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Wolfie
09-15-2014, 04:40 PM
On my GEDmatch matches there is only one person among my 1,500 matches who is an X match AND a match proper: my half-sister. I share some X chromosome matches with a few others, the largest of them being 24 cM. However, when I check the one-to-one autosomal comparison utility on those X matches using 7cm/700SNPS, I do not match ANY of them.

Is this unusual?

Thank you.

Wolfie

Ann Turner
09-15-2014, 06:45 PM
No more unusual than matching on just one autosomal chromosome.

dp
09-15-2014, 08:02 PM
Dear Wolfie,
Because of your post I got curious on X match lengths. Out of curiousity I checked my various kits. Someone matches my mom for 22.51 cM (excluding me). For me, (excluding mom) the largest match was 17 cM. For my uncle, his largest -X- match was for 16cM.
dp :-)


On my GEDmatch matches there is only one person among my 1,500 matches who is an X match AND a match proper: my half-sister. I share some X chromosome matches with a few others, the largest of them being 24 cM. However, when I check the one-to-one autosomal comparison utility on those X matches using 7cm/700SNPS, I do not match ANY of them.

Is this unusual?

Thank you.

Wolfie

dp
09-15-2014, 08:06 PM
Wolfie,
check out http://www.anthrogenica.com/showthread.php?1231-Anyone-using-Gedmatch-for-X-chromosome-comparison&p=12481&viewfull=1#post12481
dp :-)

Wolfie
09-15-2014, 08:24 PM
GEDmatch (in red letters) says that you should use the 7cM/700SNP comparison, and that if there is no match, then you should not contact the person. What would that mean regarding my X matches, whom I do not match otherwise?

dp
09-15-2014, 08:31 PM
GEDmatch (in red letters) says that you should use the 7cM/700SNP comparison, and that if there is no match, then you should not contact the person. What would that mean regarding my X matches, whom I do not match otherwise?
GEDMATCH:
To qualify as a 'match' in the genealogical time frame, results must have a largest Autosomal segment that has at least 700 SNPs and be at least 7 cM.
It must have BOTH. Results with the largest segment less than 7 cM are highlighted in pink.
In general, the results shown below use thresholds LESS than 7cm / 700 SNPs.
PLEASE verify any result shown on this list with the one-to-one comparison tool before assuming any match is real.
To check the number of SNPs, click on the 'A' on the same line to view the one-to-one comparison detail.

DP:
X & Y are allosomes. For your particular question, for segment length, I'm not sure.

dp
09-15-2014, 08:31 PM
No more unusual than matching on just one autosomal chromosome.

is "Going Through a Phase: Haplotyping the Female X Chromosomes" a paper written by you perhaps.
dp :-)

Ann Turner
09-15-2014, 08:47 PM
GEDMATCH:
To qualify as a 'match' in the genealogical time frame, results must have a largest Autosomal segment that has at least 700 SNPs and be at least 7 cM.
It must have BOTH. Results with the largest segment less than 7 cM are highlighted in pink.
In general, the results shown below use thresholds LESS than 7cm / 700 SNPs.
PLEASE verify any result shown on this list with the one-to-one comparison tool before assuming any match is real.
To check the number of SNPs, click on the 'A' on the same line to view the one-to-one comparison detail.

DP:
X & Y are allosomes. For your particular question, for segment length, I'm not sure.
In practice what you say is true for Family Tree DNA, but that's because they have never computed matches based on the X chromosome. They just display any X segments that are present in matches they've already computed, based on the autosomes. The vast majority of the X segments you see at FTDNA will be pseudo-segments.

Wolfie
09-15-2014, 08:51 PM
Thank you, Ann. I just read your article, mentioned here by David Powell. I am trying to understand this mysterious science. Before embarking on the journey to find my biological father (July 18, 2014), I had absolutely no knowledge of this sort of thing, save a tiny bit gleaned from a class in high school eons ago. I have a question for you. Is it true that full-sisters ALWAYS match the entire X? Are there ever exceptions?

Wolfie

Ann Turner
09-15-2014, 09:28 PM
Thank you, Ann. I just read your article, mentioned here by David Powell. I am trying to understand this mysterious science. Before embarking on the journey to find my biological father (July 18, 2014), I had absolutely no knowledge of this sort of thing, save a tiny bit gleaned from a class in high school eons ago. I have a question for you. Is it true that full-sisters ALWAYS match the entire X? Are there ever exceptions?

Wolfie
The exceptions are rather minor, and perhaps theoretical. The very tips of the X chromosome do exchange DNA with the very tips of the Y chromosome, and the exact points of exchange could differ with each conception. Or a mutation could occur during the creation of different sperm (but the mutation rate is exceedingly low). But basically, full sisters will be at least half-identical as far as we can tell with our current testing procedures.

Full sisters will also (usually) be completely identical for portions of the X. The mother recombines her two X chromosomes when creating an egg. In some regions her daughters will get the same stuff and in other regions they will get different stuff.

The animations at this website give a good overview of the different patterns of inheritance.

http://www.smgf.org/pages/animations.jspx

Wolfie
06-28-2017, 03:04 PM
Thanks! Finally remembered my password here.

geebee
06-29-2017, 02:13 AM
Ann is the expert here, but one reason full sisters might not have any complete matching on the X chromosome (matching on both X chromosomes in the same region) is that they might have inherited opposite X chromosomes from their mother.

As others have said, typically the X chromosome a mother passes on to a child will be a recombination of both of her X chromosomes. Typically, but not always. It's possible for her to pass on a copy of either her maternal X, with no DNA from the opposite chromosome.

This actually happened to two of my siblings -- one brother and one sister. They match across the full length of their maternal X chromosome, which happens to have come from our mother's father. (This has been confirmed by an abundance of matches to cousins on our mother's father's side, across the entire chromosome. In addition, I match these two siblings on all of the X except for about a 20 cM region surrounding the centromere, where I match cousins on our mother's mother's side.)

Now, these two siblings happened to have received an X chromosome from the same grandparent, but it could as easily have happened that they received an X chromosome from opposite grandparents. In that case, despite being full siblings, they would have no matching on the X chromosome.

Obviously, if this were two full sisters instead of a sister and a brother, they would still match each other on their paternal X chromosome. But, what this means is that you can't presume that the absence of any complete matching between sisters means they're half sisters; or on the other side, that two sisters with half matching across the entire X, must have inherited that X from a common father. As I've indicated, it is possible for siblings to inherit an identical X chromosome from their mother.

EDIT: As an aside, you might think that with six children, we'd collectively have inherited all of both of our mother's X chromosomes. We nearly did, except that none of us managed to inherited the region beginning at 144139922 and ending at 155092794 (which 23andMe shows as the end of the chromosome) from our maternal grandmother.