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Old 15th May 2018, 10:21 AM
Murphy74 Murphy74 is offline
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Question Can female 1st cousins match an entire X?

I have discovered that i may have a half sister. I did an ancestry test because i was curious about lineage and got quite the surprise! On Gedmatch our "X one to one" is:
" Largest segment = 194.5 cM
Total of segments > 7 cM = 194.5 cM Actual. "

Autosomal one to one:
"Largest segment = 176.1 cM
Total of segments > 7 cM = 1,708.6 cM
36 matching segments
Estimated number of generations to MRCA = 1.5"

I had her upload to FTDNA and the X match is 195.93 Sharing 1636 cM longest segment 170

She was adopted and has since found her bio mom.

We did a "siblingship" test at a separate company, but came back inconclusive at 74% chance of half sibling.....

I guess my question is, what are the chances that my paternal grandmother could have given the exact X to two different sons and she could actually be my first cousin? or am i just grasping at straws?

any input would be greatly appreciated.
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Old 15th May 2018, 10:31 AM
prairielad prairielad is offline
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Rarer, but possible.
Mothers two X's do not always randomly recombine to for a unique single maternal X to each child.

For example each mother (if Full siblings) will share the identical paternal X from father. If each mother passed on their full paternal X to child, each 1st cousin will show as matching along their entire X (approx 196cM)

Two of my sisters happened to receive my mothers full maternal X. They both match our maternal Grandmother along the entire length of X and do not match our maternal Grandfather at all on X. They also share their entire X from our father. They are fully identical on X.
Their children stand a greater chance of matching fully on X if each mother passed on their entire paternal X or their entire maternal X.
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Old 15th May 2018, 10:48 AM
prairielad prairielad is offline
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1708cM is high for 1st cousin..this is more in the half sibling range (Grandparent, Aunt/Uncle to niece/nephew range also)
1st cousins usually only share between 553 – 1225cM

https://dnapainter.com/tools/sharedcmv4/1708

Female Half siblings showing full X share usually suggests shared father, not always but most often.

Last edited by prairielad; 15th May 2018 at 10:51 AM.
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Old 15th May 2018, 10:54 AM
Murphy74 Murphy74 is offline
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that was going to be my next question lol. I do have a known 1st cousin (Male) whom i share 1228.9 cM with (from the same side) but thinking 1708 would really be pushing it?
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Old 15th May 2018, 11:20 AM
prairielad prairielad is offline
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Quote:
Originally Posted by vlpenney74 View Post

......

I guess my question is, what are the chances that my paternal grandmother could have given the exact X to two different sons and she could actually be my first cousin? or am i just grasping at straws?

any input would be greatly appreciated.
Quote:
Originally Posted by prairielad View Post

....

For example each mother (if Full siblings) will share the identical paternal X from father. If each mother passed on their full paternal X to child, each 1st cousin will show as matching along their entire X (approx 196cM)

....
In more in line with your question above should say,

If each son received their mothers full paternal X or maternal X (same full X from mother), then yes female paternal 1st cousin will match along the entire length of X.
As mentioned rarer, but possible....recombination of chromosomal pairs does not happen every parent to child event
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Old 15th May 2018, 12:22 PM
John McCoy John McCoy is offline
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In these situations, shared cM and even shared X chromosomes give you a limited set of possible family trees, rather than absolute proof. The next step is to find additional evidence, both traditional ("paper trail") and genetic, that will help you eliminate some of the possible trees. Finding people who match both of you is a good place to start, especially if you can compare matching kits in all combinations (the free GEDmatch web site is ideal for this). Sometimes the additional matches you need will turn up right away, at other times you will have to work hard to locate potential matches from your paper trail research and convince them to be tested. As in all genealogical research, patience and persistence are the keys to success.
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  #7  
Old 15th May 2018, 12:57 PM
georgian1950 georgian1950 is online now
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Quote:
Originally Posted by vlpenney74 View Post
She was adopted and has since found her bio mom.
What is the story on your mom?
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  #8  
Old 15th May 2018, 08:20 PM
Murphy74 Murphy74 is offline
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no story. this revelation is on my dad's side. we know this for sure as she matches with family on that side. we did not grow up together. she was discovered when i did the ancestry test.
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  #9  
Old 15th May 2018, 08:53 PM
georgian1950 georgian1950 is online now
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Quote:
Originally Posted by vlpenney74 View Post
no story. this revelation is on my dad's side. we know this for sure as she matches with family on that side. we did not grow up together. she was discovered when i did the ancestry test.
OK, if you both had the same father, you both would have the exact X-chromosome from him. No wonder you have at least a half X-match along the full X-chromosome with her.
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  #10  
Old 15th May 2018, 09:16 PM
Murphy74 Murphy74 is offline
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that's the part i am trying to confirm. whether she's a first cousin or half sibling. the evidence is pointing toward half sibling, but part of me is looking for any chance possible that she's not, as due to family dynamics...if this came out, it wouldn't end well.
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