Thursday, May 26, 2016
Interesting Facts on Jewish Ashkenazi DNA
Nadene Goldfoot
On IBDs. Identical by Descent.
Gusev et al (2011) reported that in Ashkenazi Jews “the entire segment of chromosome 6, between 25 and 35 Mb (megabases-a unit of physical distance) , is shared among individuals unrecombined at least 4-fold more than any other region in the genome”.
Peaks of IBD sharing over 5 cMs in size were seen on chromosomes 9 and 19 in Ashkenazi Jews.
AncestryDNA analysed the matches of over half a million US customers genotyped on an Illumina OmniExpress chip (~700,000 SNPs) and found that in certain regions of the genome hundreds and sometimes thousands of people were predicted to share DNA with each other. The problem was particularly pronounced in customers of Jewish ancestry and in some people of Hispanic descent, but the problem was observed across all ethnic groups. They concluded that these regions of identical DNA were likely to be attributed to a shared ethnic history rather than recent common ancestry.
Segment size:
30 cMs 0f a segment stands a 90% chance of coming from 6 generations ago
20-30 cMs 50%
12-20 cMs 20%
6-12 cMs 5%
6 cMs 1%
In a study of a European subset of the Population Reference Sample (POPRES) dataset it was estimated that for the most part IBD blocks
longer than 4 cM come from 500 to 1,500 years ago, and blocks
longer than 10 cM are within the last 500 years.
I"ll look for blocks of 10cMs or more to hope to find a common ancestor. However, "In general it will be difficult to find the genealogical connection with the majority of your matches under about 15 cMs." Then let's hope to find segments of 20 cMs or more for matches.
Resource: http://isogg.org/wiki/Identical_by_descent
The techniques of triangulation, chromosome mapping, and phasing can be used to distinguish between IBD segments and non-IBD segments.
Triangulation was used in the Halpern & Branches group of FTDNA. through GedMatch.com . http://isogg.org/wiki/Identical_by_state
The blog article by Ann Turner, https://segmentology.org/2015/10/02/anatomy-of-an-ibs-segment/, tells how to triangulate. Besides that, it explains much more. One needs to study this.
More on :
http://dna-footprints.com/the-abcs-of-dna-ibd-vs-ibs/.
When we have a DNA segment that matches another person we cannot be sure that it is a real match unless it is also a match to a third person who matches both of us at that spot. This is called triangulation. http://blog.kittycooper.com/2014/10/when-is-a-dna-segment-match-a-real-match-ibd-or-ibs-or-ibc/
So many questions come up with very wee segments that are shared. What does it mean? This is for all people, not just Ashkenazi Jews: http://isogg.org/wiki/Autosomal_DNA_statistics. Below shows the % shared between these distant cousins. These are not even of 1.0cM but less.
Small segments are prone to be false positives. What out. There is too much uncertainty surrounding small segments to base any conclusion on them. The researchers found that more than 67% of all reported segments shorter than 4 cM are false-positive segments (see FIG. 2B, below). At least 60% of 4cM segments were false-positive, and at least 33% of 5 cM segments were false-positive. The number of false-positives decreased fairly rapidly above 5 cM.
On IBDs. Identical by Descent.
Gusev et al (2011) reported that in Ashkenazi Jews “the entire segment of chromosome 6, between 25 and 35 Mb (megabases-a unit of physical distance) , is shared among individuals unrecombined at least 4-fold more than any other region in the genome”.
Peaks of IBD sharing over 5 cMs in size were seen on chromosomes 9 and 19 in Ashkenazi Jews.
AncestryDNA analysed the matches of over half a million US customers genotyped on an Illumina OmniExpress chip (~700,000 SNPs) and found that in certain regions of the genome hundreds and sometimes thousands of people were predicted to share DNA with each other. The problem was particularly pronounced in customers of Jewish ancestry and in some people of Hispanic descent, but the problem was observed across all ethnic groups. They concluded that these regions of identical DNA were likely to be attributed to a shared ethnic history rather than recent common ancestry.
Segment size:
30 cMs 0f a segment stands a 90% chance of coming from 6 generations ago
20-30 cMs 50%
12-20 cMs 20%
6-12 cMs 5%
6 cMs 1%
In a study of a European subset of the Population Reference Sample (POPRES) dataset it was estimated that for the most part IBD blocks
longer than 4 cM come from 500 to 1,500 years ago, and blocks
longer than 10 cM are within the last 500 years.
I"ll look for blocks of 10cMs or more to hope to find a common ancestor. However, "In general it will be difficult to find the genealogical connection with the majority of your matches under about 15 cMs." Then let's hope to find segments of 20 cMs or more for matches.
Resource: http://isogg.org/wiki/Identical_by_descent
The techniques of triangulation, chromosome mapping, and phasing can be used to distinguish between IBD segments and non-IBD segments.
Triangulation was used in the Halpern & Branches group of FTDNA. through GedMatch.com . http://isogg.org/wiki/Identical_by_state
The blog article by Ann Turner, https://segmentology.org/2015/10/02/anatomy-of-an-ibs-segment/, tells how to triangulate. Besides that, it explains much more. One needs to study this.
More on :
http://dna-footprints.com/the-abcs-of-dna-ibd-vs-ibs/.
When we have a DNA segment that matches another person we cannot be sure that it is a real match unless it is also a match to a third person who matches both of us at that spot. This is called triangulation. http://blog.kittycooper.com/2014/10/when-is-a-dna-segment-match-a-real-match-ibd-or-ibs-or-ibc/
So many questions come up with very wee segments that are shared. What does it mean? This is for all people, not just Ashkenazi Jews: http://isogg.org/wiki/Autosomal_DNA_statistics. Below shows the % shared between these distant cousins. These are not even of 1.0cM but less.
| 0.000763% | 0.05cMs | 8th cousins | Degree 17 |
| 0.001525% | 0.10cMs | 7th cousins once removed | Degree 16 |
Small segments are prone to be false positives. What out. There is too much uncertainty surrounding small segments to base any conclusion on them. The researchers found that more than 67% of all reported segments shorter than 4 cM are false-positive segments (see FIG. 2B, below). At least 60% of 4cM segments were false-positive, and at least 33% of 5 cM segments were false-positive. The number of false-positives decreased fairly rapidly above 5 cM.
http://thegeneticgenealogist.com/2014/12/02/small-matching-segments-friend-foe/
Labels: Ashkenazis., common ancestors, dna, familyfinder, IBDs, segments
