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Autosomal DNA Testing for Genealogy

By , About.com Guide

Contained in the nucleus of each cell are twenty-three pairs of chromosomes. Twenty-two of these matched pairs of chromosomes are called "autosomes," while the 23rd pair determines your sex (male or female). Autosomal DNA is inherited from both parents, and includes random contributions from their parents, grandparents, and so on. Therefore, your autosomes essentially contain a complete genetic record, with all branches of your ancestry at some point contributing a piece of your autosomal DNA.

Used For:

Autosomal DNA tests can be used to search for relative connections along any branch of your family tree. Unless the connection is so far back that the shared DNA has essentially been eliminated through too many generations of recombination, any autosomal match between two individuals indicates a possible genetic connection. There is nothing in this test that will tell you which branch of your family the match is on, however. Therefore, having your parents, grandparents, cousins, and other family members tested will help you to narrow down potential matches.

Available To:

Everyone (males and females)

How Autosomal DNA Testing Works:

For each of your twenty-two pairs of autosomal chromosomes, you received one from your mother and one from your father. Before they passed these chromosomes down to you, the contents were randomly jumbled in a process called "recombination" (this is why you and your siblings are all a little different from each other). Your parents, in turn, received their chromosomes from their parents (your grandparents). Your autosomal DNA, therefore, contains random bits of DNA from your great-grandparents, great-great grandparents, and so on.

Close relatives will share large fragments of DNA from a common ancestor. Connections arising from more distant relatives will result in smaller fragments of shared DNA. The smaller the fragment of shared autosomal DNA, generally the further back the connection in your family tree. Even these tiny segments of shared DNA can potentially hold a clue, however! The way in which your individual DNA has recombined through the generations also means that you may no longer carry DNA from a particular ancestor. Distant relatives often share no genetic material at all, although it is also possible to match an individual through a very distant ancestor.

How Accurate is Autosomal DNA Testing?:

The average amount of autosomal DNA shared with a relative decreases with each successive generation. Percentages are also approximate - for example a sibling may share anywhere from 47–52% of their DNA in common.

  • 50% (parents and siblings)
  • 25% (grandparents, aunts/uncles, half-siblings, double first-cousins)
  • 12.5% (first cousins)
  • 6.25% (first cousins, once removed)
  • 3.125 (second cousins, first cousins twice removed)
  • 0.781% (third cousins)
  • 0.195% (fourth cousins)

The chance that an autosomal DNA test will accurately detect a relative decreases with the distance of the relationship. For example, most autosomal DNA ancestry tests predict an accuracy rate of 90–98% when detecting a match with a 3rd cousin, but around a 45–50% chance of detecting a match with a fourth cousin. Depending on the DNA recombination, however, an autosomal test may sometimes accurately detect more distant cousins (fifth cousins and beyond). Double descent from a common distant ancestor (e.g. marriage of second cousins) may potentially increase the chance of a match.

What Autosomal DNA Test Should I Have Done?:

A number of different companies are offering autosomal DNA tests, with a few offering databases to help you use your results to connect with other potential relatives. Three of the largest include (alphabetical order):

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