Advice needed

[spanlin]

My father (deceased) was adopted. Through Y-DNA testing, we identified his paternal surname and by testing different members of his likely birth family, we were fortunate enough to narrow down his birth father to three brothers (both deceased). Brother A had a daughter that we tested and, since her results indicated that she was a 1st cousin 1xR (instead of a half aunt), we ruled out this brother (318.24 cM shared with longest block 49.97). We then tested brother B’s granddaughter (since his children are all deceased) and her results were 236.10 cM with longest block 55.05. The third brother “C” had no children. If brother B was my birth grandfather, then B’s granddaughter would be my HALF 1st cousin…..If brother C was my birth grandfather, then B’s granddaughter would be my 2nd cousin (not sure if the term half applies here)…..Is there someone else we can test to determine if our birth grandfather is “B” or “C”? We thought maybe if we tested Brother A’s GRANDDAUGHTER (the daughter of the person initially tested )….it would help us differentiate. I realize that the amount shared with both individuals is on the low side for the assumed relationship but we know we have the right family because I also share with three cousins that descend from the mother of these three brothers. Additionally, according to the ISOGG tables, I share about 1,000 cM less than I should with my full brother even though we are definitely full siblings. Any advice will be greatly appreciated.

[sjadelson]

Is that last comment correct? One *thousand* cM (not one hundred) less for your full brother, so it's something like 2400cM? That seems way off, implying that you only share 35% of your DNA with your brother.

But putting that aside for a moment, how does your brother compare to B's granddaughter?

[Rebekah Canada]

...Brother A had a daughter that we tested and, since her results indicated that she was a 1st cousin 1xR (instead of a half aunt), we ruled out this brother (318.24 cM shared with longest block 49.97)....

Hi,

For comparison, the son of my half-sister comes back as my 1st cousin. It is the same number of meiosis events.

[spanlin]

Is that last comment correct? One *thousand* cM (not one hundred) less for your full brother, so it's something like 2400cM? That seems way off, implying that you only share 35% of your DNA with your brother.

But putting that aside for a moment, how does your brother compare to B's granddaughter?

Yes....I only share 2744 cM with my full brother (and we know we are full siblings for sure)....

My brother only shares 168.35 with B's granddaughter and 267.46 with A's daughter!

[spanlin]

Mistake...

Sorry....I meant....2477!! My results were transferred from 23 and me...so, not sure if that explains the low sharing amount on FTDNA???

[Rebekah Canada]

Sorry....I meant....2477!! My results were transferred from 23 and me...so, not sure if that explains the low sharing amount on FTDNA???

Hi,

Siblings have lower than posted (ISOGG etc.) average sharing amounts at Family Tree DNA, because we only report half identical matching for segments. There are often places between full siblings where you share both a segment from your mother and a segment from your father.

That we do not count X-Segments in the autosomal sharing will also reduce the total sharing amount some.

[spanlin]

Will this explain?

So...I thought (maybe incorrectly) that since I shared less than average with my brother and, my paternal cousins are really HALF cousins, it would explain why my brother shares less cM with them than I do and (because they are half cousins), the cM shared with my might be a little lower than the ISOGG tables would suggest....

Would testing the 'granddaughter" of brother A (the daughter of the A's daughter).....help us by comparing the amount of shared cM....since she is a second cousin and we are trying to determine if B's granddaughter is our second cousin (in which case our birth grandfather is C) or our HALF 1st cousin (which would make B our birth grandfather).....OR.....is there no way to know for sure???

[Rebekah Canada]

Would testing the 'granddaughter" of brother A (the daughter of the A's daughter).....help us by comparing the amount of shared cM....since she is a second cousin and we are trying to determine if B's granddaughter is our second cousin (in which case our birth grandfather is C) or our HALF 1st cousin (which would make B our birth grandfather).....OR.....is there no way to know for sure???

Hi,

I think that you need the help of someone who is really good at sorting chromosome matching segments to help you reconstruct as much of each person's genome as possible. You can check with the adoptee groups, as they do a lot of this.

That being said, testing every 1st and 2nd cousin possible will help with phasing the data.

[mkdexter]

The numbers indicate B is closer. DNA is a probability game. If you are sure C did not have children then go back to B. You probably have a half sibling situation here where your father was a half sib to B (or C's) children. That would drop the numbers down by half the expected i.e., 450 for first cousins and so on.

Just to show you how varying the shared amounts can be, even in a half sibling situation, these are my half sisters shared amounts to my 4 grandchildren:

my half sister to my gdau1 600cM shared
my half sister to my gson1 394cM shared
my half sister to my gson2 564cM shared
my half sister to my gson3 171cM shared

Don't let the numbers sway you too much. You are either more closely related to B, or to C. Again, if C had no children you are still in the ballpark for B. You may have a father who was a half sibling and hence he was given for adoption.

You have validate more information than most people have at this point. Good luck!

Matt.

My father (deceased) was adopted. Through Y-DNA testing, we identified his paternal surname and by testing different members of his likely birth family, we were fortunate enough to narrow down his birth father to three brothers (both deceased). Brother A had a daughter that we tested and, since her results indicated that she was a 1st cousin 1xR (instead of a half aunt), we ruled out this brother (318.24 cM shared with longest block 49.97). We then tested brother B’s granddaughter (since his children are all deceased) and her results were 236.10 cM with longest block 55.05. The third brother “C” had no children. If brother B was my birth grandfather, then B’s granddaughter would be my HALF 1st cousin…..If brother C was my birth grandfather, then B’s granddaughter would be my 2nd cousin (not sure if the term half applies here)…..Is there someone else we can test to determine if our birth grandfather is “B” or “C”? We thought maybe if we tested Brother A’s GRANDDAUGHTER (the daughter of the person initially tested )….it would help us differentiate. I realize that the amount shared with both individuals is on the low side for the assumed relationship but we know we have the right family because I also share with three cousins that descend from the mother of these three brothers. Additionally, according to the ISOGG tables, I share about 1,000 cM less than I should with my full brother even though we are definitely full siblings. Any advice will be greatly appreciated.

[spanlin]

Thank You!

Thank you Rebecca and Matt! We are really thinking that our grandfather is B, but will continue to seek other cousins from that family to confirm our results. Thanks for your input!