Autosomal DNA and how much of your ancestry do you know?

+15 votes
353 views

Please see The Genetic Genealogist Blog at http://www.thegeneticgenealogist.com/2015/08/11/how-much-of-your-family-tree-do-you-know-and-why-does-that-matter/?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+TheGeneticGenealogist+%28The+Genetic+Genealogist%29

 

in which Blaine Bettinger states “No atDNA paper or proof argument should EVER make a conclusion based on shared segments without at least a sentence or two about the lack of known overlap in other family lines. Or, alternatively, addressing known overlap. That is a fatal error."

asked in The Tree House by Peter Roberts G2G6 Pilot (448k points)
Thank you, Peter. A really important read.

I have found several instances of overlap in my mother's tree as I am attempting to verify and complete her early New England lines. I can't even describe the overlap in my father's island lines!

Ann Carmel
Thanks, Peter! I added a table to my profile, although it could use some tidying. I wonder if we could do this as a tree widget? I suppose my known ancestors here on WikiTree will increase and decrease as others make and sever parent/child connections.

Karen's nice table format inspired me to create a table for my own ancestry here on WikiTree (after creating a few new profiles for ancestors who I realized I hadn't gotten around to documenting). The results are in my profile.

I modified the table headings and other format to make the table fit within the default width of the left-hand column of a WikiTree profile page, and I added some other customizations just because I wanted to. I'll share my format on Karen's free-space page, but I figured I'd post about it here first, so that you smart people can point out any problems with what I did.

2 Answers

+7 votes

Peter, thanks for pointing out this blog.  It's terrific and one that everyone interested in genetic genealogy should read.  It makes some great and very valid points.  It also points out something I always tell newbies to DNA, which is the importance of having as extensive of a family tree as you can in order to be able to successfully identify the ancestors whose DNA you're sharing with others.

In the comments, Jim Bartlett had a great one:  "My estimate is that the sweet spot for atDNA Matches is 6-8th cousins – that’s where the bulk of our Matches lie. Let’s take the 7th cousin level (256 GG grandparents, 8 gen back) – even with pretty good Trees, most genealogists may have 50% of their ancestors identified, or 1/2. With a Match who also has 1/2 identified, we’d only find a Common Ancestor maybe 1/4 of the time. And this is considering one of the very best outcomes. The point is that our Trees are the limiting factor, not the atDNA, IBS, miss-calls, etc. And that’s presuming this 7th cousin match will reply to your email/message (subtract another 50% or more), or even has a Tree of any merit (subtract another 75% or so). By the time you factor in all the issues, there’s slim pick’ns… So my constant harangue is: genetic genealogists need robust Trees (as many ancestors as possible; and I’m now adding children of ancestors, because so many of my matches cannot work their TN, KY, MO ancestors back to VA, etc. )"

answered by Darlene Athey-Hill G2G6 Pilot (265k points)
+6 votes

Example table here - I'm sure others can make it even better! I did this in Excel and then used an excel2wiki converter to get started.

I'm working in a sandbox version to make this more user-friendly by allowing us to input the number of known ancestors once and reference these inputs in the calculated cells.

http://www.wikitree.com/wiki/Space:Table_of_Known_Ancestors

For the estimated birthdates, I calculated the average birthdates of my actual ancestors for the first six or seven generations. I found there was about 30 years between generations, so I went with that for the remaining generations. I suppose most of my ancestors were having children by age 17-20 or so, but because they kept having children until age 40 or later, my average came out to about 29-30 years per generation.

To create your own table, click Edit on this free space page and then copy the table. Paste it in to your own profile and then start editing those ancestor counts and percentages! It can be hard to read the wiki markup text, so you could try viewing the original copy, noting that for example, you need to change my 38 known 4th great-grandparents to your 47, and then just finding "38" in your wikitable and replacing it with 47, finding "59.38%" and replacing it with 73.44%, etc.

answered by Karen Tobo G2G6 Pilot (112k points)
edited by Karen Tobo
Thank you, Karen. Appreciate your help. Have a few more lines to complete and then I am going to tackle this project.

 

Ann
How should we handle Pedigree Collapse?  I have my first instance at the 6th Generation, so my total possible ancestors is reduced but it also changes the DNA ratio's does it not?
Good question! I would Blaine Bettinger or Peter Roberts or Kitty Smith, but I would be inclined to count them twice. If we are calculating the percentage of known ancestry at each generation, it seems simplest to keep using powers of 2 for the generations.

Say if I knew all my great-great-grandparents but had two of them in my tree twice at that generation, I might still use the number 16 instead of 14 and know that two of them are each giving me 2/16 of my DNA instead of 1/16. Sure I might have only 14 unique great-great-grandparents, but if I have yet to discover two of them I would still say I know 14/16 of them and not 12/14. Just seems to make the math simpler. What do others think?
I completely agree, Karen.  The purpose of the chart is to identify the extent to which information is known vs still unknown.
I think we have this report available to us in WikiTree+ now. Just head to this link:
http://wikitree.sdms.si/default.htm
Scroll down to Reports and choose Ancestors Summary.

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