Limited AncestryDNA usefulness

Question

On 2 Feb 2021 Pauline Layton wrote on Layton-9:

Although it's nice to think I might have Ancestry.com matches with all the living descendants of William Layton-9, Ancestry.com tends not to show cousins beyond about 8th cousins. Any living descendants of William's sons other than my ancestor Samuel are bound to be at least 7th cousins to me. There must be younger relatives of mine who are old enough to join WikiTree and yet their DNA is too distant for Ancestry to recognize them as my eighth cousins. Unfortunately, my cousins with William Layton-9's grandfather (back in England or maybe Scotland) as the closest common ancestor are surely over Ancestry.com's event horizon.

Comments

.    Thank you for the clarification, everybody.

I do have some Y-DNA results from a test of my brother Bruce Layton-1805 that was taken by National Geographic in 2010. The first problem is that he's dead. The next problem is, I don't believe I have any living male relatives named Layton closer than 4th cousins (fellow descendants of John Layton-1811). I'm hoping some of them (if they exist) will do some DNA-testing someday.

In any case, if my own dna-test doesn't help at William Layton-9's level, what's the use of WikiTree automatically including my name on Layton-9's page as a descendant of Layton-9's whose dna can be compared against?  It's likely to be counterproductive, because my actual distant cousins might see that I'm not one of their AncestryDNA matches, and they'd think we are not related when we actually are.

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A competent genetic genealogist isn't going to think that you aren't related just because they don't share DNA with someone that's a very distant cousin.  Wikitree populates the DNA test field for your ancestors back several generations, as there is a (slim) chance you may share DNA with ancestors through that most distant relationship shown on Wikitree (which is 6th-great-grandparent).  For those times when you do indeed have DNA from that grandparent and can locate a triangulated group, you are one happy camper!  So Wikitree provides you with some information to assist you with that.

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I don’t think it’s problematic. The DNA statement only says you are likely to share DNA with Layton-9. It does not say that another descendant comparing against you is likely to share DNA with you. And regardless, if some cousin doesn’t understand how DNA works and jumps to the wrong conclusion, that’s not Wikitree’s fault.

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It is only as accurate as the trees put in but I get a lot of 8th to 12 cousin matches on Ged match most recent common ancestor. Wiki trees relationship finder is like it but without the dna match.

Answer 1

Pauline, autosomal DNA tests (like AncestryDNA) are the wrong tests to investigate descendants of your 7th great-grandparents.  The correct test is a Big Y-700 test (available only to men related on their direct paternal line - such as Layton).  

Try to find a close Layton-line male relative (father, uncle, brother, cousin) to take a Big Y-700 test (warning: it's expensive, but goes on sale several times a year).  You can also do it in stages (although the end cost is more): first have your close male relative take a Y-37 test and see which already-tested Laytons they match.  Tests can later be upgraded from the initial level to Big Y.

A Big Y-700 test (or even a Y-37 test) proves that two matching men share a common paternal line ancestor - but doesn't prove who the ancestor is.  If two matches share the Layton surname, that helps support the paper trail to a common ancestor.

In an ideal world (which rarely happens), try to get a Big Y-700 test from your line, and from a male Layton descending from two of William's sons other than Samuel (so three tests - if that's possible).  

It's still never absolute proof that you've identified the correct common Layton ancestor.  One Big Y-700 match from your close relative with someone who has a paper trail to a different son of William's is very strong supporting evidence. But a match to two other Layton lines leading back to William is the closest to proof of your paper trail that you'll probably ever get.

Comments

Thanks to Kevin and Andreas for their brief and concise replies. Getting to grips with the ISOGG cousin distance table  is key to understanding atDNA dilution and draws attention to the fact that when you get a lot of small cM distant cousin matches, there will be a much larger group of distant cousins who share no DNA at all.

Kevin's comment about using yDNA is exactly what I am trying to do, but I don't expect a person by name, just an SNP number for our 'MDCSA' - "most distant common surname ancestor" for want of a better term. The numbers game becomes important. I suspect I need about 30 'like surname' matches to prove the point and have just 6 so far, the cost of BigY being a significant brick wall.

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Just to be clear as Kevin writes " autosomal DNA tests (like AncestryDNA) are the wrong tests to investigate descendants of your 7th great-grandparents".

This refers only to your all male line 7th Grandparent (not plural because his wife cannot be determined by a Y-DNA test but only by an autosomal DNA test.

What is further confusing the less well informed readers is that Pauline writes about AncestryDNA.com which clearly doesn't offer a Y-DNA test in the first place. She explicitly writes:

"Although it's nice to think I might have Ancestry.com matches with all the living descendants of William Layton-9, Ancestry.com tends not to show cousins beyond about 8th cousins."

All the living descendants of William Layton-9 refers to male and female descendants, she doesn't write "all male living descendants".

I also don't want to split hair but technically this answer is actually wrong as Pauline is a female, so she can't use a Y-DNA test, it can be only used on her father or brothers (any direct male line descendant).

Here's a nice explanation including graphics as to which lines of your family tree are covered by autosomal, Y-DNA and mtDNA.

Answer 2

Hi Pauline,

this has nothing to do with AncestryDNA usefulness (as much as I love to criticize them) it's because you just have too many 8th cousins and even though their numbers are probably in the hundred thousands (if not more) a large majority of them doesn't have the ancestral DNA segment from your ancestor anymore or it's below the threshold that Ancestry as well as other DNA testing companies use (so they become "undetectable").

For more detailed information on the estimated number of 8th cousins and the probability of no detectable DNA please refer to Cousin statistics at Isogg.org.

Hope my answer helps, if so please upvote it or mark it as the best answer. Thank you!

Comments

Thank you Andreas for the ISOGG link on "on the estimated number of 8th cousins" - I often wonder about that.