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X-DNA Advanced Advanced topics about X-Chromosome matching and anthropologic blocks based on SNPs and/or STRs.

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  #1  
Old 9th January 2015, 04:57 PM
jeffdrake jeffdrake is offline
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Please explain!

Can anyone please tell me why in my 800+ AtDna match list, only 13 have matched me for the X chromosome?

I called FtDna support and was told that FtDna will find the X-chromosome in any AtDna test. Being a male, any X-match has to be from my maternal line. This would be a wonderful way for me to sort my AtDna matches into maternal vs. paternal lines, but with only 13 matches for X, I find it hard to believe that the X matches just aren't being caught every time.
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  #2  
Old 9th January 2015, 05:51 PM
LynCra LynCra is offline
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You also need to take into account that an aDNA match on your maternal side who has two consecutive males on their line to the common ancestor will not match you on X.
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Old 9th January 2015, 06:37 PM
dna dna is offline
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The further you go back in generations, with each generation a smaller percentage of your motherline ancestors is reflected in your own X chromosome. I think the picture http://www.thegeneticgenealogist.com.../NewChart5.jpg sums it up the best.

See also a short animation http://www.smgf.org/education/animat...hromosome.jspx but the picture is more powerful.

W.

P.S. The picture above is valid for males only. For a female the relevant picture would be different. Please reference the blogpost http://www.thegeneticgenealogist.com...-x-chromosome/ to see the differences.

Last edited by dna; 9th January 2015 at 06:39 PM.
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Old 9th January 2015, 07:17 PM
jeffdrake jeffdrake is offline
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Thanks

On the one hand, thanks for the information as you are the first people who could provide an answer my question!

But on the other hand, bummer! I was thinking (hoping, really) that the X would be a nice and simple way for me to sort my long list of AtDna matches into maternal and paternal lines. I don't know who my biological father was/is, other than a family surname (thanks to Y-DNA tests), so looking at family trees is not always helpful.

There are no such things as shortcuts in genetics it seems.
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Old 9th January 2015, 10:24 PM
dna dna is offline
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Quote:
Originally Posted by jeffdrake View Post
On the one hand, thanks for the information as you are the first people who could provide an answer my question!

But on the other hand, bummer! I was thinking (hoping, really) that the X would be a nice and simple way for me to sort my long list of AtDna matches into maternal and paternal lines. I don't know who my biological father was/is, other than a family surname (thanks to Y-DNA tests), so looking at family trees is not always helpful.

There are no such things as shortcuts in genetics it seems.
I am afraid of the CSI Effect. People not only expect results within a very short time frame (in CSI it seems like it takes minutes, hours at most), but also that everybody is in the database (maybe up to 0.1% of the US population is here, while CODIS has more than an order of magnitude better coverage), and that one can guess the exact blood relationship based on 13 (thirteen) markers...

W.
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Old 10th January 2015, 09:47 AM
josh w. josh w. is offline
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Quote:
Originally Posted by jeffdrake View Post
Can anyone please tell me why in my 800+ AtDna match list, only 13 have matched me for the X chromosome?

I called FtDna support and was told that FtDna will find the X-chromosome in any AtDna test. Being a male, any X-match has to be from my maternal line. This would be a wonderful way for me to sort my AtDna matches into maternal vs. paternal lines, but with only 13 matches for X, I find it hard to believe that the X matches just aren't being caught every time.
You could be right, it may just be bad luck in terms of who signs up for testing. I have around 4300 FF matchees and one seventh are also X matches. Only a handful involve significant X segments of 20 cMs or more.

Last edited by josh w.; 10th January 2015 at 09:50 AM.
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Old 10th January 2015, 10:53 AM
josh w. josh w. is offline
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Originally Posted by josh w. View Post
You could be right, it may just be bad luck in terms of who signs up for testing. I have around 4300 FF matchees and one seventh are also X matches. Only a handful involve significant X segments of 20 cMs or more.
Possibly I have more X's due to Ashkenazi intramarriage. However you only have around 1.5% Xs. Around 6% of your third cousins and 3% of your fourth cousins should share your X.

Last edited by josh w.; 10th January 2015 at 11:06 AM.
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Old 10th January 2015, 07:24 PM
josh w. josh w. is offline
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Originally Posted by josh w. View Post
Possibly I have more X's due to Ashkenazi intramarriage. However you only have around 1.5% Xs. Around 6% of your third cousins and 3% of your fourth cousins should share your X.
Remember that your list of FF matches is seriously incomplete beyond the third cousin level, i.e. there are many false negatives--- actual cousins who do not share enough genetic segments to be classified as FF cousins. If more of your X matches fit this category (if the actual genetic distribution was less than the expected rate), it would reduce the number of X matches

Last edited by josh w.; 10th January 2015 at 07:28 PM.
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  #9  
Old 10th January 2015, 07:57 PM
josh w. josh w. is offline
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Originally Posted by josh w. View Post
Remember that your list of FF matches is seriously incomplete beyond the third cousin level, i.e. there are many false negatives--- actual cousins who do not share enough genetic segments to be classified as FF cousins. If more of your X matches fit this category (if the actual genetic distribution was less than the expected rate), it would reduce the number of X matches
On the other hand, the number of your X matches is not vastly different from what would be expected by chance.
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  #10  
Old 11th January 2015, 10:21 AM
josh w. josh w. is offline
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Originally Posted by josh w. View Post
Possibly I have more X's due to Ashkenazi intramarriage. However you only have around 1.5% Xs. Around 6% of your third cousins and 3% of your fourth cousins should share your X.
Correction. X sharing between third cousins should be around 1% and X sharing between fourth cousins should be a fraction of 1%. I was looking at percentages for grandparental rather than cousin lines.
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