Could DNA completly not pass to one sibling.

+9 votes
Hoping to get a bit of assistance and may need to pm the details and kit numbers.

I am helping two women who neither know who their father is, they are not sisters, but estimated first cousins. And of course working through DNA and have come across a conflict.

We have Person A who is the aunt to person B.  DNA matches where I would expect it.

Person B has two brothers from two  different father's with DNA where you exoext for half siblings

Person B has 60 cm of DNA with person C, but her aunt (person A) has no match even at 2 cm in common with person C.

However it appears the brothers may also have DNA matches of person C.

I estimate the parent that gave person b this DBA likely had 120 cm.  What is the chance one sibling would get 120 cm of a dna and a full blood dna sister would get none?

To me it would seem extremely odd.. but I don't want to make an assumption and chase a red herring.
in Genealogy Help by Kristina Wheeler G2G6 (7.8k points)

3 Answers

+10 votes
It is entirely possible that it has to do with which X Chromosome person A and full dna sister of A received from joint mother. Bear in mind common mother got 1 X from father and 1 X from mother. It is random which X she gives to daughter.

Father on the other hand only gives 1 X. So it seems from this scenario that the split in the family occurred at person A's grandmother and grandfather on her mother's side. 1 sister got father/grandfather, and 1 sister got father/grandmother.
by Steven Tibbetts G2G6 Pilot (204k points)

Just to add to Steven's valuable answer: whether or not you can isolate X-chromosome sharing will depend, of course, on where you're running the comparisons. If you can see the amount of sharing (even better, map it by SNP start- and end-points) it can be a powerful asset to help you figure out who's who among some genetic cousins. But for those who haven't worked with the X-chromosome explicitly, it doesn't behave like non-recombinant DNA (yDNA, mtDNA), but it doesn't quite behave like autosomal DNA, either. It takes some background reading before diving into analysis. Some good places to start are two of Roberta Estes' blog posts, "X Marks the Spot" and "That Unruly X"; Blaine Bettinger's "Unlocking the Genealogical Secrets of the X Chromosome"; and this article from NGS Magazine by Debbie Wayne Parker.


Thank you, this has helped me understand - it's all one big learning experience!
Thank you, I will do some reading up on the recommendations as I continue to work through the DNA and trees that we find matching as I do my process of elimination.
+8 votes

Hi, Kristina.

I admit having a little difficulty--a personal problem due to irrevocably lost gray matter--following the example in the question. I got lost right away because two expected 1st cousins are mentioned at the start--neither of whom know their biological fathers--but I don't see where that fits in. The rest of the question describes cousins B and C whose results show an autosomal DNA sharing of 60 cM. Which means B and C can't be 1st cousins; can't even be 2nd cousins. Using Shared cM Project findings, not lower theoretical averages, the most likely relationship between B and C would be 3rd cousins or half-2nd cousins 1x removed...which would put the MRCA generationally back at the 2g-grandparent level, so not aunt A's parents or even her grandparents.

Either the 60 cM is a single segment among a total of about 600 to 900 cM sharing, or B and C aren't 1st cousins. Assuming it's a single segment, B's half-brothers (and here my mental sketch is that B and the brothers share the same mother, but that B has one father and the boys another) have been tested. Their results compared to aunt A's would immediately tell us whether aunt A is the sister of B's mother or her father. If aunt A is the sister of B and the brothers' mother, all of the siblings should match aunt A in the range of about 1,400 to 2,100 cM, but not match the brothers at all if aunt A was sister to B's father.

If we are at the level of 3rd cousins, not 1st, it's important to remember that meiosis and combination mean that the percentages of inheritance described as mathematical averages are only that: estimated averages. That high degree of variability is necessary for genetic diversity and survival of the species. And that's why the Shared cM Project shows that documented relationships as close as 2nd cousin 1x removed and 1st cousin 3x removed can show zero autosomal DNA sharing. And why work by Brenna Henn (former lead scientist for 23andMe) et al., shows that over 10% of our 3rd cousins will share no detectable DNA at all with us, that 54% of 4th cousins will be undetectable, and at the level of 5th cousins, 85% are DNA invisible.

Some additional detail--anonymous, of course--might help us provide more useful input.

by Edison Williams G2G6 Pilot (179k points)
Thank you - I've sent you a PM with a few more details, Sorry for the confusion in what I posted - trying to write these out in simple terms has been a struggle.

I'm new to the DNA so learning, albeit quickly.  And starting to understand why it's not the Holy Grail of genealogy and simply another useful tool that can be full of red herrings along with false negatives.

This information helps me a lot in understanding some basics, and understand that the absense of the DNA in the aunt does not mean Person C is related to the father. It simply means evolution happened and I'm back to the drawing board.
+8 votes
Well the siblings are full siblings or they aren't.  The small amount they share or don't share with person C can't change your mind about that in the face of stronger evidence.
by RJ Horace G2G6 Pilot (458k points)

Thank you for the comment, what I was trying to identify was since there is no in common DNA between Person A and C is there a possibility that person C is related to her father.

Instead I have learned that the absence of the DNA in the aunt does not mean Person C is related to the father. It simply means evolution happened and I'm back to the drawing board. :)

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