How DNA Testing Works

Who am I and where do I come from? These are two of the most fundamental questions in human existence. In the past, for many adoptees, these questions remained a mystery. But today is a different story.

DNA testing was first developed in 1985 by Dr. Alec Jeffreys. Originally it was used strictly in forensic science but today it is available to anyone with a desire to know more about their genetic makeup. Thanks to sites like AncestryDNA, Family Tree DNA, 23andMe, and MyHeritage for less than $100 and a swab of saliva it’s possible to map your family tree through multiple generations and even to find unknown living relatives.

So how does it work? Essentially there are three different DNA tests available. Y-DNA, Mitochondrial DNA (mtDNA) and autosomal DNA. Y-DNA testing is used to determine the paternal line and mtDNA testing can identify a direct maternal line. Y-DNA and mtDNA testing cannot determine an ethnic group but they can be used to establish a haplogroup (a common ancestor or patriline/matriline). 

For genetic ancestry testing, autosomal DNA can prove the most useful. Human beings have 23 pairs of autosomes (which are in addition to the sex chromosomes, X and Y). Each person inherits exactly 50% of their father’s autosomes and 50% of their mother’s autosomes. Therefore, each person also contains 25% of their grandparents’ autosomes and 12.5% of any first cousin. As you go further and further up the genealogical tree the percentages decrease with every step. The benefit of testing for autosomal DNA is that it can provide a high level of accuracy of determining everything from parent/child relationships up to a second cousin. Past second cousins, the percentage rate decreases too low to establish anything with certainty without the help of other genealogical aids.

Based on the high accuracy rate of autosome DNA testing, it should be easy to identify your biological family. The trick is there are many tests out there and each test (and testing site) reveals something different. More than 6 million people are estimated to be in some type of genetic genealogy database. The difficulty is there are several of those databases to choose from. AncestryDNA boasts the most members, but there is no guarantee that the relative you’re searching for chose that site to be tested. Additionally, the very essence of DNA testing relies on some member of your family (mother, grandfather, first cousin, etc.) having been tested at some point by someone.

The best way to get started is to choose a site and take an autosomal DNA test. From there start with your closest DNA match (even if it’s just a second cousin with a 6.25% match) and begin to build your family tree based on commonalities. Remember, genetic ancestry DNA testing does have its limitations. While it can provide a clue to biological family members, it cannot provide information on family diseases or medical conditions.