In lots of areas of life, we are told that bigger is better. But when it comes to DNA testing, is more always better? In other words, does the more genetic markers tested mean more accuracy?
In the words of Obi-Wan Kenobi, “From a certain point of view.”
Y-DNA STR Testing
Before answering this question we need to break the question into Y-DNA STR testing and Autosomal DNA SNP testing. For the Y-DNA STR test, Family Tree DNA is the only company that offers it. They offer several testing options from 37 to 67, and finally 111 markers. The more markers you test the more you'll expect to pay for the results. Each result is more accurate than the previous one in identifying how distantly related you are to other genetic matches.
However, there is a caveat. Since your data is being compared to other people’s results, your results are only going to be as accurate as of the least accurate test.
Let’s say you match with John at 37 markers with a genetic distance of 0. You get excited and take the 67 marker test, except John hasn’t. Your results still show you matching with John at 37 markers. So in this case, more markers are better IF you and your matches have tested more markers.
Family Tree DNA allows you to upgrade your Y-DNA test. If you have a specific patrilineal genealogy question you are trying to answer, start with the 37 marker test. Then, make contact with your closest matches to find out if they have done the 67 or 111 test or are willing to before you upgrade. This way, you can possibly avoid some disappointment.
While we are on Y-DNA (and mtDNA too), a SNP test, which is normally done in conjunction with an autosomal DNA test, can be used to identify your haplogroup. In this case, more is definitely better. Haplogroups are defined by the different mutations in the Y chromosome and mtDNA.
Sequencing the entire Y chromosome or mtDNA will give you the most accurate haplogroup. As Family Tree DNA now has the Big Y test, which looks at 20 times as many SNPs on the Y chromosome as other autosomal tests, it is no wonder that they have identified hundreds of new haplogroups and subclades. When it comes to haplogroups, bigger is always better.
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Autosomal DNA SNP Testing
So the next is with SNPs. SNPs and STRs are not the same things. They aren’t even comparable. Based on the ISOGG Autosomal SNP chart, the least amount of SNPs tested by one of the major companies is more than 500,000. While 500,000 is 4,500 times larger than the 111 marker Y-test, it doesn’t mean there is 4,500 times more information.
Autosomal tests can be used for different things. First, ethnicity or heritage estimates. For these estimates, a string of SNPs is being compared to reference populations. At first glance, having more SNPs would mean more strings to compare, or smaller segments of DNA being analyzed and ideally, a more precise estimate.
However, it's not just your DNA, but the DNA of the reference populations that need to be considered. You may have a test of 900,000 markers, but if the reference you are comparing it to only has 600,000 of those markers than 300,000 are effectively useless.
Which brings us to cousin matching. One on one, DNA vs. DNA. Like heritage estimates, on the surface more appears to be better, but it is actually the intersection between you and the person you are comparing to. This may be as few as 100,000 markers. So even though your test has 600,000 SNPs, the comparison with your cousin may only be 100,000 markers.
But does this affect the accuracy of relationship identification?
Simply put, not for almost all practical purposes.
MyHeritage allows upload of 23andMe v5 and LivingDNA data which share the least amount of SNPs in common with the other tests. By looking at matches of the same person (me) using the DNA tests, the identified cousins are not any different. The amount of shared DNA is different but in the same ballpark, but cousins for practical research are there regardless of which test I am using.
So when it comes to cousin matching, more SNPs don’t result in better matches. Which makes sense, because all of the companies use matching SNP and minimum segment length to determine a potential relationship.
In short, more is sometimes better. You need to check before you spend the money for more, or you may be spending money on something you aren't receiving.