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Autosomal DNA

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Autosomal DNA is a term used in genetic genealogy to describe DNA which is inherited from the autosomal chromosomes. An autosome is any of the numbered chromosomes, as opposed to the sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (the X chromosome and the Y chromosome). Autosomes are numbered roughly in relation to their sizes. That is, Chromosome 1 has approximately 2,800 genes, while chromosome 22 has approximately 750 genes. There is no established abbreviation for autosomal DNA: atDNA (more common) and auDNA are used.

Autosomes diagram.jpg

Public domain logo This image is taken from the Talking Glossary of Genetic Terms and is reproduced courtesy of the National Human Genome Research Institute.

Autosomal DNA testing

Autosomal DNA tests for genetic genealogy are provided by 23andMe, AncestryDNA, Family Tree DNA (the Family Finder test, Living DNA and MyHeritage DNA. All the genealogical DNA tests give you matches with genetic cousins and also give you admixture percentages. 23andMe and MyHeritage also provide healh and trait reports. The 23andMe health and trait reports are only available in certain countries.

The tests from Family Tree DNA, Living DNA and MyHeritage are sold worldwide.

The AncestryDNA test is sold in the United States, the United Kingdom, Ireland, Canada, Australia and New Zealand. It was launched in an additional 29 countries in February 2016.[1] As of August 2019, the AncestryDNA test is available in 34 countries.

The 23andMe test is available in 56 countries. It is now available within all 50 US states, American Samoa, Guam, and the U.S. Virgin Islands. Due to separate local regulatory requirements, 23andMe does not offer the US product in Puerto Rico and the Northern Mariana Islands.[2] Prior to FDA approval the 23andMe test was not available to customers in New York and Maryland.[3][4]

For a comparison of the autosomal DNA testing services see the autosomal DNA testing comparison chart.

Accuracy of tests

Autosomal DNA tests can be used to confirm relationships with a high level of accuracy for parent/child relationships and all relationships up to the second cousin level. For all relationships additional contextual and genealogical information is required to confirm the nature of the relationship.

For genealogical relationships between second cousins once removed and 5th cousins a more careful approach is necessary and data needs to be collected from multiple family members. For relationships at the 4th cousin once removed to 5th cousin level you may need to test 10 to 20 or more first and second cousins and see how much autosomal DNA they share with a potential 4th cousin once removed or a potential 5th cousin in order to have sufficient data to generate a statistically significant average amount of autosomal DNA that is shared among the entire group, assuming that you are dealing with a non-endogamous population. See Tim Janzen's summary at for an example of this type of quantitative approach. For endogamous populations, genealogical relationships are frequently difficult to estimate beyond about the 2nd cousin level of relationship and require careful analysis.

Genealogical relationships beyond the 5th cousin level of relationship are more difficult to prove with autosomal DNA testing and, as a general rule, these can only be approached using triangulation. In some cases Y-DNA and mtDNA data may also be of help.

Who to test?

For autosomal DNA testing one should always test the oldest generations first wherever possible - your parents, grandparents (if you are lucky), aunts and uncles. By testing yourself as well as your parents you will be able to determine which segments have been inherited from which parent, and you will also be able to rule out coincidental (Identical by state) matches. A two-parent/child trio also provides the best results for the purposes of phasing and chromosome mapping and can help you to identify false matches which are found in the child but not in either of the parents.

If you only have one parent available for testing then you should test you and your parent. You should also test your siblings because they will inherit part of your parents' DNA that you don't carry.

Your uncles and aunts will inherit part of your grandparents' DNA that your parents don't have.

The next priority should be to test other close relatives from first to third cousins to get the best representation across your ancestry. Experience suggests that the maximum return is obtained by testing second cousins. They share one set of great grandparents so when someone matches you and a second cousin you get to narrow down your matches to that specific line.

For the purposes of chromosome mapping you would need to test four 2nd cousins in order to get the same amount of DNA mapped as would be the case if you just tested one first cousin. However, the segments mapped with second cousin data would be attributed back two generations whereas the segments mapped with first cousin data can only be attributed back one generation.

AncestryDNA have written a useful article Should other family members get tested? which includes a table showing how likely it is that your close family members would have a DNA match with a cousin that you don't share DNA with.

Autosomal DNA transfers

If you have tested at AncestryDNA, MyHeritage or at 23andMe (version 3, version 4, and version 5 chips only) you can transfer your results to Family Tree DNA through their autosomal DNA transfer program. A small fee is payable to access the chromosome browser feature and the MyOrigins report.

MyHeritage accepts free transfers from all companies. Data can be uploaded here.

Introduction to Autosomal DNA

Maurice Gleeson provides an introduction to autosomal DNA testing and the matching process in the following video:

Further reading

Autosomal DNA for beginners


Blog posts

General articles

Other resources

  • DNA Adoption The DNA Adoption website has many useful resources. Although the website was primarily set up to help adoptees get the most out of their autosomal DNA results, the methodology described is equally applicable for genealogists.
  • Autosomal DNA A video from the Sorenson Molecular Genealogy Foundation now hosted on the Learn Genetics website
  • Relatedness by Dr. Erin Cline Davis, a science writer at 23andMe, Ask a geneticist, 10 October 2008.


  1. Murray J. AncestryDNA now offered in 29 new countries. Ancestry blog, 23 February 2016.
  2. Richard Hill. Message posted in the ISOGG Facebook group, 6 November 2015.
  3. NY and MD limits on 23and Me by Judy G. Russell, The Legal Genealogist, 23 December 2012.
  4. That pesky NY law by Judy G. Russell, The Legal Genealogist, 23 June 2013.

See also

Public domain logo

This article uses material in the public domain from the Talking Glossary of Genetic Terms and is reproduced courtesy of the National Human Genome Research Institute.