15.3 atDNA Testing

DNA testing has grown in popularity since the beginning of the genetic DNA testing industry in the earliest years of the twenty-first century. Although there are different kinds of DNA testing that can be utilized depending on the research question, most DNA research starts with autosomal testing, otherwise known as an atDNA test.

Autosomal DNA is “the twenty-two pairs of non-sex chromosomes found within the nucleus of every cell.”1 atDNA tests look at thousands of single nucleotide polymorphisms that are in the autosomal chromosomes. “Each of the testing companies has selected a minimum segment length threshold that must be met before two people in the testing database will be flagged as sharing DNA.”2 When two people within the database share enough single nucleotide polymorphisms, the company marks them as “matches.” The amount of shared DNA is measured in centimorgans (cM) and the higher the number of cMs, the closer the biological relationship is between the matches. In fact, “the amount of DNA shared by two people can also help estimate the genealogical relationship between those two people.”3 The Shared cM Project tool4 defines cM ranges in correlation with different biological relationships and is widely used within the genetic genealogy community.

When someone takes an atDNA test, it is common for the testing company to provide them with a list of people who are their matches. By looking at the number of shared cMs between two people, a general or estimated biological relationship can be guessed. Some companies offer an additional tool that may be referred to as “shared matches” which attempts to identify a common ancestor between the two testers. This is the start of triangulation, an important strategy for proving biological connections.

In very basic terms, if a common ancestor can be determined between two matches, then document and record research is used to trace each match back to that common ancestor. By comparing generations and the family trees created by other DNA testers, the family historian may then be able to identify the true relationship between the matches. That means when atDNA suggests two individuals share a biological connection, then traditional document research, building both family trees back to the common ancestor, gives the researcher needed information to determine the biological relationship between the two individuals.

This process may be repeated many times during a single DNA project. The more trees that triangulate with the common ancestor, the stronger the evidence of the relationships between the matches. This is just one strategy or tool in the process of genetic genealogy.

“Genealogists use the results of atDNA testing for two main purposes: cousin matching and ethnicity analysis. Ethnicity estimates provided by testing companies attempt to break down the test taker’s DNA into continental or regional sources. Although these are only estimates they can have genealogical applications.”5 Estimates are just that: estimations. As databases grow, and more people are tested, the results are adjusted to reflect new analysis and information. One tester, very excited to be identified with DNA that suggested ancestry connected to Malta, was later saddened when an update adjusted the estimate and eliminated Malta in favor of Southern Italy. Document research proved the updated region of Italy to be correct, but the initial disappointment was part of the DNA journey. Taking a DNA test is a journey. Solid, sure answers are rarely the result, and there can be many kinds of surprises.

This is a simplified overview of a complex topic. For more information, consult the resources cited in this textbook and the websites of companies that offer atDNA testing. Ancestry.com, 23andMe, and Family Tree DNA are big players for atDNA testing, but there are other options.6 MyHeritage and those with smaller databases may have specialized tools or give researchers the opportunity to upload test results from other companies to have the best resources to answer the research question. Careful consideration should be given to selecting and using the right DNA testing service.

Because of privacy laws and other issues surrounding DNA testing, not all tests are available in every country. On her The DNA Geek blog, Dr. Leah Larkin maintains a list of which atDNA tests are available in different locations.7 For international researchers and prospective testers, becoming familiar with local laws and understanding what testing is allowed will help reduce complications that could waste time and money.