Can female 1st cousins match an entire X?

[Murphy74]

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.

[prairielad]

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.

[prairielad]

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.

[Murphy74]

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?

[prairielad]

......

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.

....

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

[John McCoy]

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.

[georgian1950]

She was adopted and has since found her bio mom.

What is the story on your mom?

[Murphy74]

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.

[georgian1950]

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.

[Murphy74]

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.

[georgian1950]

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.

I thought that might be what you were up to. Maybe you should make this your own secret.

How likely is it that your father and your match's mother would have crossed paths?

[prairielad]

Another avenue to take is to see if you can round up a paternal 1st cousin of yours of each of your fathers siblings.
Testing a paternal 1st cousin of possible candidates of your matches parent may shed light on whether this match is your paternal half sister or another possible relationship.

Ultimately testing your father if possible will confirm (unless he was an identical twin)

[Murphy74]

I thought that might be what you were up to. Maybe you should make this your own secret.

How likely is it that your father and your match's mother would have crossed paths?

this IS my own secret and i am trying to figure it out all on my own. a few members of my dads family are aware of her, but so far no one knows where she falls on the tree.

and it's quite possible that paths were crossed as both were living in the same area at the time.

[Murphy74]

Another avenue to take is to see if you can round up a paternal 1st cousin of yours of each of your fathers siblings.
Testing a paternal 1st cousin of possible candidates of your matches parent may shed light on whether this match is your paternal half sister or another possible relationship.

Ultimately testing your father if possible will confirm (unless he was an identical twin)

4 of my 1st cousins and 2 of his brothers have tested on ancestry. (covers 3 brothers total) they all show as niece/cousin there is 1 brother left who hasn't tested. it's not looking good lol

am trying to avoid testing dad as it risks exposing the possible truth....but am thinking it might be the only way i will ever know for sure, and i really want to know the truth but know the ramifications of what would happen if it proves to be true.

[prairielad]

Do you have any other siblings tested?
Comparing two full siblings to a half sibling via a chromosome browser,ie) Gedmatch 3D chromosome browser (using regular browsers will require copy and pasting to compare each person to the other per chromosome)

On areas that full siblings do not match, one will have to match the suspected half sibling.
Reason, one full sibling received parents maternal chromosome segment, the other full sibling received parents paternal chromosome segment. Suspected half sibling will have to match one of them.

For every area on each chromosomes between the full siblings that they do not match each other (each received opposite maternal and paternal Grandparents DNA) , one must match suspected half sibling. If both do not match each other nor the suspected half sibling in any of these area, suspected half sibling is not a half sibling.

Following is an example of comparing each person to one another using the Regular Gedmatch one to one with Graphic (modified manually by copy and pasting to compare)