How DNA Ancestry Testing Works and How Can I Know It's Accurate

When a commercial DNA testing site like Ancestry.com or 23andme or FTDNA tests your DNA, they do not know which snippet came from which of your parents.

For example, if at a given point (a gene, in popular parlance), you have a "C" from your dad and a "T" from you mom (meaning you have brown eyes, but carry the blue-eyes gene), the testing service doesn't know which "letter" came from which parent.

What they then do is try to guess, stringing your DNA out into small chunks or strings of letters.

They then compare these to DNA in their reference database.  23andme's reference database, which is one of the best, if not the best in the world, only has about 11,000 samples in it.  To represent the whole world!


So if you have ancestry from a big country (like France or Germany) or a country that has pockets of deep isolation (like Italy), the odds -- that they have someone from your corner of the country, or your little isolated craggy valley in some mountain chain -- are small.

They then compare the little strings of letters and come up with a likelihood that you have ancestry from one of those reference populations.

23andme has the most scientific test in the business, but it gets French/German/Belgian/Dutch/Swiss/Austrian/Luxembourgisch ancestry wrong 92% of the time.  It most often shows up as "generic Northwest European."  Similarly, 23andme -- the best in the business -- can't identify Italian ancestry 50% of the time.  It often shows up incorrectly as Middle Eastern or Generic Southern European.

The moral of this story is to be patient with the science.  It's not 100% there yet.

If you have documented ancestry from one region, trust your documents.

If you don't have any cousins from a pool you were identified as, then chances are it was a miscall.  (For example, if you have documented Italian ancestry, but it says you are 1/8 Middle Eastern or 1/8 Spanish), then unless you have a known great-grandparent that is 100% such, it's probably a miscall.  (This would mean your parent would test as 1/4, by the way).

Finally, there is a series problem with testing sites, particularly FTDNA's, with the issue of timing.  If you go back far enough, we are ALL Africans, right?  Yet a DNA test telling you that you were African would not be too useful.  Do they mean recently or in the past?

Similarly, as has been well-documented, most European ancestry can be broken down into 3 big chunks: ancient hunter gatherers (Ancient Western Europeans, most similar modern population = Lithuanians); ancient farmers (Ancient Near Easterners, most similar modern populations include Greeks, Sardinians, others); and ancient pastoralists/horse rearers (Ancient Eurasian Steppe Dwellers, most similar modern populations include Ukrainians). But the migrations were really, truly all over the place.

Ancient Near Easterners are NOT modern Near Easterners.  Ancient hunter gatherers in France are NOT the modern French, etc.

If a test tells you that you have some Near Eastern blood, it often is sensing this ancient signal.

It doesn't do you much good for them to say that 6000 years ago, you had some ancestry in the Near East.  Everyone did.

Calculating Matches on Gedmatch: Why CentiMorgans (cM) are more important than SNPs

I have discovered that very very very few people know this, so it is worth posting.

The different testing companies, 23andme, Ancestry, FTDNA, etc. all test slightly different SNPs.  In other words, the "points" on the genome, the "genes" that are tested vary from company to company.

I have seen some people on Gedmatch dismiss a match because "it doesn't have enough SNPs."  Or because "it's not above the SNP threshold."

Gedmatch itself uses a 7 cM and 700 SNPs match to qualify someone as a cousin.

The SNP part is faulty thinking.

Because the testing companies don't test the same SNPs, you can have long stretches that match with a low number of SNPs.

Case in point: Someone who tested on 23andme like I did matched me for 10.0 cM and 1024 SNPs.  That same person on FTNDA matched me for 10.0 cM but just 510 SNPs.  FTDNA tested half of the SNPs that 23andme did (or half of the same set). 

This is key to grasp.  Expect closer matches to you on Gedmatch if your kits start with the same letter (i.e. M for 23andme, F for FTDNA, and A for Ancestry.)  DO NOT DISMISS LOW SNP MATCHES.