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View Full Version : Why 23andMe only tells you about the last 500-1000 years



Tomenable
29-04-17, 13:12
I noticed already some time ago, that:


23andMe experts are very careful (which is why they have all of these "Broadly something" things). Nothing is noise there. I would say that they are actually too careful with detecting admixtures.

By too careful I mean reluctant to tell you about minor or "uncertain" admixtures.

And I found similar opinions by others:


In my observation 23andme underestimates Eastern Euro admixture when someone still plots in a very West Euro countries. It might look unsure so it ends up mostly in "Broadly..." or "Unassigned", even in Speculative mode.
Which is probably why Poles usually get the same % of "North Slavic" in DNA.Land as their % of "East Euro" in 23andMe, but Westerners usually tend to get more "North Slavic" than "East Euro".

As it seems, 23andMe tend to label such admixtures as "Broadly something".
23andme shows your ethnic heritage from the last 500 years in form of bounded geographical areas you are native to . on gedmatch your genome gets broken down further in more detailed components in a larger than 500 year past timeframe.

But I wasn't sure why was this the case.

Until I found this post on Anthrogenica:

http://www.anthrogenica.com/showthread.php?4576-Do-you-trust-23andme-s-Ancestry-Composition-or-GEDmatch-calculators-more&p=145261&viewfull=1#post145261


23andMe's speculative mode greatly overestimates major components, and underestimates minor components. This is due to their methodology of snipping the genome into 100 SNP segments to compare against the limited references they have. So for example, if 60% of the the segment indicates Middle Eastern, and 40% indicates S Asian, that segment is assigned 100% Middle Eastern. In effect 40% of the segment, which is S Asian is ignored, and the whole segment is assigned Middle-Eastern.

Also, their methodology includes segment smoothing, which means if there are chunks of minor components in a segment, they are ignored.

That is how Iranians and W Asians turn out 98-100% Middle Eastern, and folks in neighboring Pakistan turn out 98-100% S Asian in speculative mode.

This naturally is unrealistic and uninformative, because you don't need a test to tell you that. Conservative mode is better with regards to inflation of major components and underestimation of minor components, but the trouble here is that people get 5-70% unassigned. This is where your minor components are hidden.

The above translates to 23andMe being useless for figuring out your minor components to any degree of accuracy.

Tomenable
29-04-17, 13:16
Some other comments:


I know I'm at least 1/8 Polish and I get 11.3% Eastern European on speculative. If I go on standard mode it drops to 4.2% and if I go onto conservative I get none at all.

When using Eurogenes Mixed mode population sharing on Gedmatch I'll usually get something like,

56.4% Orcadian + 43.6% South_Polish @ 3.91

or

77.5% North_German + 22.5% Ukrainian @ 4

It shows me as way more Eastern European than 23andme does.

FTDNA seems a bit weird, but still generally gets my ancestry right,

I get 56% for the British Isles, 20% Eastern European,

The rest is like 5% per of the other European populations + Turkey.

I have found that these genetic calculators tend to get the whole picture right. But there seems to be quite a bit of variation between them. I imagine this is because they're using different populations to compare you with, different samples, and different methods for calculating ancestry. And also because there's a great deal of overlap between Europeans.

Another one:


Some people say that 23andme predicts ancestry from the last 500 years, and that it is therefore more reliable than GEDmatch.

I don't really agree because I've noticed very peculiar things happen with the 23andme calculator. I've seen families where one parent is Sicilian and the other is Irish. The Sicilian parent scores 15% Middle Eastern, while the child scores 0%. I've yet to see a half North European, half SE European (Cretan, Sicilian, Greek islander) score significant Middle Eastern even though full members of said groups score a significant amount.

Likewise, I suspect 23andme underestimates admixture when it is small. I have seen that in people with under 10% of African, the African is greatly underestimated on 23andme.

What do others think? I think due to its algorithms, 23andme ends up giving misleading results, but it is good for telling if a given ancestral component exists... just not necessarily the amounts.

And it underestimates admixture even more if it is both small and old (= fragmented, small segments).

Basically, 23andMe was designed for people in the Americas to tell them where they ancestors lived in 1492 AD. It seems to be accurate at predicting continental breakdown of ancestry for Latin Americans. In general it seems quite accurate for people with recently mixed ancestry, but not necessarily for older mixtures.

So it tends to be less accurate for Europeans than for people in the New World. Their methodology is designed in such a way, that they can ignore minor and older admixtures. 23andMe ancestry report tells you more about recent geographical affinities than deep genetic affinities.

Twilight
01-05-17, 04:20
Some other comments:



Another one:



And it underestimates admixture even more if it is both small and old (= fragmented, small segments).

Basically, 23andMe was designed for people in the Americas to tell them where they ancestors lived in 1492 AD. It seems to be accurate at predicting continental breakdown of ancestry for Latin Americans. In general it seems quite accurate for people with recently mixed ancestry, but not necessarily for older mixtures.

So it tends to be less accurate for Europeans than for people in the New World. Their methodology is designed in such a way, that they can ignore minor and older admixtures. 23andMe ancestry report tells you more about recent geographical affinities than deep genetic affinities.

I believe 23andme has made some updates to their website since you last looked at your 23andme account, I see where you are getting at but it looks like speculative and conservative mode has been canceled. Now you will only get Standard admixture. Maciamo has written part of a page dedicated to 23andme admixtures. You can see Maciamo's 23andme interpretation in the linkbelow. :)



http://www.eupedia.com/europe/autosomal_maps_dodecad.shtml#23andMe