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I agree she refused to show her gedmatch she would score 20 percent ssa or above in those calculators, it’s very hard to determine ancestry based on single nucleotides because they are susceptible to mutations. I just see these tests as fun I don’t take them to seriously
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They do not use IBD for this part of the process:
"The algorithm we use to calculate your Ancestry Composition analyzes one small piece of your DNA at a time. For each piece of your DNA, we calculate the probability of that piece coming from 45 different populations."
My understanding is that they divide the whole DNA into segments about 300 SNPs long each, then they run ADMIXTURE calculator K45 separately on each segment, and then they assign the whole segment to its main component (for example if admixture run shows 55% West Asian 45% South Asian, they assign this segment to West Asian).
IBD is used only for assigning countries and regions, not for assigning major categories like East Euro or French-German. These are obviously done with ADMIXTURE or STRUCTURE.
The difference between 23andMe and AncestryDNA is that AncestryDNA runs ADMIXTURE on your whole DNA at once, without splitting it into segments and running on each segment.
IBD is used for Genetic Communities just like 23andMe is using it only for assigning countries and regions (without giving specific percentages).
GEDmatch is using it as well of course for family matching.
Davidski was also selling IBD analyses some time ago. Several users posted their IBD results from him - for example Grundig and Slavic Italian did.
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Nope, it seems you misunderstood what I wrote.
If you run admixture on all SNPs at once, and if you do not use segment smoothing, you will get some % of SSA (for a North African person).
But they divide your DNA into hundreds of segments (200-300 SNPs each) and run it separately on each segment, and ignore minor components from each segment.
If your SSA admixture is not recent, then no segment will be majority SSA. For example segment "A" will be 20% SSA, segment "B" will be 34% SSA, segment "C" will be 14% SSA, and so on. All of these minor percentages are ignored, because they use "segment smoothing" (they assign the whole segment to its main component).
"Unassigned" percentages is the result of segments which did not have any predominant component (for example no component was over 50% on these segments).
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Old ancestries are fragmented, which means that even if someone is racially 20% Black, no segment will be majority SSA as long as this SSA is very old.
This is how North Africans who are 20% Black - but it is from a few thousand years ago - are able to score 100% North African and 0% SSA on 23andMe.
Last edited by Peterski; 12-08-2019 at 08:43 PM.
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The reason why some North Africans score 0% SSA on 23andMe is segment smoothing.
Let's say we have five segments (numbered 1. to 5.), each one with 200 markers (SNPs):
1. - 156 North African markers, 44 SSA markers = all segment, 100% of the segment (all 200 markers) assigned to North Africa
2. - 101 North African markers, 99 SSA markers = all segment assigned to North Africa
3. - 190 North African markers, 10 SSA markers = all segment assigned to North Africa
4. - 135 North African markers, 65 SSA markers = all segment assigned to North Africa
5. - 183 North African markers, 17 SSA markers = all segment assigned to North Africa
^^^
After segment smoothing, all five segments (all 1000 markers) will be assigned fully to North Africa.
The final result is, that this person scores 100% North African and 0% SSA (on these five segments).
If there are segments in which he has for example 120 SSA markers and 80 North African, then these segments are assigned Sub-Saharan, of course.
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And now why AncestryDNA is different? Because they do not use this segment smoothing method.
And probably they don't run admixture on each segment separately, but on all of the DNA at once.
Edit:
Segment smoothing is also why Mortimer was scoring just 2% South Asian on 23andMe at one point. His South Asian markers are very diluted in his DNA.
But after recent updates, it seems they have increased his South Asian to >10% again (which is surely more accurate for him, than that ridiculous 2%).
Last edited by Peterski; 12-08-2019 at 08:43 PM.
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