There is always more to say about DNA

My connections to the Iberian Peninsula and my wife's connections to the Middle East have given me further incentive to delve further into the practical realities of genealogical DNA testing. First of all, this type of discussion can get technical really fast. What we are talking about here is haplogroups. Here is the definition of a haplogroup from the Wikipedia article, Haplogroup.

A haplotype is a group of genes in an organism that are inherited together from a single parent,[1][2] and a haplogroup (haploid from the Greek: ἁπλούς, haploûs, "onefold, simple" and English: group) is a group of similar haplotypes that share a common ancestor with a single-nucleotide polymorphism mutation.

I am going to leave in all the cross references to allow you to do your own study of this issue. Quoting from the article further:

In human genetics, the haplogroups most commonly studied are Y-chromosome (Y-DNA) haplogroups and mitochondrial DNA (mtDNA) haplogroups, both of which can be used to define genetic populations. Y-DNA is passed solely along the patrilineal line, from father to son, while mtDNA is passed down the matrilineal line, from mother to offspring of both sexes. Neither recombines, and thus Y-DNA and mtDNA change only by chance mutation at each generation with no intermixture between parents' genetic material.

In genealogical DNA testing, we encounter another type of test; the autosomal DNA test. Here is as simple an explanation of autosomal DNA as you can find from Wikipedia: Genealogical DNA test.

Autosomal DNA is the 22 pairs of chromosomes that do not contribute to sex.[2] These are inherited exactly equally from both parents and roughly equally from grandparents to about 3x great-grandparents.[3] Inheritance is more random and unequal from more distant ancestors.[4] Generally, a genealogical DNA test might test about 700,000 SNPs (single-nucleotide polymorphisms). Like mtDNA and Y-DNA SNPs, autosomal SNPs are changes at a single point in the genetic code. Autosomal DNA recombines each generation.[5] Therefore, the number of markers shared with a specific ancestor decreases by about half each generation. [I edited some typographical errors, but I am leaving in all the links]

Essentially, the Y-DNA and mtDNA tests can go back thousands of years, but the autosomal DNA test results become quickly attenuated with time. From a genealogical standpoint, the main issue is the "margin of error" with all three types of tests. If we look at the "AncestryDNA Terms and Conditions (United States)" we find the following statements:

We attempt to ensure that all Content on the Website is complete and accurate. Despite our efforts, the Content may occasionally be inaccurate or incomplete and we make no representation that the Content on the Website is complete, accurate, reliable or error-free.

The Terms and Conditions go on to explain:

We make no express warranties or representations as to the quality and accuracy of the Content, the Website or the Service, and we disclaim any implied warranties or representations, including but not limited to implied warranties of merchantability, fitness for a particular purpose, or non-infringement, to the full extent permissible under applicable law. We offer the Content, the Website and the Service on an "as is" basis and do not accept responsibility for any use of or reliance on the Website, Content or Service, or for any disruptions to or delay in the Service. In addition, we do not make any representations as to the accuracy, comprehensiveness, completeness, quality, currency, error-free nature, compatibility, security or fitness for purpose of the Website, Content or Service.

Of course, this does not address the accuracy of the AncestryDNA test at all. It simply states that they are not going to tell you how accurate the tests really are.

So how accurate are the DNA tests? Dick Eastman had a post not long ago that addressed this issue. He linked to an interesting news story on Yahoo TV entitled, "Investigation Puts Ancestry DNA Kits to the Test Among Sets of Triplets." The newscasters threw in a set of quads for good measure. The real issue with all of the tests is the margin of error.

The term "margin of error" is defined as follows from Wikipedia: Margin of error:

The margin of error is a statistic expressing the amount of random sampling error in a survey's results. It asserts a likelihood (not a certainty) that the result from a sample is close to the number one would get if the whole population had been queried. The likelihood of a result being "within the margin of error" is itself a probability, commonly 95%, though other values are sometimes used. The larger the margin of error, the less confidence one should have that the poll's reported results are close to the true figures; that is, the figures for the whole population. Margin of error applies whenever a population is incompletely sampled. 

Margin of error is often used in non-survey contexts to indicate observational error in reporting measured quantities.

For the DNA tests results to have a greater degree of believability, they should come with a clear statement of the probable margin of error since the test results are certainly incompletely sampled. There could be a number of explanations why the identical triplets came up with different percentages reported for a DNA test, but the real issue is, again, the margin of error. This is especially true when the reported relationships are based on very small percentages of shared DNA.

DNA testing still has a long way to go before it is entirely useful beyond a few generations.