X-matches with full Ashkenazim

[Michelstaedter]

I had a question regarding X-mathces on the family finder. We have been for some time trying to determine the identity of my maternal great grandmother--that is, my mother's father's mother. Her husband was fully Ashkenazi. My mother's mother was an Italian Catholic from Genoa. I just got my mother's sister tested, and, while her myOrigins results are not yet in, she is not surprisingly matching with a large number of full Ashkenazim, some on the level of 80 cM. Interestingly enough, most of her X-matches are also full Ashkenazim. Does this mean therefore that her paternal grandmother must have had some Ashkenazi ancestry, as her maternal side is entirely Northern Italian?

Many thanks.

Summary: female with non-Jewish mother, Ashkenazi paternal grandfather and unknown paternal grandmother showing hundreds of x-matches with full ashkenazim on FTDNA.

[Michelstaedter]

Anybody have any suggestions?

[prairielad]

Not familiar with how Northern Italians show up in Admixture, but as long as confirm that your maternal Grandmothers has no Jewish ancestry, then yes it would indicate that your mothers paternal Grandmother was of Jewish Ancestry also, as your Grandfather would have only receive his X from her (which he then passed on intact to your mother and Aunt)

[Michelstaedter]

Ah okay thank you this is what I suspected. Oddly enough, I have VERY few X-matches on family finder (four of them), none of whom appear to be Jewish...

What might account for this?

[prairielad]

If you are male, it is due to fact that you only have one X and women have two.

Having only one eliminates many false positive segments

it also depends how much of your mothers paternal X vs maternal X you inherited. It is possible that you inherited only her maternal X. Chromosomes pairs do not always recombine, one can receive parents maternal or paternal chromosome or a random recombination of the two(more common)

[Michelstaedter]

Ah ok. Do you think then that the 400+ Ashkenazi matches on her X-chromosome might be false positives in that case?

[khazaria]

Do you think then that the 400+ Ashkenazi matches on her X-chromosome might be false positives in that case?

Triangulation is a process that works for X-DNA just like for autosomal DNA. If your mother's X-DNA matches also match each other at the same segment location where they match her, and if those segments are somewhat lengthy, those particular matches are not false.

[Michelstaedter]

Khazaria, is there a simple way to do this in family finder?

[Michelstaedter]

I am actually fairly certain there are no way that these are "genuine" X-matches as virtually all of them are in the 6-7 cM length range, AND her MyOrigins indicates only about 1/4 Jewish Diaspora, AND I myself only have four X-matches, none of whom are Ashkenazi...

Would others agree that the probability is low?

Darn these false hope X-matches!

[Michelstaedter]

I'm still actually really confused.

1.) Are these hundreds of x-matches with full ashkenazim real? If not, why are there so many false positives reported? What is so special about this chromosome that it reports false positives when others don't?

2.) If they are indeed indicators of genuine Ashkenazi ancestry from a paternal grandmother, then why do I have no such x matches? Also, why would the my origins results indicate only 16 percent Ashkenazi, when in fact it could be closer to 5/16 or even 3/8?

[prairielad]

I'm still actually really confused.

1.) Are these hundreds of x-matches with full ashkenazim real? If not, why are there so many false positives reported? What is so special about this chromosome that it reports false positives when others don't?

One can not say if true or not without triangulation and/or phasing
Chromosome 23(X)is not used in matching, if one is declared a match on chromosomes 1 thru 22, FTDNA will also show any X matching segments with that match.

matching requires one to share at least one longest segment of 7cM/500SNPs and a Total of 20cM in shared DNA on chromosomes 1 thru 22 which include all segments 1cM/500SNPs or greater (X amounts not looked at)
Failing that criteria of 20cM total DNA shared, you will also be shown any matches who share a longest segment of 9cM or greater.

Since FTDNA shows all segments with match that are 1cM/500SNP or greater, any 1cM/500SNP or greater segment on X will be declared an X match.

Matches on X are an aide to help determine line of descent due to its unique inheritance pattern.
All chromosomes report false positive segments, it is just that all segments on X are reported on their own list. You would see just as many if not more for each chromosome if they filter each chromosome also.

2.) If they are indeed indicators of genuine Ashkenazi ancestry from a paternal grandmother, then why do I have no such x matches? Also, why would the my origins results indicate only 16 percent Ashkenazi, when in fact it could be closer to 5/16 or even 3/8?

Women on X tend to have more X matches and false segments due to fact they have two X's.
Alogrithm is looking for only 1 value of the two to match in a sequence of single letters (A,C,G or T) and with women, smaller false segments are more prevalent due to fact it creates a matching segments by zigzagging back and forth between her maternal value and paternal value.
Add to the fact that one comes from a endogamous population, many smaller segments can be true, but old and untraceable.

As for no X matches for your self with Ashkenazi, this depends on what you inherited of X from your mother. You may have inherited only her maternal chromosome, or a combination of her two, but very little of her paternal X.

Percentage wise, it just depends on how much of each Grandparent you inherited. As a rule you inherit around on average 25% of each grandparent, but depending on how recombination of your mothers maternal and paternal chromosomes recombined this can vary. ie) inherited 30% of maternal Grandmother, 20% of maternal Grandfather.

[Michelstaedter]

Many thanks for your thorough answer to someone who must seem to be a complete ignoramus in this matter.

I've read that the problem with Ashkenazi matches is that they are not usually "matches" in the useful sense of the word, but are often very distant cousins many times over, which the algorithm then transmutes into a closer relationship prediction than is actually there. As per your original post though, I'm guessing that you'd stick to the same conclusion that you first came to, namely that the paternal grandmother had to have at least *some* Jewish ancestry for so many matches to be showing on the X? I suppose the matter as to whether they are *real* matches is a different matter though?

So essentially my mother's maternal grandmother was part Jewish, but finding actual cousins to her from that line is more questionable?

[Michelstaedter]

Update

Triangulation is a process that works for X-DNA just like for autosomal DNA. If your mother's X-DNA matches also match each other at the same segment location where they match her, and if those segments are somewhat lengthy, those particular matches are not false.

Update:

So I have been combing through my aunt's x-matches, and (so far, not nearly done yet) I have found four fully Ashkenazi x-matches that all match her and each other on a segment of approximately 7.45 cM in length, from spot 150690811 to 154570039, towards the end of her x chromosome.

Is this cause for excitement?

[khazaria]

Update:

So I have been combing through my aunt's x-matches, and (so far, not nearly done yet) I have found four fully Ashkenazi x-matches that all match her and each other on a segment of approximately 7.45 cM in length, from spot 150690811 to 154570039, towards the end of her x chromosome.

Is this cause for excitement?

Good work, and I'd say that's a segment to keep an eye on as time goes on, not necessarily to achieve any specific research ends, since that is a rather short length for X-DNA matches, but just because it interests you.

[Michelstaedter]

Good work, and I'd say that's a segment to keep an eye on as time goes on, not necessarily to achieve any specific research ends, since that is a rather short length for X-DNA matches, but just because it interests you.

Indeed. I am still not sure what it means though, or if it answers part of the question about my aunt's paternal grandmother's possible Jewish origins....