan is a wonderful example, and so are the descendants of Deacon Edmund Rice of early New England). Other people end up on some little twig far removed from anyone else having the same surname who has been tested so far -- that is my experience. Still others end up in a group having a common STR signature but with many different surnames, suggesting that the STR signature became established before stable surnames became fashionable.

There is no way to predict what the results might be, but Y DNA results are bits of additional evidence that will usually be helpful in your genealogical research.

[MoberlyDrake]

The Y-DNA test is trustworthy in the sense that the results themselves are accurate.

With regard to autosomal testing the larger the DNA segment you share with a person, the more likely it is that you and the other person received that segment from a common ancestor within the last few hundred years. Very small shared segments cannot be assigned to a common ancestor with any certainty and should be ignored. But autosomal testing is completely accurate at determining close family relationships.

[Germanica]

Just had a quick message from someone on a family research group I have on Facebook. She doesn't trust DNA testing nor yDNA testing. She took a class and her teacher said there were confused too many fast mutations for it to be trustworthy.

Y-DNA doesn't actually mutate that frequently. That's why every man can be traced back to Y-Chromosomal Adam. Where/What sort of class was this, and what qualifications did the teacher have in regards to DNA? It sounds like perhaps either the teacher or the student was misunderstanding. This is a good article about how Y-DNA works and how it can be useful: https://dna-explained.com/2016/04/14...rnal-ancestor/

Not having enough knowledge myself, I am asking you folks. Just in the use of yDNA for tracing ones paternal lines, would the mutations make that much difference? Or is it valuable to find the matches that have the same mutations?

Curious and thank you.

Y-DNA helped me identify the surname of my grandfather's unknown bio father (in combination with autosomal DNA) so yes, it can absolutely be useful but it depends who else has tested, and also depends on what your goals and expectations are.

[dna]

Context, context, context... A genealogical research? A police investigation?

Y DNA would not distinguish between identical twins, but fingerprints will. Etc.


Mr. W.

[MoberlyDrake]

You might want to read the book "The Family Tree Guide to DNA Testing and Genetic Genealogy" by Blaine T. Bettinger.

Now, if you are just using a DNA test to find out the ethnicities of your ancestors, DNA tests are very accurate at the continental levels, but not very accurate at doing something like distinguishing which countries in Europe your ancestors are from. This is largely because there has been too much mixing of populations in Europe.

When my cousin did a Y-DNA test, an unexpected non-paternity event was revealed. Perhaps the teacher had such an experience and was so upset about it, that she preferred to believe the test was inaccurate, rather than face the truth.

You should always be prepared for shocking results. It doesn't usually happen, but you need to ask yourself how you will handle it if you find out that one or both of your parents wasn't your biological parent, or if you find a previously unknown half-sibling etc.

In my case the non-paternity event happened in 1872 and no one living cared at all. I was a bit upset about 10 years of researching the wrong line and all the time an money spent doing it, but it didn't bother me that my great-great-grandmother had a child by a man other than her husband. But it can really upset whole families when it involves living people!

[kevinkretz]

Thanks for the explanations, Moberly. As I understand, the Y-Chromosome matches should be direct ancestors on the paternal line - right?

I ask because the results of my Y-Chromosome test show dozens of matches (30+ with a genetic distance of 1, ~40 with a distance of 2) with people with a particular surname. That surname isn't mine and isn't that of anyone in my tree.

With this many matches with a short genetic distance, is that an indication that it's pretty likely that there's been a misidentification of a paternal link somewhere?

Kevin

[georgian1950]

I ask because the results of my Y-Chromosome test show dozens of matches (30+ with a genetic distance of 1, ~40 with a distance of 2) with people with a particular surname. That surname isn't mine and isn't that of anyone in my tree.

Kevin

How many markers did you test? With only Y-25, a GD of 1 or 2 is not necessarily close, Y-37 maybe, Y-67 yes.


Jack

[MoberlyDrake]

georgian1950 is right. It depends on how many markers you tested. As soon as I suspected a non-paternity event, I upgraded my cousin's test to 67 markers.

Back in 2010 not very many Hampton men had tested and my cousin matched an equal no. of Hamptons and Hollingsworths, but he didn't have a single match to our surname. I had already joined the project for our surname, so I joined the Hampton and the Hollingsworth projects. The administrator of the Hollingsworth project was very helpful and explained that all the Hollingsworths my cousin matched were descended from one man, born in VA in the 1700s, who apparently was the son of a Hampton, not a Hollingsworth. Luckily I didn't have to deal with that!

In 2010, once I came up with a suspected father, I had to find a descendant willing to do a Family Finder test because autosomal tests were new and not very many people had done them. I had to settle for a descendant of the suspect's brother, but he did match my mother. At that time he was her closest match.

I tested Mom at the 3 main companies and today, so many people have done DNA tests that I would be able to tell from her matches, with a bit of research, who her great-grandfather was.

When you're doing at-DNA tests, you should test the farthest generation back in the line you're interested in, e.g., grandparent rather than parent.

FTDNA is having a sale right now.

[The_Contemplator]

...her teacher said there were confused too many fast mutations for it to be trustworthy.

This line is confusing. Who was confused, the teacher or the student? Was this class from recent times or from the distant past before DNA testing got to this point?

At any rate, like the rest have said. DNA testing is trustworthy in the proper context.