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Family Finder Advanced Topics Advanced discussion about Family Tree DNA's Family Finder Product.

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  #1  
Old 7th November 2017, 05:50 PM
Frederator Frederator is offline
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Minimum

Is there any accepted calculation of the minimum number of siblings needed to test in order to fully reconstruct the genome of a parent? My best guess is 3, but that's just based on one observation of siblings whose phased kits at Gedmatch show ~2k shared paternal cM.

Also a tangential thought: Is there a significant difference in the volume of shared cM for identical twins vs. any other type of siblings?
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  #2  
Old 7th November 2017, 07:45 PM
ltd-jean-pull ltd-jean-pull is offline
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I don't think you'd ever be able to construct the full DNA of a parent. Usually some of their DNA will not have been passed to any of their children.

n= number of children
The average amount of DNA you'd be able to cover is
100(1- 1/2^n)%

e.g.
1 child - 50% of their DNA, 100(1 - 1/2)%

then on average:
2 children - 100-25% = 75%, 100(1- 1/4)%
3 children - 100-12.5% = 87.5%
4 children - 100-6.25% = 93.75%
5 children - 100-3.125% = 96.875%

With each additional child you test you halve the amount of "unknown" DNA...on average.

Last edited by ltd-jean-pull; 7th November 2017 at 07:47 PM.
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  #3  
Old 7th November 2017, 08:01 PM
Frederator Frederator is offline
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Thanks.

But I am only interested in the narrow range of their genome reflected in genealogical autosomal testing. So a bit of exaggerated imprecision in the original formulation of my question.

My sense is that for all practical intents and purposes, 5 siblings is a very safe number since the estimated loss % is so small that it most likely--though not certainly--falls in the range of un-tested SNPs.
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  #4  
Old 7th November 2017, 08:15 PM
prairielad prairielad is offline
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I have recreated my Grandparents using my mother and 2 of her siblings (have a fourth but not added it yet) and my father and 3 of his siblings. All Family Finder Raw Data.

I will figure out my average for both groups and let you know.

Note, besides siblings you will need another close relative tested also if neither parent is tested to distinguish between siblings maternal vs paternal values per chromosome.
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  #5  
Old 7th November 2017, 08:21 PM
Frederator Frederator is offline
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That is awesome.

You have all ~3,587 half-identical/~7,174 fully-identical cM that Gedmatch returns for an FF kit? From just 3 kits?

Awesome.
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  #6  
Old 7th November 2017, 08:39 PM
prairielad prairielad is offline
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Grandparents are phased per chromosome into their Parent A vs Parent B chromosomes, but I have not identified which Great Grandparent is which as of yet.

These kits are on Gedmatch
Maternal Grandmother - To knowledge only english ancestry - T645924
Maternal Grandfather - Norwegian Father, Swedish Mother - T837121
Paternal Grandmother - Born in Romania, with German paternal Grandfather - T532230
Paternal Grandfather - Scottish Father, Irish Mother. - T385374
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  #7  
Old 7th November 2017, 08:44 PM
prairielad prairielad is offline
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Quote:
Originally Posted by prairielad View Post
Grandparents are phased per chromosome into their Parent A vs Parent B chromosomes, but I have not identified which Great Grandparent is which as of yet.

These kits are on Gedmatch
Maternal Grandmother - To knowledge only english ancestry - T645924
Maternal Grandfather - Norwegian Father, Swedish Mother - T837121
Paternal Grandmother - Born in Romania, with German paternal Grandfather - T532230
Paternal Grandfather - Scottish Father, Irish Mother. - T385374
Kit T628732 is my maternal Aunt that I did not use yet. Comparing her to my maternal Grandparents shows her not matching up as full parent child due to the DNA she inherited that the other siblings did not. This DNA is currently missing from my maternal Grandparents files.
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  #8  
Old 7th November 2017, 09:13 PM
prairielad prairielad is offline
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Quote:
Originally Posted by Frederator View Post
Thanks.

But I am only interested in the narrow range of their genome reflected in genealogical autosomal testing. So a bit of exaggerated imprecision in the original formulation of my question.

My sense is that for all practical intents and purposes, 5 siblings is a very safe number since the estimated loss % is so small that it most likely--though not certainly--falls in the range of un-tested SNPs.
I have also tested myself and 5 Siblings, between the six of us we do have 100% coverage of both parents
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  #9  
Old 7th November 2017, 09:16 PM
Frederator Frederator is offline
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Thanks. This is a more thorough practical example than I could have hoped for.

But I'm not sure I understand some key points yet. How is it possible that you have phased your grandparents' kits into parent A and parent B if you haven't firmly established a kit/kits representing specific great grandparent(s) as a reference? I thought you needed a minimum of one parent (or a collection of close cousins from one side of the family) to phase a kit.

Sorry if those questions are phrased a little awkwardly. Hopefully you get my gist.
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  #10  
Old 7th November 2017, 09:41 PM
prairielad prairielad is offline
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Quote:
Originally Posted by Frederator View Post
Thanks. This is a more thorough practical example than I could have hoped for.

But I'm not sure I understand some key points yet. How is it possible that you have phased your grandparents' kits into parent A and parent B if you haven't firmly established a kit/kits representing specific great grandparent(s) as a reference? I thought you needed a minimum of one parent (or a collection of close cousins from one side of the family) to phase a kit.

Sorry if those questions are phrased a little awkwardly. Hopefully you get my gist.
Where siblings share DNA they will be one of the following
1)Full base pair matching - Sharing DNA from the same paternal Grandparent on paternal Chromosomes, and Sharing the same maternal Grandparent on maternal chromosomes.
2)Half base pair matching - Sharing either the same maternal or paternal Grandparent, but receiving the opposite Grandparent on their other chromosomes. ie Sharing maternal Grandmother on maternal chromosomes, but one inherited paternal Grandmother (fathers maternal chromosome), and the other inherited paternal Grandfather (fathers paternal chromosome).
3)Not matching - each inherited the opposite maternal and paternal grandparents.

Thus during reconstruction/phasing, the parents chromosomes are phased per chromosome into their parent A and Parent B chromosomes (which in my case, since it is my parents and their siblings results, it is my Great Grandparents)

Edit
Results in picture are phased in better detail due to using 15 Family Members kits to determine individual values. (using just 3 siblings,the individual values would not be able to be phased to precision shown)
Attached Images
File Type: jpg Reconstruction.jpg (107.9 KB, 8 views)

Last edited by prairielad; 7th November 2017 at 09:47 PM.
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