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, Family Tree DNA (the Family Finder test) and AncestryDNA. The Family Finder test and the AncestryDNA tests are both genealogical DNA tests which give you matches with genetic cousins and also give you admixture percentages. The 23andMe test is a genealogical DNA test but also provides information on health and traits. The cousin-finding element of the 23andMe test is known as DNA Relatives (formerly Relative Finder)). The 23andMe health reports were suspended pending FDA regulatory approval in November 2013, but have now been reintroduced in the UK, Ireland and Canada.

The AncestryDNA test is only available in the United States, the United Kingdom, Ireland, Canada, Australia and New Zealand. The 23andMe test is available in 56 countries. The Family Finder test is available worldwide. For further information see the article on shipping DNA kits, which includes information on shipping costs together with a list of package forwarders. Note that if you are in the US and live in New York or Maryland you will not be able to order a 23andMe test, though you will be able to purchase the AncestryDNA test or the Family Finder test.[1][2]

Autosomal tests are also offered by the Genographic Project (Geno 2.0) and BritainsDNA but these tests will only provide you with ethnicity percentages and cannot be used for genealogical purposes and do not give you matches with genetic cousins.

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 other than parent/child 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 10 to 20 or more cousins tested in order to start drawing valid conclusions, assuming that you are dealing with a non-endogamous population. 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.

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.

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

Methodology

Blog posts

General articles

Other resources

References

  1. NY and MD limits on 23and Me by Judy G. Russell, The Legal Genealogist, 23 December 2012.
  2. 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.